1911 Encyclopædia Britannica/Plants
PLANTS. In the most generally used sense, a plant is a member of the lower or vegetable order of living organized things; the term is also popularly applied to the smaller herbaceous plants, thus excluding trees and shrubs. The early use of the word is for a twig, shoot, cutting or sapling, which was the meaning of Lat. planta (for plancta, the root being that seen in planus, flat, cf. Gr. πλατύς, broad; planta thus meant a spreading shoot or sucker). Other meanings of “plant” are derived from the verb “to plant” (Lat. plantare, to fix in position or place). It is thus used of the fixtures, machinery, apparatus necessary for the carrying on of an industry or business, and in colloquial or slang use, of a swindle, a carefully arranged plot or trap laid or fixed to deceive; cf. also Plantation. In the following sections the botanical sense of the word is followed, the term being used generally as opposed to “animals.”
Classification of Plants
Some account of the history of plant classification and the development of a natural system in which an attempt is made to show the actual relationships of plants, is given in the article Botany. The plant world falls into two great divisions, the higher or flowering plants (Phanerogams), characterized by the formation of a seed, and the lower or flowerless plants (Cryptogams), in which no seed is formed but the plants are disseminated by means of unicellular bodies termed spores. The term Cryptogam is archaic, implying a hidden method of reproduction as compared with the obvious method represented by the flower of the Phanerogam; with the aid of a good microscope it is, however, easier to follow the process of fertilization in many Cryptogams than in the flowering plants. These two great divisions are moreover of unequal value, for the Cryptogams comprise several groups differing from each other by characters as marked as those which separate some of them from the Phanerogams. The following groups or sub-kingdoms are those which are now generally recognized:—
Thallophyta, are the most lowly organized plants and include a great variety of forms, the vegetative portion of which consists of a single cell or a number of cells forming a more or less branched thallus. They are characterized by the absence of that differentiation of the body into root, stem and leaf which is so marked a feature in the higher plants, and by the simplicity of their internal structure. Both sexual and asexual reproduction occur, but there is usually no definite succession of the two modes marking that alternation of sexual generation (gametophyte) and asexual generation (sporophyte) which characterizes the higher groups. The group has until recent years been regarded as comprising three classes distinguished by well-marked physiological features—the Algae (including the Seaweeds) which contain chlorophyll, the Fungi which have no chlorophyll and therefore lead a saprophytic or parasitic mode of life, and the Lichens which are composite organisms consisting of an alga and a fungus living together in a mutual parasitism (symbiosis); Bacteria were regarded as a section of Fungi. Such a system of classification, although convenient, is not the most natural one, and a sketch of the system which better expresses the relationships between the various subdivisions is given here. It has however been deemed advisable to retain the older groups for purpose of treatment in this work, and articles will be found under the headings Algae, Fungi, Bacteria, and Lichens. The study of phylogeny has suggested fourteen classes arranged in the following sequence: (1) Bacteria; (2) Cyanophyceae (Blue-green algae); (3) Flagellatae; (4) Myxomycetes (Slime-fungi); (5) Peridineae; (6) Conjugatae; (7) Diatomaceae (Diatoms); (8) Heteroconteae; (9) Chlorophyceae (Green Algae); (10) Characeae (Stoneworts); (11) Rhodophyceae (Red Algae); (12) Eumycetes (Fungi); (13) Phycomycetes (Algal fungi); (14) Phaeophyceae (Brown Algae). Bacteria (see Bacteriology) and Cyanophyceae (see Algae), which are often grouped together as Schizophyta, are from points of view of both structure and reproduction extremely simple organisms, and stand apart from the remaining groups, which are presumed to have originated directly or indirectly from the Flagellatae, a group of unicellular aquatic organisms combining animal and plant characteristics which may be regarded as the starting-point of unicellular Thallophytes on the one hand and of the Protozoa on the other. Thus simple forms included in the Heteroconteae, Chlorophyceae and Phaeophyceae show an obvious connexion with the Flagellatae; the Peridineae may be regarded as a further developed branch; the Conjugatae and Diatomaceae cannot be directly connected; the origin of the Rhodophyceae is also obscure; while the Characeae are an advanced and isolated group (see Algae). The Mycetozoa (q.v.) or Myxomycetes are a saprophytic group without chlorophyll of simple structure and isolated position. The algal fungi, Phycomycetes, are obviously derived from the Green Algae, while the remaining Fungi, the Eumycetes, appear to have sprung from the same stock as the Rhodophyceae (see Fungi). Owing to the similarity of structure and mode of life it is convenient to treat the Lichens (q.v.) as a distinct class, while recognizing that the component fungus and alga are representatives of their own classes.
The Bryophyta and Pteridophyta have sprung from the higher Thallophyta, and together form the larger group Archegoniatae, so-called from the form of the organ (archegonium) in which the egg-cell is developed. The Archegoniatae are characterized by a well-marked alternation of gametophyte and sporophyte generations; the former bears the sexual organs which are of characteristic structure and known as antheridia (male) and archegonia (female) respectively; the fertilized egg-cell on germination gives rise to the spore-bearing generation, and the spores on germination give rise directly or indirectly to a second gametophyte.
The Mosses and Liverworts (see Bryophyta) include forms with a more or less leaf-like thallus, such as many of the liverworts, and forms in which the plant shows a differentiation into a stem bearing remarkably simple leaves, as in the true mosses. They have no true roots, and their structure is purely cellular or conducting bundles of a very simple structure are present. The independent plant which is generally attached to the soil by hair-like structures is the sexual generation, the sporophyte is a stalked or sessile capsule which remains always attached to the gametophyte from which it derives the whole or part of its nourishment.
The Ferns and fern-like plants (see Pteridophyta) have on the other hand a well developed independent sporophyte which is differentiated into stem, leaf and root with highly organized internal structure including true vascular bundles. In general structure they approach the Phanerogams with which they form collectively the Vascular Plants as contrasted with the Cellular Plants—Thallophyta and Bryophyta. The gametophyte is a small thalloid structure which shows varying degrees of independence affording an interesting transition to the next group.
Spermatophyta are characterized by an extreme reduction of the gametophyte generation. The sporophyte is the plant which is differentiated into stem, leaf and root, which show a wonderful variety of form; the internal structure also shows increased complexity and variety as compared with the other roup of vascular plants, the Pteridophyta. The spores, as in the heterosporous Pteridophyta, are of two kinds—microspores (pollen grains) borne in microsporangia (pollen sacs) on special leaves (sporophylls) known as stamens, and macrospores (embryo-sac) borne in macrosporangia (ovules) on sporophylls known as carpels. The fertile leaves or sporophylls are generally aggregated on special shoots to form flowers which may contain one or both kinds. The microspores are set free from the sporangium and carried generally by wind or insect agency to the vicinity of the macrospore, which never leaves the ovule. The male gametophyte is represented by one or few cells and, except in a few primitive forms where the male cell still retains the motile character as in the Pteridophyta, is carried passively to the macrospore in a development of the pollen grain, the pollen tube. The Spermatophyta are thus land plants par excellence and have, with the few exceptions cited, lost all trace of an aquatic ancestry. Aquatic plants occur among seed plants but these are readaptations of land plants to an aquatic environment. After fertilization the female cell, now called the oospore, divides and part of it develops into the embryo (new sporophyte), which remains dormant for a time still protected by the ovule which has developed to become the seed. The seed is a new structure characteristic of this group, which is therefore often referred to as the Seed-plants. The seed is set free from the parent plant and serves as the means of dissemination (see Flower; Pollination, Fruit, and Seed). The Spermatophyta fall into two classes, Gymnosperms (q.v.) and Angiosperms (q.v.); the former are the more primitive group, appearing earlier in geological time and showing more resemblance in the course of their life-history to the Pteridophyta. A recently discovered fossil group, the Pteridospermae (see Palaeobotany) have characters intermediate between the Pteridophyta and the more primitive seed-plants.
In Gymnosperms—so-called because the ovules (and seeds) are borne on an open sporophyll or carpel—the microsporophylls and macrosporophylls are not as a rule associated in the same shoot and are generally arranged in cone-like structures; one or two small prothallial cells are formed in the germination of the microspore; the male cells are in some older members of the group motile though usually passive. The ovule is not enclosed in an ovary, and the usually solitary macrospore becomes filled with a prothallus, in the upper part of which are formed several rudimentary archegonia. The fertilized egg-cell (oospore) forms a filamentous structure, the proembryo, from a restricted basal portion of which one or more embryos develop, one only as a rule reaching maturity. The embryo consists of an axis bearing two or more cotyledons and ending below in a radicle; it lies in a generally copious food-storing tissue (endosperm) which is the remains of the female prothallus. The plant has a well-developed main root (tap-root) and a single or branched leafy stem which is provided with a means of secondary increase in thickness. The leaves are generally tough-skinned and last for more than one season.
The Angiosperms, which are much the larger class, derive their name from the fact that the carpel or carpels form a closed chamber, the ovary, in which the ovules are developed—associated with this is the development of a receptive or stigmatic surface on which the pollen grain is deposited. The sporophylls (stamens and carpels) are generally associated with other leaves, known as the perianth, to form a flower; these subsidiary leaves are protective and attractive in function and their development is correlated with the transport of pollen by insect agency (see Angiosperms; Pollination, and Flower). The male gametophyte is sometimes represented by a transitory prothallial cell; the two male cells are carried passively down into the ovary and into the mouth of the ovule by means of the pollen-tube. The female gametophyte is extremely reduced, there is a sexual apparatus of naked cells, one of which is the egg-cell which, after fusion with a male cell, divides to form a large “suspensorial” cell and a terminal embryo. Endosperm is formed as the result of the fusion of the second male cell with the so-called “definitive nucleus” of the embryo-sac (see Angiosperms). The embryo consists of an axis bearing one (Monocotyledons) or two (Dicotyledons) cotyledons, which protect the stem bud (plumule) of the future plant, and ending below in a radicle. The seed is enclosed when ripe in the fruit, a development of the ovary as a result of fertilization of the egg-cell. (A. B. R.)
Anatomy of Plants
The term “Anatomy,” originally employed in biological science to denote a description of the facts of structure revealed on cutting up an organism, whether with or without the aid of lenses for the purposes of magnification, is restricted in the present article, in accordance with a common modern use, to those facts of internal structure not concerned with the constitution of the individual cell, the structural unit of which the plant is composed.
Fig. 1.—Examples of the differentiation of the cells of plants.
A, Cell (individual) of the unicellular Green Alga Pleurococcus, as an example of an undifferentiated autonomous assimilating cell. pr., Cell protoplasm; n., nucleus; chl., chloroplast; c.w., cell-wall.
B, Plant of the primitive Siphoneous Green Alga Protosiphon botryoides. The primitive cell sends colourless tubelets (rhizoids, rh.) into the mud on which it grows. The subaerial part is tubular or ovoid, and contains the chloroplast (chl.). There are several nuclei.
C, Base of the multicellular filamentous Green Alga Chaetomorpha aerea. The basal cell has less chlorophyll than the others, and is expanded and fixed firmly to the rock on which the plant grows by the basal surface, rh, thus forming a rudimentary rhizoid.
D, Part of branched filamentous thallus of the multicellular Green Alga Oedocladium. cr. ax, Green axis creeping on the surface of damp soil; rh., colourless rhizoids penetrating the soil; asc. ax., ascending axes of green cells.
E, Vertical section of frond of the complicated Siphoneous Green Alga Halimeda. The substance of the frond is made up by a single much-branched tube, with interwoven branches. cond med., Longitudinally running comparatively colourless central (medullary) branches, which conduct food substances and support the (ass. cor.) green assimilating cortical branches, which are the ends of branches from the medulla and fit tightly together, forming the continuous surface of the plant.
F, Section through the surface tissue of the Brown Alga Cutleria multifida, showing the surface layer of assimilating cells densely packed with phaeoplasts. The layers below have progressively fewer of these, the central cells being quite colourless.
G, Section showing thick-walled cells of the cortex in a Brown Alga (seaweed). Simple pits (p.) enable conduction to take place readily from one to another.
H, Two adyacent cells (leptoids) of a food-conducting strand in Fucus (a Brown seaweed). The wall between them is perforated, giving passage to coarse strands of protoplasm.
I, End of hydroid of the thalloid Liverwort Blyttia, showing the thick lignified wall penetrated by simple pits.
An account of the structure of plants naturally begins with the cell which is the proximate unit of organic structure. The cell is essentially an individualized mass of protoplasm containing a differentiated protoplasmic body, called a nucleus. But all cells which are permanent tissue-elements of the plant body possess, in addition, a more or less rigid limiting membrane or cell-wall, consisting primarily of cellulose or some allied substance. It is the cell-walls which connect the different cells of a tissue (see below), and it is upon their characters (thickness, sculpture and constitution) that the qualities of the tissue largely depend. In many cases, indeed, after the completion of the cell-wall (which is secreted by the living cell-body) the protoplasm dies, and a tissue in which this has occurred consists solely of the dead framework of cell-walls, enclosing in the cavities, originally occupied by the protoplasm, simply water or air. In such cases the characters of the adult tissue clearly depend solely upon the characters of the cell-walls, and it is usual in plant-anatomy to speak of the wall with its enclosed cavity as “the cell,” and the contained protoplasm or other substances, if present, as cell-contents. This is in accordance with the original use of the term “cell,” which was applied in the 17th century to the cavities of plant-tissues on the analogy of the cells of honeycomb. The use of the term to mean the individualized nucleated mass of living protoplasm, which, whether with or without a limiting membrane, primitively forms the proximate histological element of the body of every organism, dates from the second quarter of the 19th century. For a more detailed description of the cell see Cytology and the section on Cytology of Plants below). In all but the very simplest forms the plant-body is built up of a number of these cells, associated in more or less definite ways. In the higher (more complicated) plants the cells differ very much among themselves, and the body is composed of definite systems of these units, each system with its own characteristic structure, depending partly on the characters of the component cells and partly on the method of association. Such a system is called a tissue-system, the word tissue being employed for any collection of cells with common structural, developmental, or functional characters to which it may be conveniently applied. The word is derived from the general resemblance of the texture of plant substance to that of a textile fabric, and dates from a period when the fundamental constitution of plant substance from individual cells was not yet discovered. It is convenient here to define the two chief types of cell-form which characterize tissues of the higher plants. The term parenchyma is applied to tissues whose cells are isodiametric or cylindrical in shape, prosenchyma tissues consisting of long narrow cells, with pointed ends.
Fig. 1a.—Examples of the differentiation of the tissue of plants.
J, End of hydroid of the Moss Mnium, showing particularly thin oblique end wall. No pits.
K, Optical section of two adjacent leptoids of the Moss Polytrichum juniperinum. The leptoids are living and nucleated. They bulge in the neighbourhood of the very thin cross-wall. Note resemblance to H and R.
L, Optical section of cell of parenchyma in the same moss. Embedded in the protoplasm are a number of starch grains.
M, Part of elongated stereid of a Moss. Note thick walls and oblique slit-like pits with opposite inclination on the two sides of the cell seen in surface view.
N, One side of the end of hydroid (tracheid) of a Pteridophyte (fern), with scalariform pits.
O, Optical section of two adjacent leptoids (sieve-tube segments) of Pteridophyte, with sieve plates (s. pl.) on oblique end-wall and side-walls.
P, Part of spiral hydroid (tracheid) of Phanerogam (Flowering Plant).
Q, Three segments of a “pitted” vessel of Phanerogam.
R, Optical section of leptoid (sieve-tube segment) of Phanerogam, with two proteid (companion) cells. s. pl., sieve-plate.
S, Optical section of part of thick-walled stereid of Phanerogam, with almost obliterated cavity and narrow slit-like oblique pits.
T, Part of vertical section through blade of typical leaf of Phanerogam. u.e., Upper epidermal cells, with (c) cuticle. (p) Assimilating (palisade) cell. sp., Assimilating (spongy) cells with large lacunae. l.e., Lower epidermis, with st., stoma
U, Absorbing cell, with process (root-hair) from piliferous layer of root of Phanerogam.
V, Endodermal cell of Phanerogam, with suberized central band on radial and transverse walls.
We may now proceed to a systematic account of the anatomy of the different groups of plants, beginning with the simplest, and passing to the more complicated forms.
Thallophyta. -The simplest members of both the Algae and the Fungi (q.v.) (the two divisions of the Thallophyta, which is the lowest of the four great groups into which the plant-kingdom is divided) have their bodies each composed of a single cell. In the Algae such a cell consists essentially of: (1) a mass of protoplasm provided with (2) a nucleus and (3) an assimilating apparatus consisting of a coloured protoplasmic body, called a chromatophore, the pigment of which in the pure green forms is chlorophyll, and which may then be called a chloroplast. The whole of these living structures are covered externally by the dead cell-membrane (fig. 1 A) It is from such a living and assimilating cell, performing as it does all the vital functions of a green plant, that, according to current theory, all the different cell-forms of a higher plant have been differentiated in the course of descent.
Among the Green Algae the differentiation of cells is comparatively slight. Many forms, even when multicellular, have all their Cell and Tissue Differentiation in Algæ. cells identical in structure and function, and are often spoken of as “physiologically unicellular.” The cells are commonly joined end to end in simple or branched filaments. Such differentiation as exists in the higher types mainly takes two directions. In the fixed forms the cell or cells which attach the plant to the substratum often have a peculiar form, containing chlorophyll and constituting a rudimentary fixing organ or rhizoid (fig. 1 C). In certain types living on damp soil, the rhizoids penetrate the substratum, and in addition to fixing the plant absorb food substances (dissolved salts) from the substratum (fig. 1 B and D).
The second type of differentiation is that between supporting axis and assimilating appendages. The cells of the axis are commonly stouter and have much less chlorophyll than those of the appendages (Draparnaldia). This differentiation is parallel with that between stem and leaf of the higher plants. In the group of the Siphoneae both these types of differentiation may exist in the single, long, branched, tube-like and multinucleate “cell” (coenocyte) which here forms the plant-body. Protosiphon (fig. 1 B) is an example parallel with Oedocladium; Bryopsis, with Draparnaldia. In Caulerpa the imitation of a higher plant by the differentiation of fixing, supporting and assimilating organs (root, stem and leaf) from different branches of the single cell is strikingly complete. In the Siphoneous family of Codiaceae the branches of the primitive cell become considerably interwoven one with another, so that a dense tissue-like structure is often produced. In this we get a further differentiation between the central tubes (branches of the primitive cell), which run in a longitudinal direction through the body, possess little or no chlorophyll, and no doubt serve to conduct food substances from one region to another, and the peripheral ones, which are directed perpendicularly to the surface of the body, ending blindly there, contain abundant chlorophyll, and are the assimilating organs (fig. 1 E).
None of the existing Red Seaweeds (Rhodophyceae) has a unicellular body. The thallus in all cases consists of a branched filament of cells placed end to end, as in many of the Green Algae. Each branch grows simply by the transverse division of its apical cell. The branches may be quite free or they may be united laterally to form a solid body of more or less firm and compact consistency. This may have a radial stem-like organization, a central cell thread giving off from every side a number of short sometimes unicellular branches, which together form a cortex round the central thread, the whole structure having a cylindrical form which only branches when one of the short cell-branches from the central thread grows out beyond the general surface and forms in its turn a new central thread, from whose cells arise new short branches. Or the thallus may have a leaf-like form, the branches from the central threads which form the midrib growing out mainly in one plane and forming a lamina, extended right and left of the midrib. Numerous variations and modifications of these forms exist. In all cases, while the internal threads which bear the cortical branches consist of elongated cells with few chromatophores, and no doubt serve mainly for conduction of food substances, the superficial cells of the branches themselves are packed with chromatophores and form the chief assimilating tissue of the plant. In the bulky forms colourless branches frequently grow out from some of the cortical cells, and, pushing among the already-formed threads in a longitudinal direction, serve to strengthen the thallus by weaving its original threads together. The cells belonging to any given thread may be recognized at an early stage of growth, because each cell is connected with its neighbours belonging to the same thread by two depressions or pits, one at each end. The common wall separating the pits of the two adjoining cells is pierced by strands of protoplasm. The whole structure, consisting of the two pits and the wall between is known as a genetic pit. Other pits, connecting cells not belonging to the same branch, are, however, formed at a later stage.
Many of the lower forms of Brown Seaweeds (Phoeophyceae) have a thallus consisting of simple or branched cell threads, as in the green and red forms. The lateral union of the branches to form a solid thallus is not, however, so common, nor is it carried to so high a pitch of elaboration as in the Rhodophyceae. In a few of the lower forms (Sphacelariaceae), and in the higher forms which possess a solid thallus, often of very large size, the plant-body is no longer formed entirely of branched cell-threads, but consists of what is called a true parenchymatous tissue, i.e. a solid mass of cells, formed by cell division in all directions of space. In the Laminariaceae this tissue is formed by cell division at what is called an intercalary growing point, i.e. a meristematic (cell-dividing) region occupying the whole of a certain transverse zone of the thallus, and cutting off new cells to add to the permanent tissue on both sides. In the Fucaceae, on the other hand, there is a single prismatic apical cell situated at the bottom of a groove at the growing apex of the thallus, which cuts off cells from its sides to add to the peripheral, and from its base to add to the central permanent cells. The whole of the tissue of the plant is formed by the division of this apical cell. In whatever way the tissues are originally formed, however, the main features of their differentiation are the same. According to a law which, as we have seen, applies also to the green and red forms, the superficial cells are packed with chromatophores and form the assimilating tissue (fig. 1, F). In these brown types with bodies of considerable thickness (Laminariaceae and Fucaceae), there is, however, a further differentiation of the internal tissues. The cells immediately subjacent to the superficial assimilating layer form a colourless, or nearly colourless, parenchymatous cortex, which acts as a food storage tissue (fig. 1, G), and surrounds a central medulla of elongated conducting cells. The latter are often swollen at the ends, so that the cross-wall separating two successive cells has a larger surface than if the cells were of uniform width along their entire length. Cells of this type are often called trumpet-hyphae (though they have no connexion with the hyphae of Fungi), and in some genera of Laminariaceae those at the periphery of the medulla simulate the sieve-tubes of the higher plants in a striking degree, even (like these latter) developing the peculiar substance callose on or in the perforated cross-walls or sieve-plates. A specialized conducting tissue of this kind, used mainly for transmitting organic substances, is always developed in plants where the region of assimilative activity is local in the plant-body, as it is in practically all the higher plants. This is the case in the Fucaceae, and in a very marked degree in the Laminariaceae in question, where the assimilative frond is borne at the end of an extremely long supporting and conducting stipe. A similar state of things exists in some of the more highly differentiated Red Seaweeds. The tissue developed to meet the demands for conduction in such cases always shows some of the characters described. It is known as leptom, each constituent cell being a leptoid (fig. 1, H). In addition to the cell types described, it is a very common occurrence in these bulky forms for rhizoid-like branches of the cells to grow out, mostly from the cells at the periphery of the medulla, and grow down between the cells, strengthening the whole tissue, as in the Rhodophyceae. This process may result in a considerable thickening of the thallus. In many Laminariaceae the thallus also grows regularly in thickness by division of its surface layer, adding to the subjacent permanent tissue and thus forming a secondary meristem.
The simpler Fungi, like the simpler Green Algae, consist of single cells or simple or branched cell-threads, but among the Tissue Differentiation in Fungi. higher kinds a massive body is often formed, particularly in connexion with the formation of spores, and this may exhibit considerable tissue-differentiation. A characteristic feature of the fungal vegetative plant-body (mycelium) is its formation from independent coenocytic tubes or cell-threads. These branch, and may be packed or interwoven to form a very solid structure; but each grows in length independently of the others and retains its own individuality, though its growth in those types with a definite external form is of course correlated with that of its neighbours and is subject to the laws governing the general form of the body. Such an independent coenocytic branch or cell-thread is called a hypha. Similar modes of growth occur among the Siphoneous Green Algae and also among the Red Seaweeds. A solid fungal body may usually be seen to consist of separate hyphae, but in some cases these are so bent and closely interwoven that an appearance like that of ordinary parenchymatous tissue is obtained in section, the structure being called pseudoparenchyma. By the formation of numerous cross-walls the resemblance to parenchyma is increased. The surface-layer of the body in the massive Fungi differs in character according to its function, which is not constant throughout the class, as in the Algae, because of the very various conditions of life to which different Fungi are exposed. In many forms its hyphae are particularly thick-walled, and may strikingly resemble the epidermis of a vascular plant. This is especially the case in the lichens (symbiotic organisms composed of a fungal mycelium in association with algal cells), which are usually exposed to very severe fluctuations in external conditions. The formation of a massive body naturally involves the localization of the absorptive region, and the function of absorption (which in the simpler forms is carried out by the whole of the vegetative part of the mycelium penetrating a solid or immersed in a liquid substratum) is subserved by the outgrowth of the hyphae of the surface-layer of that region into rhizoids, which, like those of the Algae living on soil, resemble the root-hairs of the higher plants. The internal tissue of the body of the solid higher Fungi, particularly the elongated stalks (stipes) of the fructifications of the Agarics, consists of hyphae running in a longitudinal direction, which no doubt serve for the conduction of organic food substances, just as do the “trumpet-hyphae,” similar in appearance, though not in origin, of the higher Brown Seaweeds. (In one genus (Lactarius) “milk-tubes,” recalling the laticiferous tubes of many vascular plants, are found.) These elongated hyphae are frequently thick-walled, and in some cases form a central strand, which may serve to resist longitudinal pulling strains. This is particularly marked in certain lichens of shrubby habit. The internal tissues, either consisting of obvious hyphae or of pseudoparenchyma, may also serve as a storehouse of plastic food substances.
Looking back over the progress of form and tissue-differentiation in the Thallophyta, we find that, starting from the simplest unicellular forms with no external differentiation of the body, we can trace an increase in complexity of organization everywhere determined by the principles of the division of physiological labour and of the adaptation of the organism to the needs of its environment. In the first place there is a differentiation of fixing organs, which in forms living on a soft nutrient substratum penetrate it and become absorbing organs. Secondly, in the Algae, which build up their own food from inorganic materials, we have a differentiation of supporting axes from assimilating appendages, and as the body increases in size and becomes a solid mass of cells or interwoven threads, a corresponding differentiation of a superficial assimilative system from the deep-lying parts. In both Algae and Fungi the latter are primarily supporting and food-conducting, and in some bulky Brown Seaweeds, where assimilation is strongly localized, some of the deep cells are highly specialized for the latter function. In the higher forms a storage and a mechanically-strengthening system may also be developed, and in some aerial Fungi an external protective tissue. The “hyphal” mode of growth, i.e. the formation of the thallus, whatever its external form, by branched, continuous or septate, coenocytic tubes (Siphoneae and Fungi), or by simple or branched cell-threads (Red and many Green Algae), in both cases growing mainly or entirely at the apex of each branch, is almost universal in the group, the exceptions being met with almost entirely among the higher Brown Seaweeds, in which is found parenchyma produced by the segmentation of an apical cell of the whole shoot, or by cell division in some other type of meristem.
Bryophyta.—The Bryophyta [including the Liverworts (Hepaticae) and Mosses (Musci)], the first group of mainly terrestrial plants, exhibit considerably more advanced tissue differentiation, in response to the greater complexity in the conditions of life on land. In a general way this greater complexity may be said to consist (1) in the restriction of regular absorption of water to those parts of the plant-body embedded in the soil, (2) in the evaporation of water from the parts exposed to the air (transpiration). But these two principles do not find their full expression till we come, in the ascending series, to the Vascular Plants. In the Bryophytes water is still absorbed, not only from the soil but also largely from rain, dew, &c., through the general surface of the subaerial body (thallus), or in the more differentiated forms through the leaves. The lowest Hepaticae have an extremely simple vegetative structure, little more advanced than that found in some of the higher Green Algae and very much simpler than in the large Red and Brown Seaweeds, The plant-body (thallus) is always small and normally lives in very damp air, so that the demands of terrestrial life are at a minimum. It always consists of true parenchyma, and is entirely formed by the cutting off of segments from an apical cell.
A sufficient description of the thallus of the liverworts will be found in the article Bryophyta We may note the universal Liverworts. occurrence on the lower surface of the thallus of fixing and absorbing rhizoids in accordance with the terrestrial life on soil (cf. Oedocladium among the Green Algae). The Marchantiaceae (see article Bryophyta) show considerable tissue-differentiation, possessing a distinct assimilative system of cells, consisting of branched cell threads packed with chloroplasts and arising from the basal cells of large cavities in the upper part of the thallus. These cavities are completely roofed by a layer of cells, in the centre of the roof is a pore surrounded by a ring of special cells. The whole arrangement has a strong resemblance to the lacunae, mesophyll and stomata, which form the assimilative and transpiring (water-evaporating) apparatus in the leaves of flowering plants. The frondose (thalloid) Jungermanniales show no such differentiation of an assimilating tissue, though the upper cells of the thallus usually have more chlorophyll than the rest. In three genera—Blyttia, Symphyogyna, and Hymenophytum—there are one or more strands or bundles consisting of long thick-walled fibre-like (prosenchymatous) cells, pointed at the ends and running longitudinally through the thick midrib. The walls of these cells are strongly lignified (i.e. consist of woody substance) and are irregularly but thickly studded with simple pits (see (Cytology), which are usually arranged in spirals running round the cells, and are often elongated in the direction of the spiral (fig. 1, I). These cells are not living in the adult state, though they sometimes contain the disorganized remains of protoplasm. They serve to conduct water through the thallus, the assimilating parts of which are in these forms often raised above the soil and are comparatively remote from the rhizoid-bearing (water-absorbing) region. Such differentiated water-conducting cells we call hydroids, the tissue they form hydrom. The sporogonium of the liverworts is in the simpler forms simply a spore-capsule with arrangements for the development, protection and distribution of the spores. As such its consideration falls outside the scheme of this article, but in one small and peculiar group of these plants, the Anthoceroteae, a distinct assimilating and transpiring system is found in the wall of the very long cylindrical capsule, clearly rendering the sporogonium largely independent of the supply of elaborated organic food from the thallus of the mother plant (the gametophyte). A richly chlorophyllous tissue with numerous intercellular spaces communicates with the exterior by stomata, strikingly similar to those of the vascular plants (see below). If the axis of such a sporogonium were prolonged downwards into the soil to form a fixing and absorptive root, the whole structure would become a physiologically independent plant, exhibiting in many though by no means all respects the leading features of the sporophyte or ordinary vegetative and spore-bearing individual in Pteridophytes and Phanerogams. These facts, among others, have led to the theory, plausible in some respects, of the origin of this sporophyte by descent from an Anthoceros like sporogonium (see Pteridophyta). But in the Bryophytes the sporogonium never becomes a sporophyte producing leaves and roots, and always remains dependent upon the gametophyte for its water and mineral food, and the facts give us no warrant for asserting homology (i.e. morphological identity) between the differentiated tissues of an Anthocerotean sporogonium and those of the sporophyte in the higher plants Opposed to the thalloid forms are the group of leafy Liverworts (Acrogynae), whose plant-body consists of a thin supporting stem bearing leaves. The latter are plates of green tissue one cell thick, while the stem consists of uniform more or less elongated cylindrical cells The base of the stem bears numerous cell-filaments (rhizoids) which fix the plant to the substratum upon which it is growing.
In the Mosses the plant-body (gametophyte) is always separable into a radially organized, supporting and conducting axis (stem) Mosses. and thin, flat, assimilating, and transpiring appendages (leaves). To the base of the stem are attached a number of branched cell-threads (rhizoids) which ramify in the soil, fixing the plant and absorbing water from soil. [For the histology of the comparatively simple but in many respects aberrant Bog-mosses (Sphagnaceae), see Bryophyta.] The stems of the other mosses resemble one another in their main histological features. In a few cases there is a special surface or epidermal layer, but usually all the outer layers of the stem are composed of brown, thick-walled, lignified, prosenchymatous, fibre-like cells forming a peripheral stereom (mechanical or supporting tissue) which forms the outer cortex. This passes gradually into the thinner-walled parenchyma of the inner cortex. The whole of the cortex, stereom and parenchyma alike, is commonly living, and its cells often contain starch. The centre of the stem in the forms living on soil is occupied by a strand of narrow elongated hydroids, which differ from those of the liverworts in being thin-walled, unlignified, and very seldom pitted (fig. 1, J). The hydrom strand has in most cases no connexion with the leaves, but runs straight up the stem and spreads out below the sexual organs or the foot of the sporogonium. It has been shown that it conducts water with considerable rapidity. In the stalk of the sporogonium there is a similar strand, which is of course not in direct connexion with, but continues the conduction of water from the strand of the gametophytic axis. In the aquatic, semi-aquatic, and xerophilous types, where the whole surface of the plant absorbs water, perpetually in the first two cases and during rain in the last, the hydrom strand is either much reduced or altogether absent. In accordance with the general principle already indicated, it is only where absorption is localized (i.e. where the plant lives on soil from which it absorbs its main supply of water by means of its basal rhizoids) that a water-conducting (hydrom) strand is developed. The leaves of most mosses are flat plates, each consisting of a single layer of square or oblong assimilating (chlorophyllous) cells. In many cases the cells bordering the leaf are produced into teeth, and very frequently they are thick-walled so as to form a supporting rim. The centre of the leaf is often occupied by a midrib consisting of several layers of cells. These are elongated in the direction of the length of the leaf, are always poor in chlorophyll and form a channel for conducting the products of assimilation away from the leaf into the stem. This is the first indication of a conducting foliar strand or leaf bundle and forms an approach to leptom, though it is not so specialized as the leptom of the higher Phaeophyceae. Associated with the conducting parenchyma are frequently found hydroids identical in character with those of the central strand of the stem, and no doubt serving to conduct water to or from the leaf according as the latter is acting as a transpiring or a water-absorbing organ. In a few cases the hydrom strand is continued into the cortex of the stem as a leaf-trace bundle (the anatomically demonstrable trace of the leaf in the stem). This in several cases runs vertically downwards for some distance in the outer cortex, and ends blindly—the lower end or the whole of the trace being band-shaped or star-shaped so as to present a large surface for the absorption of water from the adjacent cortical cells. In other cases the trace passes inwards and joins the central hydrom strand, so that a connected water-conducting system between stem and leaf is established.
In the highest family of mosses, Polytrichaceae, the differentiation of conducting tissue reaches a decidedly higher level. In addition to the water-conducting tissue or hydrom there is a well developed tissue (leptom) inferred to be a conducting channel for organic substances. This leptom is not so highly differentiated as in the most advanced Laminariaceae, but shows some of the characters of sieve-tubes with great distinctness. Each leptoid is an elongated living cell with nucleus and a thin layer of protoplasm lining the wall (fig. 1, K). The whole cavity of the cell is sometimes stuffed with proteid contents. The end of the cell is slightly swollen, fitting on to the similar swollen end of the next leptoid of the row exactly after the fashion of a trumpet-hypha. The end wall is usually very thin, and the protoplasm on artificial contraction commonly sticks to it just as in a sieve-tube, though no perforation of the wall has been found. Associated with the leptoids are similar cells without swollen ends and with thicker cross-walls. Besides the hydrom and leptom, and situated between them, there is a tissue which perhaps serves to conduct soluble carbohydrates, and whose cells are ordinarily full of starch. This may be called amylom. The stem in this family falls into two divisions, an underground portion bearing rhizoids and scales, the rhizome, and a leafy aerial stem forming its direct upward continuation. The leaf consists of a central midrib, several cells thick, and two wings, one cell thick. The midrib bears above a series of closely set, vertical, longitudinally-running plates of green assimilative cells over which the wings close in dry air so as to protect the assimilative and transpiring plates from excessive evaporation of water. The midrib has a strong band of stereom above and below. In its centre is a band-shaped bundle consisting of rows of leptom, hydrom and amylon cells. This bundle is continued down into the cortex of the stem as a leaf-trace, and passing very slowly through the sclerenchymatous external cortex and the parenchymatous, starchy internal cortex to join the central cylinder. The latter has a central strand consisting of files of large hydroids, separated from one another by very thin walls, each file being separated from its neighbour by stout, dark-brown walls. This is probably homologous with the hydrom cylinder in the stems of other mosses. It is surrounded by (1) a thin-walled, smaller-celled hydrom mantle; (2) an amylom sheath, (3) a leptom mantle, interrupted here and there by starch cells. These three concentric tissue mantles are evidently formed by the conjoined bases of the leaf traces, each of which is composed of the same three tissues. As the aerial stem is traced down into the underground rhizome portion, these three mantles die out almost entirely—the central hydrom strand forming the bulk of the cylinder and its elements becoming mixed with thick-walled stereids; at the same time this central hydrom-stereom strand becomes three-lobed, with deep furrows between the lobes in which the few remaining leptoids run, separated from the central mass by a few starchy cells, the remains of the amylom sheath. At the periphery of the lobes are some comparatively thin-walled living cells mixed with a few thin-walled hydroids, the remains of the thin-walled hydrom mantle of the aerial stem. Outside this are three arcs of large cells showing characters typical of the endodermis in a vascular plant; these are interrupted by strands of narrow, elongated, thick-walled cells, which send branches into the little brown scales borne by the rhizome. The surface layer of the rhizome bears rhizoids, and its whole structure strikingly resembles that of the typical root of a vascular plant. In Catharinea undulata the central hydrom cylinder of the aerial stem is a loose tissue, its interstices being filled up with thin-walled, starchy parenchyma. In Dawsonia superba, a large New Zealand moss, the hydroids of the central cylinder of the aerial stem are mixed with thick-walled stereids forming a hydrom-stereom strand somewhat like that of the rhizome in other Polytrichaceae.
The central hydrom strand in the seta of the sporogonium of most mosses has already been alluded to. Besides this there is usually a living conducting tissue, sometimes differentiated as leptom, forming a mantle round the hydrom, and bounded externally by a more or less well-differentiated endodermis, abutting on an irregularly cylindrical lacuna; the latter separates the central conducting cylinder from the cortex of the seta, which, like the cortex of the gametophyte stem, is usually differentiated into an outer thick-walled stereom and an inner starchy parenchyma. Frequently, also, a considerable differentiation of vegetative tissue occurs in the wall of the spore-capsule itself, and in some of the higher forms a special assimilating and transpiring organ situated just below the capsule at the top of the seta, with a richly lacunar chlorophyllous parenchyma and stomata like those of the wall of the capsule in the Anthocerotean liverworts. Thus the histological differentiation of the sporogonium of the higher mosses is one of considerable complexity; but there is here even less reason to suppose that these tissues have any homology (phylogenetic community of origin) with the similar ones met with in the higher plants.
The features of histological structure seen in the Bryophytic series are such as we should expect to be developed in response to the exigencies of increasing adaptation to terrestrial life on soil, and of increasing size of the plant-body. In the liverworts we find fixation of the thallus by water-absorbing rhizoids; in certain forms with a localized region of water-absorption the development of a primitive hydrom or water-conducting system; and in others with rather a massive type of thallus the differentiation of a special assimilative and transpiring system. In the more highly developed series, the mosses, this last division of labour takes the form of the differentiation of special assimilative organs, the leaves, commonly with a midrib containing elongated cells for the ready removal of the products of assimilation; and in the typical forms with a localized absorptive region, a well-developed hydrom in the axis of the plant, as well as similar hydrom strands in the leaf-midribs, are constantly met with. In higher forms the conducting strands of the leaves are continued downwards into the stem, and eventually come into connexion with the central hydrom cylinder, forming a complete cylindrical investment apparently distinct from the latter, and exhibiting a differentiation into hydrom, leptom and amylom which almost completely parallels that found among the true vascular plants. Similar differentiation, differing in some details, takes place independently in the other generation, the sporogonium. The stereom of the moss is found mainly in the outer cortex of the stem and in the midrib of the leaf.
Vascular Plants.—In the Vascular Plants (Pteridophytes, i.e. ferns, horse-tails, club mosses, &c., and Phanerogams or Flowering Plants) the main plant-body, that which we speak of in ordinary language as “the plant,” is called the sporophyte because it bears the asexual reproductive cells or spores. The gametophyte, which bears the sexual organs, is either a free-living thallus corresponding in degree of differentiation with the lower liverworts, or it is a mass of cells which always remains enclosed in a spore and is parasitic upon the sporophyte.
The body of the sporophyte in the great majority of the vascular plants shows a considerable increase in complexity over that found in the gametophyte of Bryophytes. The principal new feature in the external conformation of the body is the acquirement of “true” roots, the nearest approach to which in the lower forms we saw in the “rhizome” of Polytrichaceae. The primary root is a downward prolongation of the primary axis of the plant. From this, as well as from various parts of the shoot system, other roots may originate. The root differs from the shoot in the characters of its surface tissues, in the absence of the green assimilative pigment chlorophyll, in the arrangement of its vascular system and in the mode of growth at the apex, all features which are in direct relation to its normally subterranean life and its fixative and absorptive functions. Within the limits of the sporophyte generation the Pteridophytes and Phanerogams also differ from the Bryophytes in possessing special assimilative and transpiring organs, the leaves, though these organs are developed, as we have seen, in the gametophyte of many liverworts and of all the mosses. The leaves, again, have special histological features adapted to the performance of their special functions.
Alike in root, stem and leaf, we can trace a three-fold division of tissue systems, a division of which there are indications among the lower plants, and which is the expression of the fundamental Tissue Systems. conditions of the evolution of a bulky differentiated plant-body. From the primitive uniform mass of undifferentiated assimilating cells, which we may conceive of as the starting-point of differentiation, though such an undifferentiated body is only actually realized in the thallus of the lower Algae, there is, (1) on the one hand, a specialization of a surface layer regulating the immediate relations of the plant with its surroundings. In the typically submerged Algae and in submerged plants of every group this is the absorptive and the main assimilative layer, and may also by the production of mucilage be of use in the protection of the body in various ways. In the terrestrial plants it differs in the subterranean and subaerial parts, being in the former pre-eminently absorptive, and in the latter protective—provision at the same time being made for the gaseous interchange of oxygen and carbon dioxide necessary for respiration and feeding. This surface layer in the typically subaerial “shoot” of the sporophyte in Pteridophytes and Phanerogams is known as the epidermis, though the name is restricted by some writers, on account of developmental differences, to the surface layer of the shoot of Angiosperms, and by others extended to the surface layer of the whole plant in both these groups. On the other hand, we have (2) an internal differentiation of conducting tissue, the main features of which as seen in the gametophyte of Bryophytes have already been fully described. In the Vascular Plants this tissue is collectively known as the vascular system. The remaining tissue of the plant-body, a tissue that we must regard phylogenetically as the remnant of the undifferentiated tissue of the primitive thallus, but which often undergoes further differentiation of its own, the better to fulfil its characteristically vital functions for the whole plant, is known, from its peripheral position in relation to the primitively central conducting tissue, as (3) the cortex. Besides absorption, assimilation, conduction and protection there is another very important function for which provision has to be made in any plant-body of considerable size, especially when raised into the air, that of support. Special tissues (stereom) may be developed for this purpose in the cortex, or in immediate connexion with the conducting system, according to the varying needs of the particular type of plant-body. The important function of aeration, by which the inner living tissues of the bulky plant-body obtain the oxygen necessary for their respiration, is secured by the development of an extensive system of intercellular spaces communicating with the external air.
In relation to its characteristic function of protection, the epidermis, which, as above defined, consists of a single layer of cells Epidermis. has typically thickened and cuticularized outer walls. These serve not only to protect the plant against slight mechanical injury from without, and against the entry of smaller parasites, such as fungi and bacteria, but also and especially to prevent the evaporation of water from within.
At intervals it is interrupted by pores (stomata) leading from the air outside to the system of inter cellular spaces below. Each Stomata. stoma is surrounded by a pair of peculiarly modified epidermal cells called guard-cells (fig. 1, T), which open and close the pore according to the need for transpiration. The structure of the stomata of the sporophyte of vascular plants is fundamentally the same as that of the stomata on the sporogonium of the true mosses and of the liverwort Anthoceros. Stomata are often situated at the bottom of pits in the surface of the leaf. This arrangement is a method of checking transpiration by creating a still atmosphere above the pore of the stoma, so that water vapour collects in it and diminishes the further outflow of vapour. This type of structure, which is extremely various in its details, is found especially, as we should expect, in plants which have to economize their water supply. The stomata serve for all gaseous interchange between the plant and the surrounding air. The guard-cells contain chlorophyll, which is absent from typical epidermal cells, the latter acting as a tissue for water storage. Sometimes the epidermis is considerably more developed by tangential division of its cells, forming a many-layered water-tissue. This is found especially in plants which during certain hours of the day are unable to cover the water lost through transpiration by the supply coming from the roots. The water stored in such a time supplies the immediate need of the transpiring cells and prevents the injury which would result from their excessive depletion.
The epidermis of a very large number of species bears hairs of various kinds. The simplest type consists simply of a single Hairs. elongated cell projecting above the general level of the epidermis. Other hairs consist of a chain of cells; others, again, are branched in various ways, while yet others have the form of a flat plate of cells placed parallel to the leaf surface and inserted on a stalk. The cells of hairs may have living contents or they may simply contain air. A very common function of hairs is to diminish transpiration, by creating a still atmosphere between them, as in the case of the sunk stomata already mentioned. But hairs have a variety of other functions. They may, for instance, be glandular or stinging, as in the common stinging nettle, where the top of the hair is very brittle, easily breaking off when touched. The sharp, broken end penetrates the skin, and into the slight wound thus formed the formic acid contained by the hair is injected.
Mention may be made here of a class of epidermal organ, the hydathodes, the wide distribution and variety of which have been Hydathodes. revealed by recent research. These are special organs, usually situated on foliage leaves, for the excretion of water in liquid form when transpiration is diminished so that the pressure in the water-channels of the plant has come to exceed a certain limit. They are widely distributed, but are particularly abundant in certain tropical climates where active root absorption goes on while the air is nearly saturated with water vapour. In one type they may take the form of specially-modified single epidermal cells or multicellular hairs without any direct connexion with the vascular system. The cells concerned, like all secreting organs, have abundant protoplasm with large nuclei, and sometimes, in addition, part of the cell-wall is modified as a filter. In a second type they are situated at the ends of tracheal strands and consist of groups of richly protoplasmic cells belonging to the epidermis (as in the leaves of many ferns), or to the subjacent tissue (the commonest type in flowering plants); in this last case the cells in question are known as epithem. The epithem is penetrated by a network of fine intercellular spaces, which are normally filled with water and debouch on one or more intercellular cavities below the epidermis. Above each cavity is situated a so-called water-stoma, no doubt derived phylogenetically from an ordinary stoma, and enclosed by guard-cells which have nearly or entirely lost the power of movement. The pores of the water-stomata are the outlets of the hydathode. The epithem is frequently surrounded by a sheath of cuticularized cells. In other cases the epithem may be absent altogether, the tracheal strand debouch in directly on the lacunae of the mesophyll. This last type of hydathode is usually situated on the edge of the leaf. Some hydathodes are active glands, secreting the water they expel from the leaf. [Many other types of glands also exist, either in connexion with the epidermis or not, such as nectaries, digestive glands, oil, resin and mucilage glands, &c. They serve the most various purposes in the life of the plant, but they are not of significance in relation to the primary vital activities, and cannot be dealt with in the limits of the present article.] The typical epidermis of the shoot of a land plant does not absorb water, but some plants living in situations where they cannot depend on a regular supply from the roots (e.g. epiphytic plants and desert plants) have absorptive hairs or scales on the leaf epidermis through which rain and dew can be absorbed. Some hydathodes also are capable of absorbing as well as excreting water.
The surface layer of the root, sometimes included under the term epidermis, is fundamentally different from the epidermis Epidermis of Root. of the stem. In correspondence with its water-absorbing function it is not cuticularized, but remains usually thin-walled; the absorbing surface is increased by its cells being produced into delicate tubes which curl round and adhere firmly to particles of soil, thus at once fixing the root firmly in the soil, and enabling the hair to absorb readily the thin films of water ordinarily surrounding the particles (fig. 1, U). The root-hair ends blindly and is simply an outgrowth from a surface cell, having no cross-walls. It corresponds in function with the rhizoid of a Bryophyte. At the apex of a root, covering and protecting the delicate tissue of the growing point, is a special root-cap consisting of a number of layers of tissue whose cells break down into mucilage towards the outer surface, thus facilitating the passage of the apex as it is pushed between the particles of soil.
Fig. 2.—Transverse Sections of Leaves.
A, Dorsiventral leaf B, Isobilateral leaf.
ep, epidermis; st, stoma; mes, mesophyll; pal, palisade; spo, spongy tissue; i.sp, intercellular space; w.t., water tissue; x, xylem; ph, phloem; phlt, phloeoterma; scl, sclerenchyma.
The cortex, as has been said, is in its origin the remains of the primitive assimilating tissue of the plant, after differentiation Cortex. of the surface layer and the conducting system. It consists primitive of typical living parenchyma; but its differentiation may be extremely varied, since in the complex bodies of the higher plants its functions are numerous. In all green plants which have a special protective epidermis, the cortex of the shoot has to perform the primitive fundamental function of carbon assimilation. In the leaf shoot this function is mainly localized Mesophyll. in the cortical tissue of the leaves, known as mesophyll, which is essentially a parenchymatous tissue containing chloroplasts, and is penetrated by a system of intercellular spaces so that the surfaces of the assimilating cells are brought into contact with air to as large an extent as possible, in order to facilitate gaseous interchange between the assimilating cells and the atmosphere. At the same time the cells of the mesophyll are transpiring cells—i. e. the evaporation of water from the leaf goes on from them into the intercellular spaces. The only pathways for the gases which thus pass between the cells of the mesophyll and the outside air are the stomata. A land plant has nearly always to protect itself against over-transpiration, and for this reason the stomata of the typical dorsiventral leaf (fig. 2, A), which has distinct upper and lower faces, are placed mainly or exclusively on the lower side of the leaf, where the water vapour that escapes from them, being lighter than air, cannot pass away from the surface of the leaf, but remains in contact with it and thus tends to check further transpiration. The stomata are in direct communication with the ample system of intercellular spaces which is found in the loosely arranged mesophyll (spongy tissue) on that side. This is the main transpiring tissue, and is protected from direct illumination and consequent too great evaporation. The main assimilating tissue, on the other hand, is under the upper epidermis, where it is well illuminated, and consists of oblong cells densely packed with chloroplasts and with their long axes perpendicular to the surface (palisade tissue). The intercellular spaces are here very narrow channels between the palisade cells. Leaves whose blades are normally held in a vertical position possess palisade tissue and stomata on both sides (isobilateral leaves) (fig. 2, B), since there is no difference in the illumination and other external conditions, while those which are cylindrical or of similar shape (centric leaves) have it all round. The leaves of shade plants have little or no differentiation of palisade tissue. In fleshy leaves which contain a great bulk of tissue in relation to their chlorophyll content, the central mesophyll contains little or no chlorophyll and acts as water-storage tissue. The cortex of a young stem is usually green, and plays a more or less important part in the assimilative function. It also always possesses a well-developed lacunar system communicating with the external air through stomata (in the young stem) or lenticels (see below). This lacunar system not only enables the cells of the cortex itself to respire, but also forms channels through which air can pass to the deeper lying tissues. The cortex of the older stem of the root frequently acts as a reserve store-house for food, which generally takes the form of starch, and it also assists largely in providing the stereom of the plant. In the leaf-blade this sometimes appears as a layer of thickened subepidermal cells, the hypoderm, often also as subepidermal bundles of sclerenchymatous fibres, or as similar bundles extending right across the leaf from one epidermis to the other and thus acting as struts. Isolated cells (idioblasts), thickened in various ways, are not uncommonly found supporting the tissues of the leaf. In the larger veins of the leaf, especially in the midrib, in the petiole, and in the young stem, an extremely frequent type of mechanical tissue is collenchyma. This consists of elongated cells with cellulose walls, which are locally thickened along the original corners of the cells, reducing the lumen to a cylinder, so that a number of vertical pillars of cellulose connected by comparatively thin walls form the framework of the tissue. his tissue remains living and is usually formed quite early, just below the epidermis, where it provides the first peripheral support for a still growing stem or petiole. Sclerenchyma may be formed later in various positions in the cortex, according to local needs. Scattered single stereids or bundles of fibres are not uncommon in the cortex of the root.
The innermost layer of the cortex, abutting on the central cylinder of the stem or on the bundles of the leaves, is called the Phloeoterma. phloeoterma, and is often differentiated. In the leaf-blade it takes the form of special parenchymatous sheaths to the bundles. The cells of these sheaths are often distinguished from the rest of the mesophyll by containing little or no chlorophyll. Occasionally, however, they are particularly rich in chloroplasts. These bundle sheaths are important in the conduction of carbohydrates away from the assimilating cells to other parts of the plant. Rarely in the leaf, frequently in the stem (particularly in Pteridophytes), and universally in the root, the phloeoterma is developed as an endodermis (see below). In other cases it does not differ histologically from the parenchyma of the rest of the cortex, though it is often distinguished by containing particularly abundant starch, in which case it is known as a starch sheath.
One of the most striking characters common to the two highest groups of plants, the Pteridophytes and Phanerogams, is the Vascular System. possession of a double (hydrom-leptom) conducting system, such as we saw among the highest mosses, but with sharply characterized and peculiar features, probably indicating common descent throughout both these groups. It is confined to the sporophyte, which forms the leafy plant in these groups, and is known as the vascular system. Associated with it are other tissues, consisting of parenchyma, mainly starchy, and in the Phanerogams particularly, of special stereom. The whole tissue system is known as the stelar system (from the way in which in primitive forms it runs through the whole axis of the plant in the form of a column). The stelar system of Vascular Plants has no direct phylogenetic connexion with that of the mosses. The origin of the Pteridophyta (q.v.) is very obscure, but it may be regarded as certain that it is not to be sought among the mosses, which are an extremely specialized and peculiarly differentiated group. Furthermore, both the hydrom and leptom of Pteridophytes have marked peculiarities to which no parallel is to be found among the Bryophytes. Hence we must conclude that the conducting system of the Pteridophytes has had an entirely separate evolution. All the surviving forms, however, have a completely established double system with the specific characters alluded to, and since there is every reason to believe that the conditions of evolution of the primitive Pteridophyte must have been essentially similar to those of the Bryophytes, the various stages in the evolution of the conducting system of the latter (p. 732) are very useful to compare with the arrangements met with in the former.
The hydroid of a Pteridophyte or of a Phanerogam is characteristically a dead, usually elongated, cell containing air and water, and Tissue Elements. either thin-walled with lignified (woody) spiral (fig. 1, P) or annular thickenings, or with thick lignified walls, incompletely perforate by pits (fig. 1, Q.) (usually bordered pits) of various shapes, e.g. the pits may be separated by a network of thickenings when the tracheid is reticulate or the may be transversely elongated and separated by bars of thickening like the rungs of a ladder (scalariform thickening). When, in place of a number of such cells called tracheids, we have a continuous tube with the same kind of wall thickening, but composed of a number of cells whose cross walls have disappeared, the resulting structure is called a vessel. Vessels are common in the Angiospermous group of Flowering Plants. The scalariform hydroids of Ferns (fig. 1, N) have been quite recently shown to possess a peculiar structure. The whole of the middle lamella or originally formed cell-wall separating one from another disappears before the adult state is reached, so that the walls of the hydroids consist of a framework of lignified bars with open communication between the cell cavities. The tracheids or vessels, indifferently called tracheal elements, together with the immediately associated cells (usually amylom in Pteridophytes) constitute the xylem of the plant. This is a morphological term given to the particular type of hydrom found in both Pteridophytes and Phanerogams, together with the parenchyma or stereom, or both, included within the boundaries of the hydrom tissue strand. The leptoid of a Pteridophyte (fig 1, O) is also an elongated cell, with a thin lining of protoplasm, but destitute of a nucleus, and always in communication with the next cell of the leptom strand by perforations (in Pteridophytes often not easily demonstrable), through which originally pass strings of protoplasm which are bored out by a ferment and converted into relatively coarse “slime strings,” along which pass, we must suppose, the organic substances which it is the special function of the leptoids to conduct from one part of the plant to another. The peculiar substance called callose, chemically allied to cellulose, is frequently formed over the surface of the perforated end-walls. The structure formed by a number of such cells placed end to end is called a sieve-tube (obviously comparable with a xylem-vessel), and the end-wall or area of end-wall occupied by a group of perforations, a sieve-plate. When the sieve-tube has ceased to Function and the protoplasm, slime strings, and callose have disappeared, the perforations through which the slime strings passed are left as relatively large holes, easily visible in some cases with low powers of the microscope, piercing the sieve-plate. The sieve-tubes, with their accompanying parenchyma or stereom, constitute the tissue called phloem. This is the term for a morphologically defined tissue system, i.e the leptom found in Pteridophytes and Phanerogams with its associated cells, and is entirely parallel with the xylem. The sieve-tubes differ, however, from the tracheids in being immediately associated, apparently constantly, not with starchy parenchyma, but with parenchymatous cells, containing particularly abundant proteid contents, which seem to have a function intimately connected with the conducting function of the sieve-tubes, and which we may call proteid-cells. In the Angiosperms there are always sister-cells of sieve-tube segments and are called companion-cells (fig. 1, R.).
The xylem and phloem are nearly always found in closing association in strands of various shapes in all the three main organs of the sporophyte—root, stem and leaf—and form a connected tissue-system running through the whole body. In the primary axis of the plant among Pteridophytes and many Phanerogams, at any rate in its first formed part, the xylem and phloem are associated in the form of a cylinder (stele), with xylem occupying the centre, and the phloem (in the upward-growing part or primary Arrangement in Strands: the Central Cylinder. stem) forming a mantle at the periphery (fig. 4). In the downward growing part of the axis (primary root), however, the peripheral mantle of phloem is interrupted, the xylem coming to the surface of the cylinder along (usually) two or (sometimes) more vertical lines. Such an arrangement of vascular tissue is called radial, and is characteristic of all roots (figs. 3 and 10). The cylinder is surrounded by a mantle of one or more layers of parenchymatous cells, the pericycle, and the xylem is generally separated from the phloem in the stem by a similar layer, the mesocycle (corresponding with the amylom sheath in mosses). The pericycle and mesocycle together form the conjunctive tissue of the stele in these simplest types. When the diameter of the stele is greater, parenchymatous conjunctive tissue often occupies its centre and is frequently called the pith. In the root the mesocycle, like the phloem, is interrupted, and runs into the pericycle where the xylem touches the latter (fig. 3). The whole cylinder is enclosed by the peculiarly differentiated innermost cell-layer of the cortex, known as the endodermis. This layer has its cells closely united and sealed to one another, so to speak, by the conversion of the radial and transverse walls (which separate each cell from the other cells of the layer), or of a band running in the centre of these, into corky substance (fig 1, V.), so that the endodermal cells cannot be split apart to admit of the formation of inter cellular spaces, and an air-tight sheath is formed round the cylinder. Such a vascular cylinder is called a haplostele, and the axis containing it is said to be haplostelic. In the stele of the root the strands of tracheids along the lines where the xylem touches the pericycle are spiral or annular, and are the xylem elements first formed when the cylinder is developing. Each strand of spiral or annular first-formed tracheids is called a protoxylem strand, as distinct from the metaxylem or rest of the xylem, which consists of thick-walled tracheids, the pits of which are often scalariform. The thin-walled spiral or annular tracheae of the protoxylem allow of longitudinal stretching brought about by the active growth in length of the neighbouring living parenchymatous cells of a growing organ. During the process the thin walls are stretched and the turns of the spiral become pulled apart without rupturing the wall of the tracheid or vessel. If the pitted type of tracheal element were similarly stretched its continuously thickened walls would resist the stretching and eventually break. Hence such tracheae are only laid down in organs whose growth in length has ceased. The stele is called monarch, diarch, . . . polyarch according as it contains one, two, . . . or many protoxylems. When the protoxylem strands are situated at the periphery of the stele, abutting on the pericycle, as in all roots, and many of the more primitive Pteridophyte stems, the stele is said to be exarch. When there is a single protoxylem strand in the centre of the stele, or when, as is more commonly the case, there are several protoxylem strands situated at the internal limit of the xylem, the centre of the stem being occupied by parenchyma, the stele is endarch. This is the case in the stems of most Phanerogams and of some Pteridophytes. When the protoxylems have an intermediate position the stele is mesarch (many Pteridophytes and some of the more primitive Phanerogams). In many cases external protophloem, usually consisting of narrow sieve-tubes often wit swollen walls, can be distinguished from metaphloem.
As the primitive stele of a Pteridophyte is traced upwards from the primary root into the stem, the phloem becomes continuous Evolution of the Stele in Pteridophytes. round the xylem. At the same time the stele becomes more bulky, all its elements increasing in number (fig. 4). Soon a bundle goes off to the first leaf. This consists of a few xylem elements, a segment of phloem, pericycle, and usually an arc of endodermis, which closes round the bundle as it detaches itself from the stele. As the stele is traced farther upwards it becomes bulkier, as do the successive leaf-bundles which leave it. In many Pteridophytes the solid haplostele is maintained throughout the axis. In others a central parenchyma or primitive pith—a new region of the primitive stelar conjunctive—appears in the centre of the xylem. In most ferns internal phloem appears instead of a parenchymatous pith (fig. 5). Sometimes this condition, that of the amphiphloic haplostele, is maintained throughout the adult stem (Lindsaya). In the majority of ferns, at a higher level, after the stele has increased greatly in diameter, a large-celled true pith or medulla, resembling the cortex in its characters, and quite distinct from conjunctive, from which it is separated by an internal endodermis, appears in the centre. These successive new tissues, appearing in the centre of the stele, as the stem of a higher fern is traced upwards from its first formed parts, are all in continuity with the respective corresponding external tissues at the point of origin of each leaf trace (see below). Where internal phloem is present this is separated from the internal endodermis by an endocycle or “internal pericycle,” as it is sometimes called, and from the xylem by an internal mesocycle—these two layers, together with the outer mesocycle and pericycle, constituting the conjunctive tissue of the now hollow cylindrical stele. (The conjunctive frequently forms a connected whole with bands of starchy xylem-parenchyma, which, when the xylem is bulky, usually appear among the tracheids, the phloem also often being penetrated by similar bands of phloem-parenchyma.)
Figs. 3-15—Types of Stele in Vascular Plants. Fig. 3.—Diarch stele of root of a Fern. Fig. 4.—Haplostele of stem of young Fern. Fig. 5.—Amphiphloic haplostele of stem of young Fern. Fig. 6.—Solenostele of stem of Fern showing detachment of leaf-trace and leaf-gap. Fig. 7.—Dictyostele of Fern. Fig. 8.—Tricyclic solenostele of Matonia. Fig. 9.—Tricyclic dictyostele of Danæa. Fig. 10.—Diarch haplostele of Selaginella. Fig. 11.—Tristelic stem of Selaginella. Fig. 12.—Modified haplostele of Lycopodium. Fig. 13.—Typical siphonostele of dicotyledon. Fig. 14.—Stele of monocotyledon. Fig. 15.—Polyarch root of Veratrum (a monocotyledon).
Explanation of Lettering: st. stele; mst. meristele; l.t. leaf-trace; l.g. leaf-gap; cor. cortex; p.t. peristelar tissue; p.l. peristelar lacuna; end. endodermis; p.c. passage cell; per. pericycle; ph. phloem; mes. mesocycle, x. xylem; px. protoxylem; mx. metaxylem; p. pith; scl. p. sclerised pith; c. cambium; p.m.r. primary, medullary, ray.
In the other groups of Pteridophytes internal phloem is not found and an internal endodermis but rarely. The centre of the Siphonostely. stele is however often occupied by a large-celled pith resembling the cortex in structure, the cortex and pith together being classed as ground tissue. To this type of stele having a “ground-tissue pith,” whether with or without internal phloem, is given the name siphonostele to distinguish it from the solid haplostele characteristic of the root, the first-formed portion of the stem, and in the more primitive Pteridophytes, of the whole of the axis. The type of siphonostele characteristic of many ferns, in which are found internal phloem, and an internal endodermis separating the vascular conjunctive from the pith, is known as a solenostele. The solenostele of the ferns is broken by the departure of each leaf-bundle, the outer and inner endodermis joining so that the stele becomes horseshoe-shaped and the cortex continuous with the pith (fig. 6). Such a break is known as a leaf-gap. A little above the departure of the leaf-bundle the stele again closings up, only to be again broken by the departure of the next leaf-bundle. Where the leaves are crowded, a given leaf-gap is not closed before the next ones appear, and the solenostele thus becomes split up into a number of segments, sometimes band-shaped or semilunar, sometimes isodiametric Dictyostely. in cross-section (fig. 7). In the after case each segment of the solenostele frequently resembles a haplostele, the segments of inner endodermis, pericycle, phloem and mesocycle joining with the corresponding outer segments to form a nearly concentric structure. For this reason a stem in which the vascular system has this type of structure used to be spoken of as polystelic, the term “stele” being transferred from the primary central cylinder of the axis and applied to the vascular strands just described. In this use the term loses, of course, its morphological value, and it is better to call such a segment of a broken-up stele a meristele, the whole solenostele with overlapping leaf-gaps being called a dictyostele. The splitting up of the vascular tube into separate strands does not depend wholly upon the occurrence of leaf-gaps. In some forms other gaps (perforations) appear in the vascular tube placing the pith and cortex in communication. In other cases the leaf-gaps are very broad and long, the meristeles separating them being reduced to comparatively slender strands, while there is present in each gap a network of fine vascular threads, some of which run out to the leaf, while others form cross-connexions between these “leaf-trace” strands and also with the main cauline meristeles. Finally the cauline meristeles themselves may be resolved into a number of fine threads. Such a structure may be spoken of as a dissected dictyostele.
In some solenostelic ferns, and in in any dictyostelic ones additional vascular strands are present which do not form part of the primary Polycycly. vascular tube. They usually run freely in the pith and join the primary tube in the neighbourhood of the leaf-gaps. Sometimes a complete internal vascular cylinder, having the same structure as the primary one, and concentric with it, occurs in the pith, and others may appear, internal to the first (Matonia, Saccoloma). Junctions of the first internal cylinder are made with the primary (external) cylinder at the leaf-gaps, and of the second internal cylinder with the first in the same neighbourhood (fig. 8). In dictyostelic ferns similar internal (dictyostelic) cylinders are found in some forms, and occasionally a large series of such concentric cylinders is developed (Marattiaceae) (fig. 9). In such cases the vascular system is said to be polycyclic in contrast with the ordinary monocyclic condition. These internal strands or cylinders are to be regarded as peculiar types of elaboration of the stele, and probably act as reservoirs for water-storage which can be drawn upon when the water supply from the root is deficient.
The vascular supply of the leaf (leaf-trace) consists of a single strand only in the haplostelic and some of the more primitive Leaf-trace and Petlolar Strands. siphonostelic forms. In the “microphyllous” groups of Pteridophytes (Lycopodiales and Equisetalis) in which the leaves are small relatively to the stem, the single bundle destined for each leaf is a small strand whose departure causes no disturbance in the cauline stele. In the “megaphyllous” forms, on the other hand, (Ferns) whose leaves are large relatively to the stem, the departure of the correspondingly large trace causes a gap (leaf-gap) in the vascular cylinder, as already described. In the haplostelic ferns the leaf-trace appears as a single strand with a tendency to assume the shape of a horseshoe on cross-section, and this type is also found in the more primitive solenostelic types. In the more highly developed forms, as already indicated, the leaf-trace is split up into a number of strands which leave the base and sides of the leaf-gap independently. In the petiole these strands may increase in number by branching, and though usually reducible to the outline of the primitive “horseshoe,” more or less elaborated, they may in some of the complex polycylic dictyostelic types (Marattiaceae) be arranged in several concentric circles, thus imitating the arrangement of strands formed in the stem. The evolution of the vascular structure of the petiole in the higher ferns is strikingly parallel with that of the stem, except in some few special cases.
There is good reason to believe that the haplostele is primitive in the evolution of the vascular system. It is found in most of Parallel of Ontogeny with Phylogeny. those Pteridophytes which we have other reasons for considering as primitive types, and essentially the same type is found, as we have seen, in the independently developed primitive conducting system of the moss-stem. This type of stem is therefore often spoken of as protostelic. In the Ferns there is clear evidence that the amphiphloic haplostele or protostele succeeded the simple (ectophloic) protostele in evolution, and that this in its turn gave rise to the solenostele, which was again succeeded by the dictyostele. Polycycly was derived independently from monocycly in solenostelic and in dictyostelic forms. In the formation of the stem of any fern characterized in the adult condition by one of the more advanced types of vascular structure all stages of increase in complexity from the haplostele of the first-formed stem to the particular condition characteristic of the adult stem are gradually passed through by a series of changes exactly parallel with those which we are led to suppose, from the evidence obtained by a comparison of the adult forms, must have taken place in the evolution of the race. There is no more striking case in the plant kingdom of the parallel between ontogeny (development of the individual) and phylogeny (development of the race) so well known in many groups of animals.
The stele of most Lycopods is a more or less modified protostele, but in the genus Lycopodium a peculiar arrangement of the xylem Aberrant Stelar Systems of Pteridophytes. and phloem is found, in which the latter, instead of being stelar confined to a peripheral mantle of tissue, forms bands running across the stele and alternating with similar bands of xylem (fig. 12). In Selaginella the stelar system shows profounder modifications. In some forms we find a simple protostele, exarch-polyarch in one species (S. spinosa), exarch-diarch in several (fig. 10). In other species, however, a peculiar type of polystely is met with, in which the original diarch stele gives rise to so-called dorsal and ventral stelar “cords” which at first lie on the surface of the primary stele, but eventually, at a higher level separate from it and form distinct “secondary” steles resembling the primary one. Similar cords may be formed on, and may separate from, these secondary steles, thus giving rise to a series of steles arranged in a single file (fig. 11). In the creeping stem of one species (S. Lyallii) a polycyclic solenostele is found exactly parallel with that of the rhizome of ferns. The gaps in the outer tubular stele, however, are formed by the departure of aerial branch-traces, instead of leaf-traces as in the ferns. The first formed portion of the stem in all species of Selaginella which have been investigated possesses an exarch haplostele. The stele of Equisetum is of a very peculiar type whose relations are not completely clear. It consists of a ring of endarch collateral bundles, surrounding a hollow pith. The protoxylem of each is a leaf-trace, while the metaxylem consisting of a right and a left portion forms a quite distinct cauline system. All the metaxylems join at the nodes into a complete ring of xylem. The whole stele may be surrounded by a common external endodermis; sometimes there is an internal endodermis in addition, separating the bundles from the pith; while in other cases each bundle possesses a separate endodermis surrounding it. At the nodes the relation of the endodermis to the bundles undergoes rather complex but definite changes. It is probable that this type of stele is a modification of a primitive protostele, in which the main mass of stelar xylem has become much reduced and incidentally separated from the leaf-traces.
During recent years a number of fossil (Carboniferous and Permian) plants have been very thoroughly investigated in the light of modern Stelar System of Cycadofilices. anatomical knowledge, and as a result it has become clear that in those times a large series of plants existed intermediate in structure between the modern ferns and the modern Gymnosperms (especially Cycads), and to these the general name “Cycadofilices” has been applied. We now know that many at least of the Cycadofilices bore seeds, of a type much more complex than that of most modern seed plants, and in some cases approximating to the seeds of existing Cycads. Among the Cycadofilices a series of stages is found leading from the primitive fern-protostele to the type of siphonostele characteristic of the Cycads which agrees in essentials in all the Spermophytes. The main events in this transition appear to have been (1) disappearance of the central xylem of the protostele and replacement by pith, leading to the survival of a number of (mesarch) collateral bundles (see below) at the periphery of the stele; (2) passage from mesarchy to endarchy of these bundles correlated with a great increase in secondary thickening of the stele. The leaves of the more primitive members of this series were entirely fern-like and possessed a fern-like vascular strand; while in the later members, including the modern Cycads, the leaf bundles, remaining unaffected by secondary thickening, are mesarch, while those of the stem-stele have become endarch. Besides the types forming this series, there are a number of others (Medulloseae and allied forms) which show numerous, often very complex, types of stelar structure, in some cases polystelic, whose origin and relationship with the simpler and better known types is frequently obscure. Among the existing Cycads, though the type of vascular system conforms on the whole with that of the other existing seed-plants, peculiar structures are often found (e.g. indications of polystely, frequent occurrence of extra-stelar concentric bundles, “anomalous” secondary thickening) which recall these complex types of stelar structure in the fossil Cycadofilices.
The typical structure of the vascular cylinder of the adult primary stem in the Gymnosperms and Dicotyledons is, like Structure of the Stele in Seed-plants. that of the higher ferns, a hollow cylinder of vascular tissue enclosing a central parenchymatous pith. But, unlike the ferns, there is in the seed-plants no internal phloem (except as a special development in certain families) and no internal endodermis. The xylem and phloem also, rarely form perfectly continuous layers as they do in a solenostelic fern. The vascular tissue is typically separable into distinct collateral bundles (figs. 13, 23), the xylem of which is usually wedge-shaped in cross-section with the protoxylem elements at the inner extremity, while the phloem forms a band on the outer side of the xylem, and separated from it by a band of conjunctive tissue (mesodesm). These collateral bundles are separated from one another by bands of conjunctive tissues called primary medullary rays, which may be quite narrow or of considerable width. When the pith is large celled, the xylems of the bundles are separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex. The inner layer of the cortex (phloeoterma) may form a well-marked endodermis, or differ in other ways from the rest of the cortex. The pericycle, medullary rays, endocycle and mesoderm all form parts of one tissue system, the external conjunctive, and are only topographically separable. The external conjunctive is usually a living comparatively small-celled tissue, whose cells are considerably elongated in the direction of the stem-axis and frequently contain abundant starch. Certain regions of it, particularly the whole or part of the pericycle, but sometimes also the endocycle, are typically converted into thick-walled hard (sclerenchymatous) tissue usually of the prosenchymatous (fibrous) type, which is important in strengthening the stem, particularly in enabling it to resist bending strains. The relatively peripheral position in the stem of the pericycle is important in this connexion. Various secondary meristems (see p. 741) also arise in the external conjunctive.
Most of the collateral bundles of this spermophytic type of siphonostele are leaf-trace bundles, i.e. they can be traced upwards from any given point till they are found to pass out of the cylinder, travel through the cortex of the stem and enter a leaf. The remaining bundles (compensation bundles) which go to make up the cylinder are such as have branched off from the leaf-traces, and will, after joining with others similarly given off, themselves form the traces of leaves situated at a higher level on the stem. Purely cauline vascular strands (i.e. confined to the stem) such as are found in the dictyosteles of ferns are rare in the flowering plants. The leaf trace of any given leaf rarely consists of a single bundle only (unifascicular); the number of bundles of any given trace is always odd; they may either be situated all together before they leave the stele or they may be distributed at intervals round the stele. The median bundles of the trace are typically the largest, and at any given level of the stem the bundles destined for the next leaf above are as a whole larger than the others which are destined to supply higher leaves. Leaf-gaps are formed in essentially the same way as in the ferns, but when in the case of a plurifascicular trace the bundles are distributed at intervals round the cylinder it is obvious that several gaps must be formed as the different bundles leave the stele. The gaps, are, however, often filled as they are formed by the development of external conjunctive tissue immediately above the points at which the bundles begin to bend out of the stele, so that sharply defined open gaps such as occur in fern-steles are but rarely met with in flowering plants. The constitution of the stele of a flowering plant entirely from endarch collateral bundles, which are either themselves leaf-traces or will form leaf-traces after junction with other similar bundles, is the great characteristic of the stem-stele of flowering plants. These collateral bundles are obviously highly individualized. The external conjunctive tissue is often arranged in relation to each bundle separately, the pericyclic fibres for instance, already referred to, being often confined to the bands of pericyclic tissue abutting on the phloem of each bundle, while the cortex and pith frequently form rays in the intervals between the adjacent bundles.
In some cases this individualization is carried further, the cortex and pith becoming continuous between the bundles which appear Aberrant Types of Stele in Angiosperms. as isolated strands embedded in a general ground-tissue. Each bundle has its own investment of tissue corresponding with external conjunctive, and now called peridesm. The bundles sometimes keep their arrangement in a ring corresponding with the stele, though the continuous cylinder no longer exists (species of Ranunculus). This condition is known as astely. In some astelic stems (Nymphaeaceae) the number of bundles is greatly increased and they are scattered throughout the ground tissue. A “polystelic” condition arises in some members of this order by the association of collateral bundles round common centres. A similar phenomenon is seen in two widely separated genera of flowering plants: Primula § Auricula and Gunnera (Halorageae).
The monocotyledons, one of the primary divisions of angiosperms, Monocotyledonous Type. typically possess large leaves with broad sheathing bases containing a very great number of bundles. This results in the number of bundles present at any given level of the stem being enormously increased. These bundles are scattered in a definite though not superficially obvious order through the conjunctive tissue of the stele, which occupies nearly the whole diameter of the stem, the cortex being reduced to a very narrow layer, or disappearing altogether (fig. 3). The mass of conjunctive tissue is developed as a large-celled “ground-tissue,” and round each bundle there is a “peridesm” which is often fibrous (fig. 16). It is possible to suppose that this condition is derived from the astelic condition already referred to, but the evidence on the whole leads to the conclusion that it has arisen by an increase in the number of the bundles within the stele, the individuality of the bundle asserting itself after its escape from the original bundle-ring of the primitive cylinder.
In the stems of many water-plants various stages of reduction of the vascular system, especially of the xylem, are met with, and Reduced Haplostelic Type. very often this reduction leads to the formation of a compact stele in which the individuality of the separate bundles may be suppressed, so that a closed cylinder of xylem surrounds a pith. The phloem is generally unreduced, and there is normally a well marked endodermis (fig. 17). In other cases the reduction goes much further, till the endodermis eventually comes to surround nothing but an intercellular channel formed in place of the stelar tissue.
In the blade of a typical leaf of a vascular plant—essentially a thin plate of assimilating tissue—the vascular system takes the Stelar Tissue of Leaf and Root. form of a number of separate, usually branching and anastomosing strands. These, with their associated stereom, form a kind of framework which is of great importance in supporting the mesophyll; but also, and chiefly, they provide a number of channels, penetrating every part of the leaf, along which water and dissolved salts are conveyed to, and elaborated food-substances from, the mesophyll cells. The bundle-system is of course continuous with that of the petiole and stem. The leaf-bundles are always collateral (the phloem being turned downwards and the xylem upwards), even in Ferns, where the petiolar strands are concentric, and they have the ordinary mesodesm and peridesm of the collateral bundle. The latter is often sclerized, especially opposite the phloem, and to a less extent opposite the xylem, as in the stem. As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath.
Fig. 17.—Transverse section of the stele of the stem of a water-plant (Naias); l. intercellular channel representing xylem; ph. phloem; e. endodermis.
Fig. 18.—Vertical section of a Palm-stem, showing the vascular bundles, fv, curving inwards and then outwards.
The structure of the stele of the primary root as it is found in most Pteridophytes and many Phanerogams has been already described. The radial structure is characteristic of all root-steles, which have in essential points a remarkably uniform structure throughout the vascular plants, a fact no doubt largely dependent on the very uniform conditions under which they live. While the stele of the primary root in both Gymnosperms and Angiosperms is usually diarch or tetrarch, the large primary root-steles of many adventitious roots are frequently polyarch, sometimes with a very large number of protoxylems. Such a stele seldom has the centre filled up with xylem, this being replaced by a large-celled pith, so that a siphonostelic structure is acquired (fig. 15). Sometimes, however, the centre of a bulky root stele has strands of metaxylem (to which may be added strands of metaphloem) scattered through it, the interstices being filled with conjunctive. The conjunctive of a root-stele possessing a pith is often sclerized between the pith and the pericycle. Sometimes all the parenchyma within the stele undergoes this change. In the roots of some palms and orchids a “polystelic” structure obtains.
In certain families of Angiosperms a peculiar tissue, called laticiferous tissue is met with. This takes the form of long usually richly branched tubes which penetrate the other tissues of the plant mainly in a longitudinal direction. They possess a delicate Laticiferous Tissue. layer of protoplasm, with numerous small nuclei lining the walls, while the interior of the tube (corresponding with the cell-vacuole) contains a fluid called latex, consisting of an emulsion of fine granules and drops of very various substances suspended in a watery medium in which various other substances (salts, sugars, rubber-producers, tannins, alkaloids and various enzymes) are dissolved. Of the suspended substances, grains of caoutchouc, drops of resin and oil, proteid crystals and starch grains may be mentioned. Of this varied mixture of substances some are undoubtedly plastic (i.e. of use in constructing new plant-tissue), others are apparently end-products of metabolism, in other words excreta, though they are not actually cast out from the plant-body. The relation of the laticiferous tissue to the assimilating cells under which they often end, and the fact that where this tissue is richly developed the conducting parenchyma of the bundles, and sometimes also the sieve-tubes, are poorly developed, as well as various other facts, point to the conclusion that the laticiferous system has an important function in conducting plastic substances, in addition to acting as an excretory reservoir. As a secondary function we may recognize, in certain cases, the power of closing wounds, which results from the rapid coagulation of exuded latex in contact with the air. The use of certain plants as rubber-producers (notably Hevea brasiliensis, the Para rubber-tree) depends on this property. The trees are regularly tapped and the coagulated latex which exudes is collected and worked up into rubber. Opium is obtained from the latex of the opium poppy (Papaver somniferum), which contains the alkaloid morphine.
|(After Haberlandt. From Vines' Text-Book of Botany, by permission.)|
|Fig. 19.—A portion of a la
Laticiferous tissue is of two kinds: (1) laticiferous cells (coenocytes) (fig. 19) which branch but do not anastomose, and the apices of which keep pace in their growth with that of the other tissues of the plant (Apocynaceae, most Euphorbiaceae, &c.); (2) laticiferous vessels (fig. 20) which are formed from rows of meristematic cells, the walls separating the cells breaking down, so that a network of laticiferous tubes arises (Papaveraceae, Hevea, &c.). In some cases (Allium, Convolvulaceae, &c.) rows of cells with latex-like contents occur, but the walls separating the individual cells do not break down.
The body of a vascular plant is developed in the first place by repeated division of the fertilized egg and the growth of Development of Primary Tissue. the products of division. The body thus formed is called the embryo, and this develops into the adult plant, not by continued growth of all its parts as in an animal, but by localization of the regions of cell-division and growth, such a localized region being called a growing-point. This localization takes place first at the two free ends of the primary axis, the descending part of which is the primary root, and the ascending the primary shoot. Later, the axis branches by the formation of new growing-points, and in this way the complex system of axes forming the body of the ordinary vascular plant is built up. In the flowering plants the embryo, after developing up to a certain point, stops growing and rests, enclosed within the seed. It is only on “germination” of the latter that the development of the embryo into the free plant is begun. In the Pteridophytes, on the other hand, development from the egg is continuous.
The triple division of tissues is laid down in most cases at a very early period of development—in the flowering plants usually before the resting stage is reached. In many Pteridophytes the first leaf is formed very early, and the first vascular strand is developed at its base, usually becoming continuous with the cylinder of the root; the strand of the second leaf is formed in a similar way and runs down to join that of the first, so that the stem stele is formed by the joined bases of the leaf-traces. In other cases, however, a continuous primitive stele is developed, extending from the primary stem to the primary root, the leaf-traces arising later. This is correlated with the comparatively late formation and small development of the first leaves. The evidence scarcely admits of a decision as to which of these methods is to be regarded as primitive in descent. In the seed-forming plants (Phanerogams) one or more primary leaves (cotyledons) are already formed in the resting embryo. In cases where the development of the embryo is advanced at the resting period, traces run from the cotyledons and determine the symmetry of the stele of the primitive axis, the upper part of which often shows stem-structure, in some respects at least, and is called the hypocotyledonary stem or hypocotyl, while the lower part is the primary root (radicle). In other cases the root structure of the stele continues up to the cotyledonary node, though the hypocotyl is still to be distinguished from the primary root by the character of its epidermis. On germination of the seed the radicle first grows out, increasing in size as a whole, and soon adding to its tissues by cell division at its apical growing-point. The hypocotyl usually elongates, by its cells increasing very greatly in the longitudinal direction both in number and size, so that the cotyledons are raised into the air as the first foliage-leaves. Further growth in length of the stem is thenceforward confined to the apical growing point situated between the cotyledons. In other cases this growing-point becomes active at once, there being little or no elongation of the hypocotyl and the cotyledon or cotyledons remaining in the seed.
(After Sachs. From Vines' Text-Book of Botany, by permission.)
Fig. 20.—Laticiferous Vessels from the cortex of the root Scorzonera hispanica, tangential section.
A, Slightly magnified. B, A small portion highly magnified.
The structure of the growing-points or apical meristems varies much in different cases. In most Pteridophytes there is a single Growing-points. large apical cell at the end of each stem and root axis. This usually has the form of a tetrahedron, with its base occupying the surface of the body of the axis and its apex pointing towards the interior. In the stem, segments are successively cut off from the sides of the tetrahedron, and by their subsequent division the body of the stem is produced. In the root exactly the same thing occurs, but segments are cut off also from the base of the tetrahedron, and by the division of these the root-cap is formed (fig. 21). In both stem and root early walls separate the cortex from the stele. The epidermis in the stem and the surface layer of the root soon becomes differentiated from the underlying tissue. In some Pteridophyte stems the apical cell is wedge-shaped, in others prismatic; in the latter case segments are cut off from the end of the prism turned towards the body of the stem. In other cases, again, a group of two or four prismatic cells takes the place of the apical cell. Segments are then cut off from the outer sides of these initial cells. In most of the Phanerogams the apical (or primary) meristem, instead of consisting of a single apical cell or a group of initials, is stratified—i.e. there is more than one layer of initials (fig. 22). Throughout the Angiosperms the epidermis of the shoot originates from separate initials, which never divide tangentially, so that the young shoot is covered by a single layer of dividing cells, the dermatogen. Below this are the initials of the cortex and central cylinder. Whether these are always in layers which remain separate is not known, but it is certain that in many cases such layers cannot be distinguished. This, however, may be due to irregularity of division and displacement of the cells by irregular tensions destroying the obvious layered arrangement. In some cases there is a perfectly definite line of separation between the young cylinder (plerome) and young cortex (periblem), the latter having one or more layers of initials at the actual apex. This clear separation between periblem and plerome is mostly found in plants whose stem-apex forms a naked cone, the leaves being produced relatively late, so that the stele of the young stem is obvious above the youngest leaf-traces (fig. 22). Where the leaves are developed early, they often quite overshadow the actual apex of the stem, and, the rapid formation of leaf-tissue disturbs the obviousness of, and perhaps actually destroys, the stratified arrangement of the shoot initials. In this case also, the differentiation of leaf-bundles, which typically begins at the base of the leaf and extends upwards into the leaf and downwards into the stem, is the first phenomenon in the development of vascular tissue, and is seen at a higher level than the formation of a stele. The latter is produced (except in cases of complete astely where a cylinder is never formed) after a number of leaf-traces have appeared on different sides of the stem so as to form a circle as seen in transverse section, the spaces intervening between adjacent bundles becoming bridged by small-celled tissue closing the cylinder. In this tissue fresh bundles may become differentiated, and what remains of it becomes the rays of the fully-formed stele. Many cases exist which are intermediate between the two extreme types described. In these the stele becomes obvious in transverse section at about the same level as that at which the first leaf-traces are developed. Where a large-celled pith is developed this often becomes obvious very early, and in some cases it appears to have separate initials situated below those of the hollow vascular cylinder. In some cases where there is apparently a well-marked plerome at the apex, this is really the young pith, the distinction between the stelar and cortical initials, if it exists, being, as is so often the case, impossible to make out. The young tissue of the stelar cylinder, in the case of the modified siphonostele characteristic of the dicotyledonous stem, differs from the adjoining pith and cortex in its narrow elongated cells, a difference produced by the stopping of transverse and the increased frequency of longitudinal divisions. This is especially the case in the young vascular bundles themselves (desmogen strands). The protoxylem and protophloem are developed a few cells from the inner and outer margins respectively of the desmogen strand, the desmogenic tissue left over giving rise to the segments of endocycle and pericycle capping the bundle. Differentiation of the xylem progresses outwards, of the phloem inwards, but the two tissues never meet in the centre. Sometimes development stops altogether, and a layer of undifferentiated parenchyma (the mesodesm) is left between them; or it may continue indefinitely, the central cells keeping pace by their tangential division with the differentiation of tissue on each side. In this case the formation of the primary bundle passes straight over into the formation of secondary tissue by a cumbium, and no line can be drawn between the two processes. The differentiation of the stelar stereom, which usually takes the form of a sclerized pericycle, and may extend to the endocycle and parts of the rays, takes place in most cases later than the formation of the primary vascular strand. In the very frequent cases where the bundles have considerable individuality, the fibrous “pericyclic” cap very clearly has a common origin from the same strand of tissue as the vascular elements themselves. In such cases it is part of the peridesm or sheath of elongated narrow-celled tissue surrounding the individual bundle.
|(After De Bary. From Vines' Text-Book of Botany, by permission.)|
Fig. 22.—Median Longitudinal Section of the Growing Point of the Stem of Hippuris vulgaris, showing a many-layered meristem. (× 225.)
|l, Rudiment of leaf; d, dermatogen.|
The separation of layers in the apical meristem of the root is usually very much more obvious than in that of the stem. The outermost is the calyptrogen, which gives rise to the root-cap, and in Dicotyledons to the piliferous layer as well. The periblem, one cell thick at the apex, produces the cortex, to which the piliferous layer belongs in Monocotyledons; and the plerome, which is nearly always sharply separated from the periblem, gives rise to the vascular cylinder. In a few cases the boundaries of the different layers are not traceable. The protoxylems and the phloem strands are developed alternately, just within the outer limit of the young cylinder. The differentiation of metaxylem follows according to the type of root-stele, and, finally, any stereom there may be is developed. Differentiation is very much more rapid—i.e. the tissues are completely formed much nearer to the apex, than is the case in the stem. This is owing to the elongating region (in which protoxylem and protophloem alone are differentiated) being very much shorter than in the stem. The root hairs grow out from the cells of the piliferous layer immediately behind the elongating region.
The branches of the stem arise by multiplication of the cells of the epidermis and cortex at a given spot, giving rise to a protuberance, at the end of which an apical meristem is established. The vascular system is connected in various ways with that of the parent axis by the differentiation of bundle-connexions across the cortex of the latter. This is known as exogenous branch-formation. In the root, on the other hand, the origin of branches is endogenous. The cells of the pericycle, usually opposite a protoxylem strand, divide tangentially and give rise to a new growing-point. The new root thus laid down burrows through the cortex of the mother-root and finally emerges into the soil. The connexions of its stele with that of the parent axis are made across the pericycle of the latter. Its cortex is never in connexion with the cortex of the parent, but with its pericycle. Adventitious roots, arising from stems, usually take origin in the pericycle, but sometimes from other parts of the conjunctive.
In most of the existing Pteridophytes, in the Monocotyledons, and in annual plants among the Dicotyledons, there is no Secondary Tissues. further growth of much structural importance in the tissues after differentiation from the primary meristems. But in nearly all perennial Dicotyledons, in all dicotyledonous and gymnospermous trees and shrubs, and in fossil Pteridophytes belonging to all the great groups, certain layers of cells remain meristematic among the permanent tissues, or after passing through a resting stage reacquire meristematic properties, and give rise to secondary tissues. Such meristematic layers are called secondary meristems. There are two chief secondary meristems, the cambium and the phellogen. The formation of secondary tissues is characteristic of most woody plants, to whatever class they belong. Every great group or phylum of vascular plants, when it has become dominant in the vegetation of the world, has produced members with the tree habit arising by the formation of a thick woody trunk, in most cases by the activity of a cambium.
The cambium in the typical case, which is by far the most frequent, continues the primary differentiation of xylem and phloem in the desmogen strand (see above), or arises in the resting mesodesm or mesocycle and adds new (secondary) xylem and phloem to the primary tissues. New tangential walls arise in the cells which are the seat of cambial activity, and an initial layer of cells is established which cuts off tissue mother-cells on the inside and outside, alternately contributing to the xylem and to the phloem. A tissue mother-cell of the xylem may, in the most advanced types of Dicotyledons, give rise to—(1) a tracheid; (2) a segment of a vessel; (3) a xylem-fibre; or (4) a vertical file of xylem-parenchyma cells. In the last case the mother-cell divides by a number of horizontal walls. A tissue mother-cell of the phloem may give rise to (1) a segment of a sieve-tube with its companion cell or cells; (2) a phloem fibre; (3) a single phloem-parenchyma (cambiform) cell, or a vertical file of short parenchyma cells. At certain points the cambium does not give rise to xylem and phloem elements, but cuts off cells on both sides which elongate radially and divide by horizontal walls. When a given initial cell of the cambium has once begun to produce cells of this sort it continues the process, so that a radial plate of parenchyma cells is formed stretching in one plane through the xylem and phloem. Such a cell-plate is called a medullary ray. It is essentially a living tissue, and serves to place all the living cells of the secondary vascular tissues in communication. It conducts plastic substances inwards from the cortex, and its cells are frequently full of starch, which they store in winter. They are accompanied by intercellular channels serving for the conduction of oxygen to, and carbon dioxide from, the living cells in the interior of the wood, which would otherwise be cut off from the means of respiration. The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether. The parenchyma is often arranged in tangential bands between the layers of sieve-tubes and tracheal elements. The xylem parenchyma is often found in strands associated with the tracheal elements. These strands are not isolated, but form a connected network through the wood. The xylem parenchyma cells are connected, as are the medullary ray cells, with the tracheal elements by one-sided bordered pits—i.e. pits with a border on the tracheal element side, and simple on the parenchyma cell side. The fibres are frequently found in tangential bands between similar bands of tracheae or sieve-tubes. The fibrous bands are generally formed towards the end of the year's growth in thickness. The fibres belong to the same morphological category as the parenchyma, various transitions being found between them; thus there may be thin-walled cells of the shape of fibres, or ordinary fibres may be divided into a number of superposed cells. These intermediate cells, like the ordinary parenchyma, frequently store starch, and the fibres themselves, though usually dead, sometimes retain their protoplasm, and in that case may also be used for starch accumulations. The vessels and tracheids are very various in size, shape and structure in different plants. They are nearly always aggregated in strands, which, like those of the parenchyma, are not isolated, but are connected with one another. In a few cases some of the tracheids have very thick walls and reduced cavities, functioning as mechanical rather than as water-conducting elements. All transitions are found between such forms and typical tracheids. These fibre-tracheids are easily confused on superficial view with the true wood-fibres belonging to the parenchymatous system, but their pits are always bordered, though in the extreme type they are reduced to mere slits in the wall. The sieve-tubes of the secondary phloem usually have very oblique end-walls bearing a row of sieve-plates; plates also occur on the radial side walls
The tissue-elements just described are found only in the more complicated secondary vascular tissues of certain Dicotyledons. A considerable evolution in complexity can be traced in passing from the simplest forms of xylem and phloem found in the primary vascular tissues both among Pteridophytes and Phanerogams to these highly differentiated types. In the simplest condition we have merely tracheae and sieve-tubes, respectively associated with parenchyma, which in the former case is usually amylom, i.e. consists of starch-containing cells, and in the latter of proteid cells. This type is found in nearly all Pteridophytes and, so far as is known, in Cycadofilices, both in primary and secondary tissue. The stereom is furnished either by cortical cells or by the tracheal elements, in a few cases by fibres which are probably homologous with sieve-tubes. Among Gymnosperms the secondary xylem is similarly simple, consisting of tracheids which act as stereom as well as hydrom, and a little amylom; while the phloem-parenchyma sometimes undergoes a differentiation, part being developed as amylom, part as proteid cells immediately associated with the sieve-tube. In other cases the proteid cells of the secondary phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cell-rows of the medullary rays, the middle rows consisting of ordinary starchy cells. The top and bottom rows of the xylem rays are often developed as irregularly-thickened radially-elongated, tracheids which serve for the radial conduction of water, and communicate with the ordinary tracheids of the secondary xylem by large bordered pits. The primary vascular tissues of Angiosperms are likewise nearly always simple, consisting merely of tracheae and sieve-tubes often associated with amylom. A characteristic peculiarity, both in the primary and secondary tissue, is that the proteid cells of the phloem are here always sister-cells of the leptoids and are known as companion-cells. In the secondary tissues of Dicotyledons we may have, as already described, considerably more differentiation of the cells, all the varieties being referable, however, on the one hand to the tracheal or sieve-tube type, on the other to the parenchyma type. The main feature is the development of special vascular stereom and storage tissue. In some cases special secreting tissues, resin ducts, oil glands, laticiferous tissue, crystal sacs, &c., may be developed among the ordinary secondary vascular elements.
|(from Green's Vegetable Physiology, by permission)|
Fig. 23.—Section of part of hypocotyledonary stem of Ricinus communis.
a, Starch sheath, at the extremities of the figure its cells are represented as empty; b, cambium layer.
The limit of each year's increment of secondary wood, in those plants whose yearly activity is interrupted by a regular winter Annual Rings. or dry season, is marked by a more or less distinct line, which is produced by the sharp contrast between the wood formed in the late summer of one year (characterized by the sparseness or small diameter of the tracheal elements, or by the preponderance of fibres, or by a combination of these characters, giving a denseness to the wood) and the loose spring wood of the next year, with its absence of fibres, or its numerous large tracheae. The abundance of water conducting channels is in relation to the need for a large and rapid supply of water to the unfolding leaves in the spring and early summer. In Gymnosperms, where vessels and fibres are absent the late summer wood is composed of radially narrow thick-walled tracheids, the wood of the succeeding spring being wide-celled and thin-walled, so that the limit of the years growth is very well marked. The older wood of a large tree forming a cylinder in the centre of the trunk frequently undergoes marked changes in character. The living elements die, and the walls of all the cells often become hardened, owing to the deposit in them of special substances. Wood thus altered is known as heart-wood, or duramen, as distinguished from the young sap-wood, or alburnum, which, forming a cylinder next the cambium, remains alive and carries on the active functions of the xylem, particularly the conduction of water. The heart-wood ceases to be of any use to the tree except as a support, but owing to its dryness and hardness it alone is of much use for industrial purposes. The great hardness of teak is due to the silica deposited in the heart-wood, and the special colouring matters of various woods, such as satinwood, ebony, &c., are confined to the heart-wood. In some cases the heart-wood, instead of becoming specially hard, remains soft and easily rots, so that the trunk of the tree frequently becomes hollow, as is commonly the case in the willow. Heart-wood is first formed at very different epochs in the life of a tree, according to the species—e.g. after fifteen to twenty years in the oak, forty years in the ash, &c.
In many annual plants no cambium is formed at all, and the same is true of most perennial Pteridophytes and Monocotyledons. Cambium in Stems. When the vascular tissue of such plants is arranged in separate bundles these are said to be closed. The bundles of plants which form cambium are, on the contrary, called open. In stems with open bundles the formation of cambium and secondary tissue may be confined to these, when it is said to be entirely fascicular. In that case either very little secondary tissue is formed, as in the gourds, some Ranunculaceae, &c., or a considerable amount may be produced (clematis, barberry, ivy). In the latter event the cells of the primary rays are either merely stretched radially, or they divide to keep pace with the growth of the bundles. If this division occurs by means of a localized secondary meristem connecting the cambial layers of adjacent bundles, an interfascicular is formed in addition to the fascicular cambium. The interfascicular cambium may form nothing but parenchymatous tissue, producing merely continuations of the primary rays. Such rays are usually broader and more conspicuous than the secondary rays formed within the wedges of wood opposite the primary bundles, and are distinguished as principal rays from these narrower subordinate or fascicular rays (fig. 24). This is the typical case in most trees where the primary bundles are close together. Where the primary bundles are farther apart, so that the primary rays are wider, the interfascicular cambium may form several fairly broad (principal) secondary rays in continuation of certain radial bands of the primary ray, and between these, wedges of secondary xylem and phloem: or, finally, secondary xylem and phloem may be formed by the whole circumference of the cambium, fascicular and interfascicular alike, interrupted only by narrow secondary rays, which have no relation to the primary ones.
In a good many cases, sometimes in isolated genera or species, sometimes characteristic of whole families, so-called anomalous cambial layers are formed in the stem, either as an extension of, or in addition to, the original cambial cylinder. They are frequently associated with irregularities in the activity of the original cambium. Irregularity of cambium occurs in various families of woody dicotyledonous plants, mostly among the woody climbers, known as lianes, characteristic of tropical and sub-tropical forests. In the simplest cases the cambium produces xylem more freely along certain tracts of the circumference than along others, so that the stem loses its original cylindrical form and becomes elliptical or lobed in section. In others the secondary phloem is produced more abundantly in those places where the secondary xylem is deficient, so that the stem remains cylindrical in section, the phloem occupying the bays left in the xylem mass. Sometimes in such cases the cambium ceases to be active round these bays and joins across the outside of the bay, where it resumes its normal activity, thus isolating a phloem strand, or, as it is sometimes called, a phloem island, in the midst of the xylem. The significance of these phenomena, which present many minor modifications in different cases, is not fully understood, but one purpose of the formation of phloem promontories and islands seems to be the protection of the sieve-tubes from crushing by the often considerable peripheral pressure that is exercised on the stems of these lianes. Sometimes the original cambial ring is broken into several arcs, each of which is completed into an independent circle, so that several independent secondary vascular cylinders are formed. The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem. The activity of the new cambium is often associated with the stoppage of the original one. Sometimes the activity of the successive cambiums simply results in the formation of concentric rings or arcs of secondary xylem and phloem. In other cases a most intricate arrangement of secondary tissue masses is produced, quite impossible to interpret unless all stages of their development have been followed. Sometimes in lianes the whole stem breaks up into separate woody strands, often twisted like the strands of a rope, and running into one another at intervals. An ordinary cambium is scarcely ever found in the Monocotyledons, but in certain woody forms a secondary meristem is formed outside the primary bundles, and gives rise externally to a little secondary cortex, and internally to a secondary parenchyma in which are developed numerous zones of additional bundles, usually of concentric structure, with phloem surrounded by xylem.
The cambium in the root, which is found generally in those plants which possess a cambium in the stem, always begins in the conjunctive Cambium in Roots. tissue internal to the primary phloems, and forms new (secondary) phloem in contact with the primary, and secondary xylem internally. In roots which thicken but slightly, whose cambium usually appears late, it is confined to these regions. If the development of secondary tissues is to proceed further, arcs of cambium are formed in the pericycle external to the primary xylems, and the two sets of cambial arcs join, forming a continuous, wavy line on transverse section, with bays opposite the primary phloems and promontories opposite the primary xylems. Owing to the resistance offered by the hard first-formed secondary xylem, the bays are pushed outwards as growth proceeds, and the wavy line becomes a circle. Opposite the primary xylems, the cambium either (a) forms parenchyma on both sides, making a broad, secondary (principal) ray, which interrupts the vascular ring and is divided at its inner extremity by the islet of primary xylem; or (b) forms secondary xylem and phloem in the ordinary way, completing the vascular ring. In either case, narrow, secondary rays are formed at intervals, just as in the stem. Thus the structure of an old thickened root approximates to that of an old thickened stem, and so far as the vascular tissue is concerned can often only be distinguished from the latter by the position and orientation of the primary xylems. The cambium of the primary root, together with the tissues which it forms, is always directly continuous with that of the primary stem, just in the same way as the tissues of the primary stele. The so-called anomalous cambiums in roots follow the same lines as those of the stem.
In nearly all plants which produce secondary vascular tissues by means of a cambium there is another layer of secondary meristem Phellogen and Periderm. arising externally to, but in uite the same fashion as, the cambium, and producing like the latter an external and an internal secondary tissue. This is the phellogen, and the whole of the tissue it gives rise to is known as periderm. The phellogen derives its name from the fact that its external product is the characteristic tissue known as cork. This consists typically of close-fitting layers of cells with completely suberized walls, intended to replace the epidermis as the external protective layer of the plant when the latter, incapable as it is of further growth after its original formation, is broken and cast off by the increase in thickness of the stem through the activity of the cambium. Cork is also formed similarly in the root after the latter has passed through its primary stage as an absorptive organ, and its structure is becoming assimilated to that of the stem. The internal tissue formed by the phellogen is known as phelloderm, and consists usually of ordinary parenchyma. The phellogen may arise, in the first place, in any tissue of the axis external to the actual vascular tissues—i.e. in the epidermis itself (rarely), in any layer of the cortex, or in the pericycle. Its most usual seat of origin in the stem is the external layer of the cortex immediately below the epidermis; in the root, the pericycle. All the tissues external to the cork are cast off by the plant. The extent of development of the phelloderm is dependent upon whether the phellogen has a superficial or a deep-seated origin. In the former case the formation of phelloderm is trivial in amount; in the latter, considerable, since this tissue has to replace the cast-off cortex, as a metabolic and particularly a storage tissue.
Provision is made for gaseous interchange between the internal tissues and the external air after the formation of cork, by the development Lenticels. of lenticels. These are special organs which interrupt the continuity of the impermeable layer of ordinary cork-cells. A lenticel is formed by the phellogen at a given spot dividin very actively and giving rise to a loose tissue of rounded cells which soon lose their contents, and between which air can pass to the tissues below (fig. 25). A lenticel appears to the naked eye as a rounded or elongated scar, often forming a distinct prominence on the surface of the organ. The lenticels of the stem are usually formed beneath stomata, whose function they take up after the stomata have been ruptured and cast off with the rest of the epidermis. Both cork and phelloderm may be differentiated in various ways. The former often has its cells lignified, and may consist of alternate layers of hard and soft cells. The latter may develop stereom, and may also be the seat of origin of new formations of various kinds—e.g. supplementary vascular bundles, anomalous cambial zones, &c. It is often enormously developed and forms a very important tissue in roots. In the stem of a tree the original phellogen is replaced by successive new phellogenic layers of deeper and deeper origin, each forming its own layer of cork. Eventually the new phellogens reach the level of the secondary phloem, and are formed in the parenchyma of the latter, keeping pace in their inward march with the formation of fresh secondary phloem by the cambium. The complex system of dead and dying tissues cut off by these successive periderms, together with the latter themselves—in fact, everything outside the innermost phellogen, constitutes what is often known botanically as the bark of the tree. Rhytidome is, however, a preferable term, as the word bark has long been established in popular usage to mean all the tissue that can easily be peeled off—i.e. everything down to the wood of the tree. The rough surface of the bark of many trees is due to the successive phellogens not arising in regular concentric zones, but forming in arcs which join with the earlier-formed arcs and thus causing the bark to come off in flakes or thick chunks. A layer of cork is regularly formed in most Phanerogams across the base of the petiole before leaf fall, so as to cover the wound caused by the separation of the leaf from the stem. Special “wound-cork” is also often formed round accidental injuries so as to prevent the rotting of the tissues by the soaking in of rain and the entrance of fungal spores and bacteria. A peculiar modification of periderm is rormed by the phellogen in the submerged organs (roots or stems) of many aquatic or marsh-loving plants. This may take various forms and may cover the whole of the organ or be localized in special regions; but its cells are always living and are separated by very large intercellular spaces containing air. This tissue is called aerenchym, and no doubt its function is to facilitate the respiration of the organs on which it is formed and to which the access of oxygen is difficult. In other cases, a similar formation of spongy but dead periderm tissue may occur for the same purpose in special patches, called pneumatodes, on the roots of certain trees living in marshy places, which rise above the soil in order to obtain air.
|(From Vines' Text-Book of Botany, by permission.)|
Fig. 24.—Lenticel in the transverse section of a twig of Elder. (× 300.)
E, epidermis; q, phellogen, l, cells, and pl, the phellogen of the lenticel; lc, cortical parenchyma, containing chlorophyll.
History and Bibliography.-The study of plant anatomy was begun in the middle of the seventeenth century as a direct result of the construction of microscopes, with which a clear view of the structure of plant tissues could be obtained. The Englishman Grew and the Italian Malpighi almost simultaneously published illustrated works on the subject, in which they described, for the most part very accurately, what they saw with the new instruments. The subject was practically dormant for nearly a century and a half, largely owing to the dominance of classificatory botany under the influence of Linnaeus. It was revived by several German workers, prominent among whom were Treviranus and Link, and later Moldenhawer, as well as by the Frenchmen Mirbel, at the beginning of the 19th century. The new work largely centred round a discussion of the nature and origin of vessels, conspicuous features in young plant tissues which thus acquired an importance in the contemporary literature out of proportion to their real significance in the construction of the vascular plant. The whole of the writings of this time are dominated by a preoccupation with the functions of the different tissues, in itself an excellent standpoint for investigation, but frequently leading in the case of these early investigators to one-sided and distorted views of the facts of structure. The pioneer of modern plant anatomy was Hugo von Mohl (fl. 1840), who carefully investigated and described the facts of anatomical structure without attempting to fit them into preconceived views of their meaning. He produced a solid body of accurately described facts which has formed the secure groundwork of subsequent advance. From Mohl down to the eighth decade of the century the study of anatomy was entirely in the hands of a group of German investigators, prominent among whom were several of the most eminent founders of modern scientific botany—such, for instance, as Nageli, Sanio and De Bary. To the first we owe the secure foundation of our knowledge of the structure and course of the vascular strands of the higher plants (“Ueber den Bau und die Anordnung der Gefässbündel bei den Stamm und Wurzel der Phanerogamen, ” Beiträge zur wissenschaftlichen Botanik, Heft i., Leipzig, 1859); to the second the establishment of the sound morphological doctrine of the central cylinder of the axis as the starting-point for the consideration of the general arrangement of the tissues, and the first clear distinction between primary and secondary tissues (Botanische Zeitung, 1861 and 1863); to the last the putting together of the facts of plant anatomy known up to the middle of the eighth decade of the century in that great encyclopedia of plant anatomy, the Vergleichende Anatomie der Vegetationsorgane bei den Phanerogamen und Farnen (Stuttgart, 1876; Eng. trans., Comparative Anatomy of the Vegetative Organs of the Phanerogams and Ferns, Oxford, 1882). In 1870-1871 Van Tieghem published his great work, “Sur la Racine,” Ann. sci. nat. bot. (Paris). This was not only in itself an important contribution to plant anatomy, but served as the starting-point of a series of researches by Van Tieghem and his pupils, which has considerably advanced our knowledge of the details of histology, and also culminated in the foundation of the doctrine of the stele (Van Tieghem and Douliot, “Sur la polystélie,” Ann. sci. nat. bot., 1887; Van Tieghem, Traité de botanique (2nd ed. Paris, 1889-1891). This has had a most important effect on the development in recent years of morphological anatomy.
In the progress of the last three decades, since the publication of De Bary's great work, five or six main lines of advance can be Modem Progress of the Subject. distinguished. First, the knowledge of the details of histology has of course advanced greatly in the direction through the ceaseless activity of very numerous, mainly German, workers, though no fundamentally new types of tissue have been discovered. Secondly, the histology of fossil plants, particularly woody plants of the carboniferous period, has been placed on a sound basis, assimilated with general histological doctrine, and has considerably enlarged our conceptions of plant anatomy as a whole, though again without revealing any entirely new types of structure. This branch of the subject, founded by Corda, Göppert, Stenzel and others in Germany, was enormously advanced by Williamson's work on the Coal Measures plants, recorded in the magnificent series of memoirs, “Researches on the Organization of Fossil Plants of the Coal Measures” (Phil. Trans. Roy. Soc., vols. i.-xix., 1871-1893). The work of Solms Laubach in Germany, Renault and Bertrand in France, and in recent years, of Zeiller in France, and Scott, Seward and others in England, has advanced our knowledge of the anatomy of fossil plants in an important degree. While convincing us that the plants of past ages in the earth's history were exposed to very similar conditions of life, and made very much the same adaptive responses as their modern representatives, one of the main results of this line of work has been to reveal important data enabling us to fill various gaps in our morphological knowledge and to obtain a more complete picture of the evolution of tissues in the vascular plants. One of the most striking incidents in the progress has been the recognition within the last few years of the existence of an extinct group of plants lying on the borderland between Filicales and Gymnosperms, and known as the Cycadofilices, a group in which, curiously enough, the reproductive organs remained undiscovered for some time after the anatomy of the vegetative organs was sufficiently well known to afford clear evidence of their true affinities. Thirdly, we have to record very considerable progress in our knowledge of distinctively morphological anatomy, i.e. the study of tissues from the standpoint of evolution. The Russian plant-anatomist, Russow, may be said to have founded the consideration of plant tissues from the point of view of descent (Vergleichende Untersuchungen über die Leitbundelkryptogamen, St Petersburg, 1872; and Betrachtungen über Leitbundel und Grundgewebe, Dorpat, 1875). He was ably followed by Strasburger (Ueber den Bau und die Verrichtungen der Leitungsbahnen in den Pflanzen, Jena, 1891), Haberlandt and others. The explicit adoption of this point of view has had the effect of clearing up and rendering definite the older morphological doctrines, which for the most part had no fixed criterion by which they could be tested.
Since about 1895 this branch has been most actively pursued in England, where the work of Boodle and of Gwynne-Vaughan especially on Ferns) has been the most important, leading to a coherent theory of the evolution of the vascular system in these plants (Tansley, Evolution of the Filicinean Vascular System, Cambridge, 1908); and in America, where Jeffrey has published important papers on the morphology of the vascular tissues of the various groups of Pteridophytes and Phanerogams and has sought to express his conclusions in a general morphological theory with appropriate terminology. As a result of this activity Van Tieghem's so-called “Stelar theory” has been revised and modified in the light of more extended and detailed anatomical and developmental knowledge. Schoute's Die Stelar-Theorie (Groningen, 1902), gives an important critical account of this subject.
Fourthly, attention must be called to the great development of what is called “Systematic Anatomy,” i.e. the study of the anatomical features characteristic of the smaller groups of flowering plants, i.e. the orders, families, genera and species. Radlkofer (1883) was the first to call attention to the great importance of this method in systematic botany, as providing fresh characters on which to base a natural classification. Solereder's great work, Systematische Anatomie der Dicotyledonen (Stuttgart, 1898-1908; Eng. trans., Systematic Anatomy of Dicotyledons, Oxford, 1998), brings together so many of the facts as are at present known in an orderly arrangement. Theoretically this branch of the subject should connect with and form the completion of “morphological anatomy,” but the field has not yet been sufficiently explored to allow of the necessary synthesis. The true relation of “systematic” to “ecological” anatomy (see below) also awaits proper elucidation.
Fifthly, we have to record the foundation of the modern study of “physiological anatomy” (i.e. the study of the specific functions of the various tissues) by Schwendener (Das mechanische Princip im Bau der Monocotylen, 1874, and other works), followed by numerous pupils and others, among whom Haberlandt (Physiologische Pflanzen-Anatomie, Leipzig, 1st ed., 1884, 4th ed., 1909, and other works) is pre-eminent. The pursuit of this study has not only thrown valuable light on the economy of the plant as a whole, but forms an indispensable condition of the advance of morphological anatomy. A great deal of work still remains to be done in this department, which at the present time affords one of the most promising fields of anatomical investigation.
Finally we may mention “ecological anatomy,” i.e. the study of anatomical features directly related to the habitat. A very considerable body of knowledge relating to this subject already exists, but further work on experimental lines is urgently required to enable us to understand the actual economy of plants growing under different conditions of life and the true relation of the hereditary anatomical characters which form the subject matter of “systematic anatomy” to those which vary according to the conditions in which the individual plant is placed. On these lines the future of anatomical study presents almost inexhaustible possibilities. (A. G. T.)
Physiology of Plants
The so-called vegetable physiology of a generation ago was in arrear of animal, and particularly of human, physiology, the study of the latter being followed by many more observers, and from its relative degree of advancement being the more capable of rapid development. It was fully recognized by its followers that the dominating influence in the structure and working of the body was the protoplasm, and the division of labour which it exhibited, with the accompanying or resulting differentiation into various tissues, was the special subject of investigation. Many who followed the study of vegetable structure did not at that time give an equal prominence to this view. The early histological researches of botanists led them to the recognition of the vegetable cell, and the leading writers in the middle of the 19th century pointed out the probable identity of Von Mohl's “protoplasm” with the “sarcode” of zoologists. They laid great stress on the nitrogenous nature of protoplasm, and noted that it preceded the formation of the cell-membrane. But by the ordinary student of thirty years later their work was to some extent overlooked, and the cell-wall assumed a prominence to which it was not entitled. The study of the differentiation of protoplasm was at that time seldom undertaken, and no particular attention was paid either to fixing it, to enable staining methods to be accurately applied to it, or to studying the action of chemical reagents upon it. It is only comparatively recently that the methods of histological investigation used by animal physiologists have been carefully and systematically applied to the study of the vegetable organisms. They have, however, been attended with wonderful results, and have revolutionized the whole study of vegetable structure. They have emphasized the statements of Von Mohl, Cohn, and other writers alluded to, that the protoplasm is here also the dominant factor of the body, and that all the peculiarities of the cell-wall can only be interpreted in the light of the needs of the living substance.
The Nature of the Organization of the Plant, and the Relations of the Cell-Membrane and the Protoplasm.—This view of the structure of the plant and this method of investigation lead us to a greatly modified conception of its organization, and afford more completely an explanation of the peculiarities of form found in the vegetable kingdom.
The study of simple organisms, many of which consist of nothing but a little mass of protoplasm, exhibiting a very rudimentary degree of differentiation, so far as our methods enable us to determine any at all, shows that the duties of existence can be discharged in the absence of any cell-wall. Those organisms which possess the latter are a little higher in the scale of life than those which remain unclothed by it, but a comparison of the behaviour of the two quickly enables us to say that the membrane is of but secondary importance, and that for those which possess it, it is nothing more than a protective covering for the living substance. Its physical properties, permeability by water, extensibility and elasticity, receive their interpretation in the needs of the latter. We come, accordingly, to regard it as practically an exoskeleton, and its functions as distinctly subordinate to those of the protoplasm which it clothes. If we pass a little higher up the scale of life we meet with forms consisting of two or more cells, each of which contains a similar minute mass of living substance. A study of them shows that each is practically independent of the others; in fact, the connexion between them is so slight that they can separate and each become free without the slightest disadvantage to another. So long as they are connected together mechanically they have apparently the power of influencing one another in various ways, and of passing liquid or gaseous materials from one to another. The conjoined organism is, in fact, a colony or association of the protoplasmic units, though each unit retains its independence. When we pass, again, from these to examine more bulky, and consequently more complex, plants, we find that the differences which can be observed between them and the simple lowly forms are capable of being referred to the increased number of the protoplasmic units and the consequent enlarged bulk of the mass or colony. Every plant is thus found to be composed of a number of these protoplasmic units, or, as they may preferably be termed, protoplasts, all of which are at first exactly alike in appearance and in properties. This is evident in the case of such plants as have a body consisting of filaments or plates of cells, and is little less conspicuous in those whose mass is but small, though the cells are evidently capable of computation in three dimensions. It does not at first appear to be the same with the bulkier plants, such as the ordinary green herbs, shrubs or trees, but a study of their earlier development indicates that they do not at the outset differ in any way from the simple undifferentiated forms. Each commences its existence as a simple naked protoplast, in the embroyo-sac or the archegonium, as the case may be. After the curious fusion with another similar protoplast, which constitutes what we call fertilization, the next stage in complexity already noted may be observed, the protoplasm becoming clothed by a cell-membrane. Very soon the single cell gives rise to a chain of cells, and this in turn to a cell mass, the individual units of which are at first quite uniform. With increase of number, however, and consequently enlargement of bulk in the colony, differentiation becomes compulsory. The requirements of the several pro top lasts must be met by supplies from without, and, as many of them are deep seated, varieties of need arise, so that various members of the colony are set apart for special duties, masses of them being devoted to the discharge of one function, others to that of another, and so on. Such limitations of the powers and properties of the individuals have for their object the well-being of the community of which those individuals are constituents.
Physiological and Morphological Differentiation.—The first indication of this differentiation in the vegetative body of the plant can be seen not only in the terrestrial green plants which have been particularly referred to, but also in the bulkier seaweeds. It is an extension of the first differentiation which was observable in the simple protoplasts first discussed, the formation, that is, of a protective covering. Fucus and its allies, which form conspicuous members of the larger Algae, have their external cells much smaller, more closely put together, and generally much denser than the rest of their tissue. In the lowly as well as the higher green plants we have evidence of specialization of the external protoplasts for the same purpose, which takes various shapes and shows different degrees of completeness, culminating in the elaborate barks which clothe our forest trees.
The second prominent differentiation which presents itself takes the form of a provision to supply the living substance with water. This is a primal necessity of the protoplast, and every cell gives evidence of its need by adopting one of the various ways in which such need is supplied. What little differentiation can be found to exist in the protoplasm of the simple unicellular organism shows the importance of an adequate water-supply, and indeed, the dependence of life upon it. The naked cells which have been alluded to live in water, and call therefore for no differentiation in connexion with this necessity; but those which are surrounded by a cell-wall always develop within themselves a vacuole or cavity which occupies the greater part of their interior, and the hydrostatic pressure of whose contents keeps the protoplasm in contact with the membrane, setting up a condition of turgidity.
The need for a constant supply of water is partly based upon the constitution of protoplasm, so far as we know it. The apparently structureless substance is saturated with it; and if once a cell is completely dried, even at a low temperature, in the enormous majority of cases its life is gone and the restoration of water fails to enable it to recover. Besides this intimate relationship, however, we can point to other features of the necessity for a constantly renewed water supply. The protoplasm derives its food from substances in solution in the water; the various waste products which are incident to its life are excreted into it, and so removed from the sphere of its activity. The raw materials from which the food is constructed are absorbed from the exterior in solution in water, and the latter is the medium through which the gaseous constituents necessary for life reach the protoplasm. Moreover, growth is essentially dependent upon water-supply. There is little wonder, then, that in a colony of pro top lasts such as constitute a large plant a considerable degree of differentiation is evident, bearing upon the question of water supply. Certain cells of the exterior are set apart for absorption of water from the soil, this being the source from which supplies are derived. Others are devoted to the work of carrying it to the protoplasts situated in the interior and at the extremities of the plant, a conducting system of considerable complexity being the result.
Other collections of cells are in many cases set apart for giving rigidity and strength to the mass of the plant. It is evident that as the latter increases in bulk, more and more attention must be paid to the dangers of uprooting by winds and storms. Various mechanisms have been adopted in different cases, some connected with the subterranean and others with the sub-aerial portions of the plant. Another kind of differentiation in such a cell-mass as we are dealing with is the setting apart of particular groups of cells for various metabolic purposes. We have the formation of numerous mechanisms which have arisen in connexion with the question of food supply, which may not only involve particular cells, but also lead to differentiation in the protoplasm of those cells, as in the development of the chloroplastids of the leaves and other green parts.
The inter-relations of the members of a large colony of protoplasts such as constitute a tree, demand much adjustment. Relations with the exterior are continually changing, and the needs of different regions of the interior are continually varying, from time to time. Two features which are essentially protoplasmic assume a great importance when we consider these relations. They are the power of receiving impressions or stimuli from the exterior, and of communicating with each other, with the view of co-ordinating a suitable response. We have nothing structural which corresponds to the former of these. In this matter, differentiation has proceeded very differently in animals and plants respectively, no nerves or sense organs being structurally recognizable. Communication between the various protoplasts of the colony is, however, carried on by means of fine protoplasmic threads, which are continuous through the cell-walls.
All the peculiarities of structure which we encounter consequently support the view with which we started, that the protoplasm of the plant is the dominant factor in vegetable structure, and that there need be but one subject of physiology, which must embrace the behaviour of protoplasm wherever found. There can be no doubt that there is no fundamental difference between the living substance of animals and plants, for many forms exist which cannot be referred with certainty to either kingdom. Free-swimming organisms without cell-membranes exist in both, and from them series of forms can be traced in both directions. Cellulose, the material of which vegetable cell-walls are almost universally composed, at any rate in their early condition, is known to occur, though only seldom, among animal organisms. Such forms as Volvox and the group of the Myxomycetes have been continually referred to both kingdoms, and their true systematic position is still a subject of controversy. All physiology, consequently, must be based upon the identity of the protoplasm of all living beings.
This method of study has to a large extent modified our ideas of the relative importance of the parts of such an organism as a large tree. The interest with which we regard the latter no longer turns upon the details of the structure of its trunk, limbs and roots, to which the living substance of the more superficial parts was subordinated. Instead of regarding these as only ministering to the construction of the bulky portions, the living protoplasts take the first place as the essential portion of the tree, and all the other features are important mainly as ministering to their individual well-being and to their multiplication. The latter feature is the growth of the tree, the well-being of the protoplasts is its life and health. The interest passes from the bulky dense interior, with the elaborate features of its cell-walls, to the superficial parts, where its life is in evidence. We see herein the reason for the great subdivision of the body, with its finely cut twigs and their ultimate expansions, the leaves, and we recognize that this subdivision is only an expression of the need to place the living substance in direct relationship with the environment. The formation and gradually increasing thickness of its bark are explained by the continually increasing need of adequate protection to the living cortex, under the strain of the increasing framework which the enormous multiplication of its living protoplasts demands, and the development of which leads to continual rupture of the exterior. The increasing development of the wood as the tree grows older is largely due to the demands for the conduction of water and mineral matters dissolved in it, which are made by the increased number of leaves which from year to year it bears, and which must each be put into communication with the central mass by the formation of new vascular bundles. Similar considerations apply to the peculiar features of the root-system. All these points of structure can only be correctly interpreted after a consideration of the needs of the individual protoplasts, and of the large colony of which they are members.
Gaseous Interchanges and their Mechanism.—Another feature of the construction of the plant has in recent years come into greater prominence than was formerly the case. The organism is largely dependent for its vital processes upon gaseous interchanges. It must receive a large constituent of what ultimately becomes its food from the air which surrounds it, and it must also take in from the same source the oxygen of its respiratory processes. On the other hand, the aerial environment presents considerable danger to the young and tender parts, where the protoplasts are most exposed to extremes of heat, cold, wet, &c. These must in some way be harmonized. No doubt the primary object of the cell-wall of even the humblest protoplast is protection, and this too is the meaning of the coarser tegumentary structures of a bulkier plant. These vary considerably in completeness with its age; in its younger parts the outer cells wall undergoes the change known as cuticularization, the material being changed both in chemical composition and in physical properties. The corky layers which take so prominent a share in the formation of the bark are similarly modified and subserve the same purpose. But these protective layers are in the main impermeable by gases and by either liquid or vapour, and prevent the access of either to the protoplasts which need them. Investigations carried out by Blackman, and by Brown and Escombe, have shown clearly that the view put forward by Boussingault, that such absorption of gases takes place through the cuticular covering of the younger parts of the plant, is erroneous and can no longer be supported. The difficulty is solved by the provision of a complete system of minute intercellular spaces which form a continuous series of delicate canals between the cells, extending throughout the whole substance of the plant. Every protoplast, except in the very young regions, has part of its surface abutting on these, so that its wall is accessible to the gases necessary for its vital processes. There is no need for cuticularization here, as the external dangerous influences do not reach the interior, and the processes of absorption which Boussingault attributed to the external cuticularized cells can take place freely through the delicate cell-walls of the interior, saturated as these are with water. This system of channels is in communication with the outer atmosphere through numerous small apertures, known as stomata, which are abundant upon the leaves and young twigs, and gaseous interchange between the plant and the air is by their assistance rendered constant and safe. This system of intercellular spaces, extending throughout the plant, constitutes a reservoir, charged with an atmosphere which differs somewhat in its composition from the external air, its gaseous constituents varying from time to time and from place to place, in consequence of the interchanges between itself and the protoplasts. It constitutes practically the exterior environment of the protoplasts, though it is ramifying through the interior of the plant.
The importance of this provision in the case of aquatic vascular plants of sturdy bulk is even greater than in that of terrestrial organisms, as their environment offers considerable obstacles to the renewal of the air in their interior. They are without stomata on their submerged portions, and the entry of gases can only take place by diffusion from the water through their external cells, which are not cuticularized. Those which are only partially submerged bear stomata on their exposed portions, so that their environment approximates towards that of a terrestrial plant, but the communication even in their case is much less easy and complete, so that they need a much larger reservoir of air in their interior. This is secured by the development of much larger intercellular spaces, amounting to lacunæ or passages of very considerable size, which are found ramifying in different ways in their interior.
Transpiration.—In the case of terrestrial plants, the continual renewal of the water contained in the vacuoles of the protoplasts demands a copious and continuous evaporation. This serves a double purpose, bringing up from the soil continually a supply of the soluble mineral matters necessary for their metabolic processes, which only enter the plant in solutions of extreme dilution, and at the same time keeping the plant cool by the process of evaporation. The latter function has been found to be of extreme importance in the case of plants exposed to the direct access of the sun's rays, the heat of which would rapidly cause the death of the protoplasts were it not employed in the evaporation of the water. Brown and Escombe have shown that the amount of solar energy taken up by a green leaf may often be fifty times as much as it can utilize in the constructive processes of which it is the seat. If the heat were allowed to accumulate in the leaf unchecked, they have computed that its temperature would rise during bright sunshine at the rate of more than 12° C. per minute, with of course very rapidly fatal results. What is not used in the constructive processes is employed in the evaporation of the water, the leaf being thus kept cool. Whether the leaf is brightly or only moderately illuminated, the same relative proportions of the total energy absorbed are devoted to the purposes of composition and construction respectively. This large evaporation which constitutes the so-called transpiration of plants, takes place not into the external air but into this same inter cellular space system, being possible only through the delicate cell-walls upon which it abuts, as the external coating, whether bark, cork or cuticle, is impermeable by watery vapour. The latter ultimately reaches the external air by diffusion through the stomata, whose dimensions vary in proportion as the amount of water in the epidermal cells becomes greater or less.
Mechanism and Function of Stomata.—It is not quite exact to speak of either the gaseous interchanges or the transpiration as taking place through the stomata. The entry of gases into, and exit from, the cells, as well as the actual exhalation of watery vapour from the latter, take place in the intercellular space system of which the stomata are the outlets. The opening and closing of the stomata is the result of variation in the turgidity of their guard cells, which is immediately affected by the condition of turgidity of the cells of the epidermis contiguous to them. The amount of watery vapour in the air passing through a stoma has no effect upon it, as the surfaces of the guard cells abutting on the air chamber are strongly cuticularized, and therefore impermeable. The only way in which their turgidity is modified is by the entry of water into them from the contiguous cells of the general epidermis and its subsequent withdrawal through the same channel. This opening and closing of the stomata must be looked upon as having a direct bearing only on the emission of watery vapour. There is a distinct advantage in the regulation of this escape, and the mechanism is directly connected with the greater or smaller quantity of water in the plant, and especially in its epidermal cells. This power of varying the area of the apertures by which gases enter the internal reservoirs is not advantageous to the gaseous interchanges—indeed it may be directly the reverse. It may lead to an incipient asphyxiation, as the supply of oxygen may be greatly interfered with and the escape of carbon dioxide may be almost stopped. It may at other times lead to great difficulties in the supply of the gaseous constituents which are used in the manufacture of food. The importance of transpiration, is, however, so great, that these risks must be run.
The Ascent of Water in Trees.—The supply of water to the peripheral protoplasts of a tree is consequently of the first importance. The means by which such a supply is ensured are by no means clearly understood, but many agencies are probably at work. The natural source of the water is in all cases the soil, and few plants normally obtain any from elsewhere. The water of the soil, which in well-drained soil is met with in the form of delicate films surrounding the particles of solid matter, is absorbed into the plant by the delicate hairs borne by the young roots, the entry being effected by a process of modified osmosis. Multitudes of such hairs on the branches of the roots cause the entry of great quantities of water, which by a subsequent similar osmotic action accumulates in the cortex of the roots. The great turgidity which is thus caused exerts a considerable hydrostatic pressure on the stele of the root, the vessels of the wood of which are sometimes filled with water, but at other times contain air, and this often under a pressure less than the ordinary atmospheric pressure. This pressure of the turgid cortex on the central stele is known as root pressure, and is of very considerable amount. This pressure leads to the filling of the vessels of the wood of both root and stem in the early part of the year, before the leaves have expanded, and gives rise to the exudation of fluid known as bleeding when young stems are cut in early spring.
Root pressure is one of the forces co-operating in the forcing of the water upwards. The evaporation which is associated with transpiration is no doubt another, but by themselves they are insufficient to explain the process of lifting water to the tops of tall trees. There is at present also a want of agreement among botanists as to the path which the water takes in the structural elements of the tree, two views being held. The older is that the water travels in the woody cell-walls of the vascular bundles, mainly under the action of the forces of root pressure and transpiration, and that the cavities of the vessels contain only air. The other is that the vessels are not empty, but that the water travels in their cavities, which contain columns of water in the course of which are large bubbles of air. On this view the water flows upwards under the influence of variations of pressure and tension in the vessels. These forces however fail to furnish a complete explanation of the ascent of the current, and others have been thought to supplement them, which have more or less weight. Westermaier and Godlewski put forward the view that the living cells of the medullary rays of the wood, by a species of osmosis, act as a kind of pumping apparatus, by the aid of which the water is lifted to the top of the tree, a series of pumping-stations being formed. Though this at first met with some acceptance, Strasburger showed that the action goes on in great lengths of stem the cells of which have been killed by poison or by the action of heat. More recently, Dixon and Joly in Dublin and Askenasy in Germany have suggested the action of another force. They have shown that columns of water of very small diameter can so resist tensile strain that they can be lifted bodily instead of flowing along the channel. They suggest that the forces causing the movement are complex, and draw particular attention to the pull upwards in consequence of disturbances in the leaves. In these we have (1) the evaporation from the damp delicate cell-walls into the intercellular spaces; (2) the imbibition by the cell-wall of water from the vacuole; (3) osmotic action, consequent upon the subsequent increased concentration of the cell sap, drawing water from the wood cells or vessels which abut upon the leaf parenchyma. They do not, of course, deny the co-operation of the other forces which have been suggested, except so far as these are inconsistent with the motion of the water in the form of separate columns rather than a flowing stream. This view requires the existence of certain anatomical arrangements to secure the isolation of the separate columns, and cannot be said to be fully established.
Nature of the Food of Plants.—The recognition of the fundamental identity of the living substance in animals and plants has directed attention to the manner in which plants are nourished, and especially to the exact nature of their food. The idea was till recently currently accepted, that anything which plants absorbed from without, and which went to build up their organic substance, or to supply them with energy, or to exert some beneficial influence upon their metabolism, constituted their food. Now, as the materials which plants absorb are carbon dioxide from the air, and various inorganic compounds from the soil, together with water, it is clear that if this view is correct, vegetable protoplasm must be fed in a very different way from animal, and on very different materials. A study of the whole vegetable kingdom, however, negatives the theory that the compounds absorbed are in the strict sense to be called food. Fungal and phanerogamic parasites can make no use of such substances as carbon dioxide, but draw elaborated products from the bodies of their hosts. Those Fungi which are saprophytic can only live when supplied with organic compounds of some complexity, which they derive from decomposing animal or vegetable matter. Even in the higher flowering plants, in which the processes of the absorption of substances from the environment has been most fully studied, there is a stage in their life in which the nutritive processes approximate very closely to those of the group last mentioned. When the young sporophyte first begins its independent life—when, that is, it exists in the form of the embryo in the seed—its living substance has no power of utilizing the simple inorganic compounds spoken of. Its nutritive pabulum is supplied to it in the shape of certain complex organic substances which have been stored in some part or other of the seed, sometimes even in its own tissues, by the parent plant from which it springs. When the tuber of a potato begins to germinate the shoots which it puts out derive their food from the accumulated store of nutritive material which has been laid up in the cells of the tuber. If we examine the seat of active growth in a young root or twig, we find that the cells in which the organic substance, the protoplasm, of the plant is being formed and increased, are not supplied with carbon dioxide and mineral matter, but with such elaborated material as sugar and proteid substances, or others closely allied to them.
Identity of the Food of Animals and Plants.—It is evidently to the actual seats of consumption of food, and of consequent nutrition and increase of living substance, that we should turn when we wish to inquire what are the nutritive materials of plants. If we go back to the first instance cited, the embryo in the seed and its development during germination, we can ascertain what is necessary for its life by inquiring what are the materials which are deposited in the seed, and which become exhausted by consumption as growth and development proceed. We find them to consist of representatives of the great classes of foodstuffs on which animal protoplasm is nourished, and whose presence renders seeds such valuable material for animal consumption. The are mainly carbohydrates such as starch and sugar, proteids in the form of globulin's or albumoses, and in many cases fats and oils, while certain other bodies of similar nutritive value are less widely distributed.
The differences between the nutritive processes of the animal and the plant are not therefore fundamental, as they were formerly held to be. The general vegetable protoplasm has not the capacity of being nourished by inorganic substances which are denied to the living substance of the animal world. Differences connected with the mode of supply of nutritive material do exist, but they are mainly correlated with the structure of the organisms, which makes the method of absorption different. The cell-walls of plants render the entry of solid material into the organism impossible. The food must enter in solution in order to pass the walls. Moreover, the stationary habit of plants, and the almost total absence of locomotion, makes it impossible for them to seek their food.
The Special Apparatus of Plants for constructing Food.—The explanation of the apparent difference of food supply is very simple. Plants are furnished with a constructive mechanism by which they are enabled to fabricate the food on which they live from the inorganic, gaseous and liquid matters which they absorb. The fact of such absorption does not render these substances food; they are taken in not as food, but as raw materials to be subjected to the action of this constructive mechanism, and by it to be converted into substances that can nourish protoplasm, both vegetable and animal. It is sometimes forgotten, when discussing questions of animal nutrition, that all the food materials of all living organisms are prepared originally from inorganic substances in exactly the same way, in exactly the same place, and by the same machinery, which is the chlorophyll apparatus of the vegetable kingdom. A consideration of these facts emphasizes still more fully the view with which we set out, that all living substance is fundamentally the same, though differentiated both anatomically and physiologic all in many directions and in different degrees. All is nourished alike on materials originally prepared by a mechanism attached to the higher vegetable organism, and capable of being dissociated, in theory at least, from its own special means of nutrition, if by the latter term we understand the appropriation by the protoplasm of the materials so constructed.
The chlorophyll apparatus of plants demands a certain description. It consists essentially of a number of minute corpuscles or plastids, the protoplasmic substance of which is impregnated with a green colouring matter. These bodies, known technically as chloroplasts, are found embedded in the protoplasm of the cells of the mesophyll of foliage leaves, of certain of the cells of some of the leaves of the flower, and of the cortex of the young twigs and petioles. Usually they are absent from the cells of the epidermis, though in some of the lower plants they are met with there also. The plastids are not rigidly embedded in the cytoplasm, but are capable of a certain amount of movement therein. Each is a small protoplasmic body, in the meshes of whose substance the green colouring matter chlorophyll is contained in some form of solution. Various solvents, such as benzene, alcohol and chloroform, will dissolve out the pigment, leaving the plastid colourless. Chlorophyll is not soluble in water, nor in acids or alkalies without decomposition.
These plastids are especially charged with the duty of manufacturing carbohydrates from the carbon dioxide which the air contains, and which is absorbed from it after it has entered the intercellular passages and has so reached the cells containing the plastids. This action is found to take place only in the presence of light, preferably moderate sunlight. The reason for the distribution of the chloroplasts described above is consequently seen. The relation of the chlorophyll to light has been studied by many observers. If a solution of the pigment is placed in the path of a beam of light which is then allowed to fall on a prism, the resulting spectrum will be found to be modified. Instead of presenting the appearance of a continuous band in which all the colours are represented, it is interrupted by seven vertical dark spaces. The rays which in the absence of the solution of chlorophyll would have occupied those spaces have no power to pass through it, or in other words chlorophyll absorbs those particular rays of light which are missing.
The absorption of these rays implies that the pigment absorbs radiant energy from the sun, and gives us some explanation of its power of constructing the carbohydrates which has been mentioned as the special work of the apparatus. The working of it is not at all completely understood at present, nor can we say exactly what is the part played by the pigment and what is the rôle of the protoplasm of the plastid. It is not certain either whether the action of the chlorophyll apparatus is confined to the manufacture of carbohydrates or whether it is concerned, and if so how far, with the construction of proteids also.
As the action of the chlorophyll apparatus is directly dependent upon light, and the immediate result of its activity is the building up of complex compounds, it has become usual to speak of the processes it sets up under the name of photosynthesis.
Photosynthesis.—In the presence of light and when the plant is subjected to a suitable temperature, photosynthesis commences, provided that the plant has access to air containing its normal amount of carbon dioxide, about 3 parts, or rather less, in 10,000. The process involves the inter-action of water also, and this, as we have seen, is always present in the cell. In addition, certain inorganic salts, particularly certain compounds of potassium, are apparently necessary, but they seem to take no part in the chemical changes which take place. The original hypothesis of Baeyer suggested that the course of events is the following: the carbon dioxide is decomposed into carbon monoxide and oxygen, while water is simultaneously split up into hydrogen and oxygen; the hydrogen and the carbon monoxide unite to form formaldehyde and the oxygen is exhaled. This explanation is unsatisfactory from many points of view, but till quite recently no acceptable alternative has been advanced. There is no evidence that carbon monoxide is ever produced, indeed there are strong reasons for disbelieving in its occurrence. The formation of formaldehyde has till recently not been satisfactorily proved, though it has been obtained from certain leaves by distillation. Certain Algae have been found capable of forming nutritive carbohydrates in darkness, when supplied with a compound of this body with sodium-hydrogen-sulphite. But it is certain that it can only be present in a cell in very small amount at any moment, for an extremely dilute solution acts as a poison to protoplasm. If formed, as it probably is, it is immediately changed into some more complex combination, and so rendered incapable of exerting its poisonous action.
Baeyer's hypothesis was entertained by botanists partly because it explained the gaseous interchanges accompanying photosynthesis. These show that a definite intake of carbon dioxide is always accompanied by an exhalation of an equal volume of oxygen.
Recent investigations have confirmed Baeyer's view of the formation of formaldehyde, but a different explanation has been recently advanced. The first chemical change suggested is an interaction between carbon dioxide and water, under the influence of light acting through chlorophyll, which leads to the simultaneous formation of formaldehyde and hydrogen peroxide. The formaldehyde at once undergoes a process of condensation or polymerization by the protoplasm of the plastid, while the hydrogen peroxide is said to be decomposed into water and free oxygen by another agency in the cell, of the nature of one of the enzymes of which we shall speak later.
Polymerization of the aldehyde was also a feature of Baeyer's hypothesis, so that this view does not very materially differ from those he advanced. More emphasis is, however, now laid on the action of the plastid in polymerization, while the initial stages are still not definitely explained.
The steps which lead from the appearance of formaldehyde to that of the first well-defined carbohydrate are again matters of speculation. There are many possibilities, but no definite body of simpler composition than a sugar has so far been detected. Nor is the nature of the first formed sugar certain; the general opinion has been that it is a simple hexose such as glucose or fructose, C6H12O6. Brown and Morris in 1892 advanced strong reasons for thinking that cane-sugar, C12H22O11, is the first carbohydrate synthesized, and that the hexoses found in the plant result from the decomposition of this. The whole story of the different sugars existing in the plant—their relations and their several functions—requires renewed investigation.
The first visible carbohydrate formed, one which appears so rapidly on the commencement of photosynthesis as to have been regarded as the first evidence of the setting up of the process, is starch. This is met with in the form of small granular specks in the substance of the chloroplast, specks which assume a blue colour when treated with a solution of iodine. Its very prompt appearance, as soon as the apparatus became active, led to the opinion formerly held, that the work of the latter was complete only when the starch was formed. We have seen that the starch is preceded by the formation of sugar, and its appearance is now interpreted as a sign of surplus manufacture. As much sugar as is produced in excess of the immediate requirements of the cell is converted into the insoluble form of starch by the plastids of the chlorophyll apparatus, and is so withdrawn from the sphere of action, thereby enabling the construction of further quantities of sugar to take place. The presence of too much sugar in solution in the sap of the cell inhibits the activity of the chloroplasts; hence the necessity for its removal. Starch, indeed, wherever it appears in the plant seems to be a reserve store of carbohydrate material, deposited where it is found for longer or shorter periods till it is needed for consumption. The readiness with which it is converted into sugar fits it especially to be a reserve or stored material.
Proteid Formation.—We have seen that it has been suggested that the chlorophyll apparatus may perhaps be concerned in the manufacture of proteids as well as of carbohydrates. If not, there must exist in the green plant, side by side with it, another mechanism which is concerned with the manufacture of the complex compounds in which nitrogen is present The independence of the two is suggested by the fact that fungi can live, thrive and grow in nutritive media which contain carbohydrates together with certain salts of ammonia, but which are free from proteids. It is certain that their protoplasm cannot be nourished by inorganic compounds of nitrogen, any more than that of animals. We must therefore surmise their possession of a mechanism which can construct proteids, if supplied with these compounds of nitrogen together with sugar.
The probability is that this mechanism is to be found in green plants in the leaves—at any rate there is a certain body of evidence pointing in this direction. It may be, however, that there is no special mechanism, but that this power is a particular differentiation of a physiological kind, existing in all vegetable protoplasm, or in that of certain cells. The idea of an identity of protoplasm does not involve a denial of special powers developed in it in different situations, and the possession of such a power by the vegetable cell is not more striking than the location of the powers of co-ordination and thought in the protoplasm of cells of the human brain.
But if we accept either view we have still to examine the process of construction in detail, with a view to ascertaining the stages by which proteid is built up. Here unfortunately we find ourselves in the region of speculation and hypothesis rather than in that of fact. The nitrogen is absorbed by the plant in some form of combination from the soil. The green plant prefers as a rule nitrates of various metals, such as calcium, magnesium or potassium. The fungus seems to do better when supplied with compounds of ammonia. The nitrogen of the atmosphere is not called into requisition, except by a few plants and under special conditions, as will be explained later. The fate of these inorganic compounds has not been certainly traced, but they give rise later on to the presence in the plant of various amino acid amides, such as leucin, glycin, asparagin, &c. That these are stages on the way to proteids as been inferred from the fact that when proteids are split up by various means, and especially by the digestive secretions, these nitrogen-containing acids are among the products which result.
While we know little of the processes of proteid-construction, we are almost completely in the dark also as to what are the particular proteids which are first constructed.
Opinions are conflicting also as to the conditions under which proteids are formed. There is a certain amount of evidence that at any rate in some cases light is necessary, and that the violet rays of the spectrum are chiefly concerned. But the subject requires elucidation from both chemical and biological points of view.
The normal green plant is seen thus to be in possession of a complete machinery for the manufacture of its own flood. The way in which such food when manufactured is incorporated into, and enabled to build up, the living substance is again hidden in obscurity. This is, however, also the case with the nutrition of animal protoplasm.
The building up and nutrition of the living substance by the foods manufactured or absorbed is properly spoken of as the assimilation of such food. Up to very recently the original absorption and subsequent treatment of the carbon dioxide and the compounds of nitrogen has been called by the same term. We frequently find the expression used, “the ‘assimilation’ of carbon dioxide, or of nitrogen.” As this is not the incorporation of either into the living substance, but is only its manufacture into the complex substances which we find in the plant, it seems preferable to limit the term “assimilation” to the processes by which foods are actually taken into the protoplasm.
Symbiosis.—Though green plants thus possess a very complete mechanism for the manufacture of their different foodstuffs, it is not always exercised to the fullest extent. Many of them are known to supplement it, and some almost entirely to replace it, by absorbing the food they need in a fully prepared condition from their environment. It may be that they procure it from decomposing organic matter in the soil, or they may get it by absorption from other plants to which they attach themselves, or they may in rare cases obtain it by preying upon insect life. The power of green plants, not even specialized in any of these directions, to absorb certain carbohydrates, particularly sugars, from the soil was demonstrated by Acton in 1889. Similar observat1ons have been made in the case of various compounds of nitrogen, though these have not been so complex as the proteids. It was formerly the custom to regard as parasites all these plants which inserted roots or root-like organs into the tissues of other plants and absorbed the contents of the latter. The most conspicuous case, perhaps, of all these is the mistletoe, which flourishes luxuriantly upon the apple, the poplar and other trees. Bonnier has drawn attention to the fact that the mistletoe in its turn, remaining green in the winter, contributes food material to its host when the latter has lost its leaves. The relationship thus existing he showed to be mutually beneficial, each at one time or another supplying the necessities of the other. Such a relationship is known as symbiosis, and the large majority of the cases of so called parasitism among green plants can be referred to it. Bonnier showed that the same relationship could be proved in the cases of such plants as the rattle (Rhinanthus), the eye-bright (Euphrasia), and other members of the Natural Orders, Scrophulariaceae and Santalaceae, which effect a union between their roots and the roots of other plants growing near them. The union taking place underground, while the bulk of both partners in the symbiosis rises into the air, renders the association a little difficult to see, but there is no doubt that the plants in question do afford each other assistance, forming, as it were, a kind of partnership. The most pronounced case of parasitism, that of Cuscuta, the dodder, which infests particularly clover fields, appears to differ only in degree from those mentioned, for the plant, bare of leaves as it is, yet contains a little chlorophyll. The advantages it can offer to its host are, however, infinitesimal when compared with the injury it does
it. Many other cases of symbiosis have been investigated with some completeness, especially those in which lower plants than the Phanerogams are concerned. The relations of the Alga and the Fungus, which have formed a close associationship in the structure known as the Lichen, were established many years ago. Since about 1880 our knowledge of the species which can enter into such relationships has been materially extended, and the fungal constituents of the Lichens are known to include Basidiomycetes as well as Ascomycetes.
Mycorhizas.—The most interesting cases, however, in which Fungi form symbiotic relationships with green plants have been discovered in connexion with forest trees. The roots of many of the latter, while growing freely in the soil are found to be surrounded with a dense feltwork of fungal mycelium, which sometimes forms a mass of considerable size. The plants showing it are not all forest trees, but include also some Pteridophytes and some of the prothallia of the Ferns, Club-mosses, Liverworts and Horsetails. The true nature of the relationship was first recognized by Pfeffer in 1877, but few cases were known till recent years. Very complete examination, however, has now been made of many instances, and the name mycorhiza has been given to the symbiotic union. Two classes are recognized. In the first, which are called ectotropic, the fungal filaments form a thick felt or sheath round the root, either completely enclosing it or leaving the apex free. They seldom penetrate the living cells, though they do so in a few cases. The root-hairs penetrate between masses of the hyphae of the Fungus. This type of mycorhiza is found among the Poplars, Oaks and Fir trees. The other type is called endotropic. The fungal filaments either penetrate the epidermis of the root, or enter it from the stem and ramify in the interior. Some make their way through the cells of the outer part of the cortex towards the root-tip, and form a mycelium or feltwork of hyphae, which generally occupies two or three layers of cells. From this branches pass into the middle region of the cortex and ramify through the interior half of its cells. They often cause a considerable hypertrophy of the tissue. From the outer cortical mycelium, again, branches pass through the epidermis and grow out in the soil. In such cases the roots of the plants are usually found spreading in soils which contain a large amount of humus, or decaying vegetable matter. The organic compounds of the latter are absorbed by the protruding fungal filaments, which take the place of root-hairs, the tree ceasing to develop the latter. The food so absorbed passes to the outer cortical mycelium, and from this to the inner hyphae, which appear to be the organs of the interchange of substance, for they are attracted to the neighbourhood of the nuclei of the cells, which they enter, and in which they form agglomerations of interwoven filaments. The prothalli of the Pteridophytes, which form similar symbioses, show a somewhat different mode of arrangement, the Fungi occupying the external or the lower layers of the thalloid body.
The discovery of the widespread occurrence of this mycorhizal symbiosis must be held to be one of the most important results of research upon the nutritive processes of plants during the closing decade of the 19th century. Among green plants the symbionts include Conifers, Orchids, Heaths, Oaks, Poplars and Beeches, though all do not derive equal advantages from the association. Monotropas afford an extreme case of it, having lost their chlorophyll almost entirely, and come to depend upon the Fungi for their nutriment. The fungal constituents vary considerably. Each species of green plant may form a mycorhiza with two or three different Fungi, and a single species of Fungus may enter into symbiosis with several green plants. The Fungi that have been discovered taking part in the union include Eurotium, Pythium, Boletus, Agaricus, Lactarius, Penicillium and many others of less frequent occurrence. All the known species belong to the Oomycetes, the Pyrenomycetes, the Hymenomycetes or the Gasteromycetes. The habit of forming mycorhizas is found more frequently in warm climates than cold; indeed, the percentage of the flora exhibiting this peculiarity seems to increase with a certain regularity from the Arctic Circle to the equator.
Fixation of Nitrogen.—Another, and perhaps an even more important, instance of symbiotic association has come to the front during the same period. It is an alliance between the plants of the Natural Order Leguminosae and certain bacterium-like forms which find a home within the tissues of their roots. The importance of the symbiosis can only be understood by considering the relationship in which plants stand with regard to the free nitrogen of the air. Long ago the view that this gas might be the source of the combined nitrogen found in different forms within the plant, was critically examined, particularly by Boussingault, and later by Lawes and Gilbert and by Pugh, and it was ascertained to be erroneous, the plants only taking nitrogen into their substance when it is presented to their roots in the form of nitrates of various metals, or compounds of ammonia. Many writers in recent years, among whom may be named especially Hellriegel and Wilfarth, Lawes and Gilbert, and Schlœsing and Laurent, have shown that the Leguminosae as a group form conspicuous exceptions to this rule. While they are quite capable of taking up nitrates from the soil where and so long as these are present, they can grow and thrive in soil which contains no combined nitrogen at all, deriving their supplies of this element in these cases from the air. The phenomena have been the subject of very careful and critical examination for many years, and may be regarded as satisfactorily established. The power of fixing atmospheric nitrogen by the higher plants seems to be confined to this solitary group, though it has been stated by various observers with more or less emphasis that it is shared by others. Frank has claimed to have found oats, buckbeans, spurry, turnips, mustard, potatoes and Norway maples exercising it; Nobbe and others have imputed its possession to Elaeagnus. There is little direct evidence pointing to this extension of the power, and many experimenters directly contradict the statements of Frank.
The power exercised by the Leguminosae is associated with the presence of curious tubercular swellings upon their roots, which are developed at a very early age, as they are cultivated in ordinary soil. If experimental plants are grown in sterilized soil, these swellings do not appear, and the plant can then use no atmospheric nitrogen. The swellings have been found to be due to a curious hypertrophy of the tissue of the part, the cells being filled with an immense number of minute bacterium-like organisms of V, X or Y shape. The development of these structures has been studied by many observers, both in England and on the continent of Europe. They appear to be present in large numbers in the soil, and to infect the Leguminous pant by attacking its root-hairs. One of these hairs can be seen to be penetrated at a particular spot, and the entering body is then found to grow along the length of the hair till it reaches the cortex of the root. It has the appearance of a delicate tube which has granular contents, and is provided with an apex that appears to be open. The wall of the tube is very thin and delicate, and does not seem to be composed of cellulose or any modification of it. Careful staining shows that the granular substance of the interior really consists of a large number of delicate rod-like bodies. As the tube grows down the hair it maintains its own independence, and does not fuse with the contents of the root-hair, whose protoplasm remains quite distinct and separate. After making its way into the interior, the intruder sets up a considerable hypertrophy of the tissue, causing the formation of a tubercle, which soon shows a certain differentiation, branches of the vascular bundles of the root being supplied to it. The rod-like bodies from the interior of the tube, which has considerable resemblance to the zoogloea of many Bacteria, are liberated into the interior of the cells of the tubercle and fill it, increasing by a process of branching and fission. When this stage is reached the invading tubes and their ramifications frequently disappear, leaving the cells full of the bacterioids, as they have been called. When the root dies later such of these as remain are discharged into the soil, and are then ready to infect new plants. In some cases the zoogloea thread or tube has not been seen, the organism consisting entirely of the bacterioids.
This peculiar relationship suggests at once a symbiosis, the Fungus gaining its nutriment mainly or entirely from the green plant, while the latter in some way or other is able to utilize the free nitrogen of the air. The exact way in which the utilization or fixation of the nitrogen is effected remains undecided. Two views are still receiving certain support, though the second of them appears the more probable. These are: (1) That the green plant is so stimulated by the symbiotic association which leads to the hypertrophy, that it is able to fix the nitrogen or cause it to enter into combination. (2) That the fixation of the gas is carried out by the fungal organism either in the soil or in the plant, and the nitrogenous substance so produced is absorbed by the organism, which is in turn consumed by the green plant. Certain evidence which supports this view will be referred to later.
Whichever opinion is held on this point, there seems no room for doubt that the fixation of the nitrogen is concerned only with the root, and that the green leaves take no part in it. The nodules, in particular, appear to play the important part in the process. Marshall Ward has directed attention to several points of their structure which bear out this view. They are supplied with a regular system of conducting vascular bundles communicating with those of the roots. Their cells during the period of incubation of the symbiotic organism are abundantly supplied with starch. The cells in which the fungoid organism is vigorously flourishing are exceedingly active, showing large size, brilliant nuclei, protoplasm and vacuole, all of which give signs of intense metabolic activity. The sap in these active tissues is alkaline, which has been interpreted as being in accordance with Lœw's suggestion that the living protoplasm in presence of an alkali and free nitrogen can build up ammonium nitrate, or some similar body. It is, however, at present entirely unknown what substances are formed at the expense of the atmospheric nitrogen.
The idea that the atmospheric nitrogen is gradually being made use of by plants, although it is clearly not easily or commonly utilized, has been growing steadily. Besides the phenomena of the symbiosis just discussed, certain experiments tend to show that we have a constant fixation of this gas in the soil by various Bacteria. Researches which have been carried out since 1885 by Berthelot, Andrée, Laurent and Schlœsing, and more recently by Kossowitsch, seem to establish the fact, though the details of the process remain undiscovered. Berthelot imputes it to the action of several species of soil Bacteria and Fungi, including the Bacterium of the Leguminosae, when the latter is cultivated free from its ordinary host. Laurent and Schlœsing affirm that the free nitrogen of the air can be fixed by a number of humble green plants, principally lowly green Algae. They must be exposed freely to light and air during the process, or they fail to effect it. Frank has stated that Penicillium cladiosporioides can flourish in a medium to which no nitrogen but that of the atmosphere has access. Kossowitsch claims to have proved that fixation of nitrogen takes place under the influence of a symbiosis of certain Algae and soil Bacteria, the process being much facilitated by the presence of sugar. The Algae include Nostoc, Cystococcus, Cylindrospermum and a few other forms. In the symbiosis the Algae are supplied with nitrogen by the bacteria, and in turn they construct carbohydrate material, art of which goes to the microbes. This is supported by the fact that if the mixed culture is placed in the light there is a greater fixation than when it is left in darkness. If there is a plentiful supply of carbon dioxide, more nitrogen is fixed.
Nitrification and Denitrification in the Soil.—Another aspect of the nitrogen question has been the subject of much investigation and controversy since 1877. The round of changes which nitrogenous organic matter undergoes in the soil, and how it is ultimately made use of again by plants, presents some curious features. We have seen that when nitrogenous matter is present in the condition of humus, some plants can absorb it by their roots or by the aid of mycorhizas. But the changes in it in the usual course of nature are much more profound than these. It becomes in the soil the prey of various microbes. Ammonia appears immediately as a product of the disruption of the nitrogen-containing organic molecule. Later, oxidation processes take place, and the ammonia gives rise to nitrates which are absorbed by plants. These two processes go on successively rather than simultaneously, so that it is only towards the end of the decomposition of the organic matter that nitrification of the ammonia which is formed is set up. In this process of nitrification we can distinguish two phases, first the formation of nitrites, and secondly their oxidation to nitrates. The researches of Warington in England and Winogradsky on the Continent have satisfactorily shown that two distinct organisms are concerned in it, and that probably more than one species of each exists. One of them comprising the genera Nitrosomonas and Nitrosococcus, has the power of oxidizing salts of ammonium to the condition of compounds of nitrous acid. When in a pure culture this stage has been reached no further oxidation takes place. The oxidation of the nitrites into nitrates is effected by another organism, much smaller than the first. The name Nitrobacter has been given to this genus, most of our knowledge of which is due to the researches of Winogradsky.
The two kinds of organism are usually both present in the same soil, those of the second type immediately oxidizing the nitrites which those of the first form from ammonium salts. The Nitrobacter forms not only cannot oxidize the latter bodies, but they are very injuriously affected by the presence of free ammonia. When cultivated upon a suitable nutritive material in the laboratory, the organism was killed by the presence of .015% of this gas, and seriously inconvenienced by one-third as much. Except in this respect, however, the two classes show great similarity. A very interesting peculiarity attaching to them is their distaste for organic nutriment. They can be cultivated most readily on masses of gelatinous silica impregnated with the appropriate compounds of nitrogen, and their growth takes place most copiously in the absence of light. They need a little carbonate in the nutrient material, and the source of the carbon which is found in the increased bulk of the plant is partly that and partly the carbon dioxide of the air.
We have in these plants a power which appears special to them, in the possession of some mechanism for the construction of organic substance which differs essentially from the chlorophyll apparatus of green plants, and yet brings about substantially similar results. The steps by which this carbon dioxide is built up into a compound capable of being assimilated by the protoplasm of the cells are not known. The energy for the purpose appears to be supplied by the oxidation of the molecules containing nitrogen, so that it is dependent upon such oxidation taking place. Winogradsky has investigated this point with great care, and he has come to the conclusion that about 35 milligrammes of nitrogen are oxidized for each milligramme of carbon absorbed and fixed.
Deposition and Digestion of Reserve Materials in Plants and Animals.—As we have seen, the tendency of recent research is to prove the identity of the mode of nutrition of vegetable and animal organisms. The material on which they feed is of the same description and its treatment in the body is precisely similar. In both groups we find the presence of nutritive material in two forms, one specially fitted for transport, the other for storage. We have seen that in the plant the processes of construction go on in the seats of manufacture faster than those of consumption. We have the surplus sugar, for instance, deposited as starch in the chloroplasts themselves. The manufacture goes on very actively so long as light shines upon the leaves, and we find towards night a very great surplus stored in the cells. This excess of manufacture is one of the features of plant life, and is exhibited, though in various degrees, by all green plants. The accumulated material is made to minister to the need of the plant in various ways; it may be by increasing the bulk of the plant, as by the formation of the wood of the trunk, branches and roots; or it may be by laying up a store of nutritive materials for purposes of propagation, as in tubers, corms, seeds, &c. In any case the surplus is continuously being removed from the seats of its construction and deposited for longer or shorter periods in other parts of the structure, usually near the regions at which its ultimate consumption will take place. We have the deposition of starch, aleurone grains, amorphous proteids, fats, &c., in the neighbourhood of growing points, cambium rings and phellogens; also the more prolonged storage in tubers, seeds and other reproductive bodies. Turning to the animal, we meet with similar provisions in the storage of glycogen in the liver and other parts, of fat in various internal regions, and so on. In both we find the reserve of food, so far as it is in excess of immediate need, existing in two conditions, the one suitable for transport, the other for storage, and we see continually the transformation of the one into the other. The formation of the storage form at the expense of the travelling stream is due to the activity of some protoplasmic structure—it may be a plastid or the general protoplasm of the cell—and is a process of secretion. The converse process is one of a true digestion, which deserves the name no less because it is intracellular. We find processes of digestion strictly comparable to those of the alimentary canal of an animal in the case of the insectivorous Nepenthes, Drosera and other similar plants, and in the saprophytic Fungi. Those which now concern us recall the utilization of the glycogen of the liver, the stored fats and proteids of other parts of the animal body being like them intracellular.
Enzymes.—The agents which effect the digestive changes in plants have been studied with much care. They have been found to be mainly enzymes, which are in many cases identical with those of animal origin. A vast number of them have been discovered and investigated, and the majority call for a brief notice. Their number, indeed, renders it necessary to classify them, and rather to look at groups of them than to examine them one by one. They are usually classified according to the materials on which they work, and we may here notice es especially four principal groups, the members of which take part in the digestion of reserve materials as well as in the processes of external digestion. These decompose respectively carbohydrates, glucosides, proteids and fats or oils. The action of the enzyme in nearly every case is one of hydration, the body acted on being made to take up water and to undergo a subsequent decomposition.
Among those which act on carbohydrates the most important are: the two varieties of diastase, which convert starch into maltose or malt sugar; inulase, which forms fructose from inulin; invertase, which converts cane sugar into glucose (grape sugar) and fructose; glucase or maltase, which produces grape sugar from maltose; and cytase, which hydrolyses cellulose. Another enzyme which does not appear to be concerned with digestion so directly as the others is pectase, which forms vegetable jelly from pectic substances occurring in the cell-wall.
The enzymes which act upon glucosides are many; the best known are emulsin and myrosin, which split up respectively amygdalin, the special glucoside of certain plants of the Rosaceae; and sinigrin, which has a wide distribution among those of the Cruciferae. Others of less frequent occurrence are erythrozym, rhamnase and gaultherase.
The proteolytic enzymes, or those which digest proteids, are usually divided into two groups, one which breaks down ordinary proteids into diffusible bodies, known as peptones, which are themselves proteid in character. Such an enzyme is the pepsin of the stomach of the higher animals. The other group attacks these peptones and breaks them down into the amino-acids of which we have spoken before. This group is represented by the erepsin of the pancreas and other organs. A third enzyme, the trypsin of the pancreas, possesses the power of both pepsin and erepsin. The relationships existing between these enzymes are still the subjects of experiment, and we cannot regard them as exhaustively examined. It is not quite certain whether a true pepsin exists in plants, but many trypsins have been discovered, and one form of erepsin, at least, is very widespread. Among the trypsins we have the papäin of the Papaw fruit (Carica Papaya), the bromelin of the Pine-apple, and the enzymes present in many germinating seeds, in the seedlings of several plants, and in other parts. Another enzyme, rennet, which in the animal body is proteolytic, is frequently met with in plants, but its function has not been ascertained.
The digestion of fat or oil has not been adequately investigated, but its decomposition in germinating seeds has been found to be due to an enzyme, which has been called lipase. It splits it into a fatty acid and glycerin, but seems to have no further action. The details of the further transformations have not yet been completely followed.
Oxidases.—Another class of enzymes has been discovered in both animals and plants, but they do not apparently take any part in digestion. They set up a process of oxidation in the substances which they attack, and have consequently been named oxidases. Very little is known about them.
In many cases the digestion of reserve food materials is effected by the direct action of the protoplasm, without the intervention of enzymes. This property of living substance can be proved in the case of the cells of the higher plants, but it is especially prominent in many of the more lowly organisms, such as the Bacteria. The processes of putrefaction may be alluded to as affording an instance of such a power in the vegetable organisms. At the same time it must be remembered that the secretion of enzymes by Bacteria is of widespread occurrence.
Supply and Distribution of Energy in Plants.—It is well known that one of the conditions of life is the maintenance of the process which is known as respiration. It is marked by the constant and continuous absorption of a certain quantity of oxygen and by the exhalation of a certain volume of carbon dioxide and water vapour. There is no direct connexion between the two, the oxygen is absorbed almost immediately by the protoplasm, and appears to enter into some kind of chemical union with it. The protoplasm is in a condition of instability and is continually breaking down to a certain extent, giving rise to various substances of different degrees of complexity, some of which are again built up by it into its own substances, and others, more simple in composition, are given off. Of these carbon dioxide and water are the most prominent. These respiratory processes are associated with the liberation of energy by the protoplasm, energy which it applies to various purposes. The assimilation of complex foods consequently may be regarded as supplying the protoplasm with a potential store of energy, as well as building up its substance. Whenever complex bodies are built up from simple ones we have an absorption of energy in some form and its conversion into potential energy; whenever decomposition of complex bodies into simpler ones takes place we have the liberation of some or all of the energy that was used in their construction.
Since about 1880 considerable attention has been directed to the question of the supply, distribution and expenditure of energy in the vegetable kingdom. This is an extremely important question, since the supply of energy to the animal world has been found to depend entirely upon the vegetable one. The supply of energy to the several protoplasts which make up the body of a plant is as necessary as is the transport to them of the food they need; indeed, the two things are inseparably connected. The source of energy which is the only one accessible to the ordinary plant as a whole is the radiant energy of the rays of the sun, and its absorption is mainly due to the properties of chlorophyll. This colouring matter, as shown by its absorption spectrum, picks out of the ordinary beam of light a large proportion of its red and blue rays, together with some of the green and yellow. This energy is obtained especially by the chloroplastids, and part of it is at once devoted to the construction of carbohydrate material, being thus turned from the kinetic to the potential condition. The other constructive processes, which are dependent partly upon the oxidation of the carbohydrates so formed, and therefore upon an expenditure of part of such energy, also mark the storage of energy in the potential form. Indeed, the construction of protoplasm itself indicates the same thing. Thus even in these constructive processes there occurs a constant passage of energy backwards and forwards from the kinetic to the potential condition and vice versa. The outcome of the whole round of changes, however, is the fixation of a certain part of the radiant energy absorbed by the chlorophyll. The rays of the visible spectrum do not supply all the energy which the plant obtains. It has been suggested by several botanists, with considerable plaus1b1l1ty, that the ultra-violet or chemical rays can be absorbed and utilized by the protoplasm without the intervention of any pigment such as chlorophyll. There is some evidence pointing to the existence of this power in the cells of the higher plants. Again, we have evidence of the power of plants to avail themselves of the heat rays. There is, no doubt, a direct interchange of heat between the plant and the air, which in many cases results in a gain of heat by the plant. Indeed, the tendency to absorb heat in this way, either from the air or directly from the sunlight, has already been pointed out as a danger which needs to be averted by transpiration.
There is probably but little transformation of one form of kinetic energy into another in the plant. It has been suggested that the red pigment Anthocyan, which is found very commonly in young developing shoots, petioles and midribs, effects a conversion of light rays into heating ones, so facilitating the metabolic processes of the plant. This is, however, rather a matter of speculation. The various electrical phenomena of plants also are obscure.
Certain plants possess another source of energy which is common to them and the animal world. This is the absorption of elaborated compounds from their environment, by whose decomposition the potential energy expended in their construction can be liberated. Such a source is commonly met with among the Fungi, the insectivorous plants, and such of the higher plants as have a saprophytic habit. This source is not, however, anything new, for the elaborated compounds so absorbed have been primarily constructed by other plants through the mechanism which has just been described.
The question of the distribution of this stored energy to the separate protoplasts of the plant can be seen to be the same problem as the distribution of the food. The material and the energy go together, the decomposition of the one in the cell setting free the other, which is used at once in the vital processes of the cell, being in fact largely employed in constructing protoplasm or storing various products. The actual liberation in any cell is only very gradual, and generally takes the form of heat. The metabolic changes in the cells, however, concern other decompositions side by side with those which involve the building up of protoplasm from the products of which it feeds. So long as food is supplied the living substance is the seat of transformations which are continually proceeding, being partially decomposed and again constructed, the new food being incorporated into it. The changes involve a continual liberation of energy, which in most cases is caused by the respiration of the protoplasm and the oxidation of the substances it contains. The need of the protoplasm for oxygen has already been spoken of: in its absence death soon supervenes, respiration being stopped. Respiration, indeed, is the expression of the liberation of the potential energy of the protoplasm itself. It is not certain how far substances in the protoplasm are directly oxidized without entering into the composition of the living substance, though this appears to take place. Even their oxidation, however, is effected by the protoplasm acting as an oxygen carrier.
The supply of oxygen to a plant is thus seen to be as directly connected with the utilization of the energy of a cell as is that of food concerned in its nutrition. If the access of oxygen to a protoplast is interfered with its normal respiration soon ceases, but frequently other changes supervene. The partial asphyxiation or suffocation stimulates the protoplasm to set up a new and perhaps supplementary series of decompositions, which result in the liberation of energy just as do those of the respiratory process. One of the constant features of respiration—the exhalation of carbon dioxide—can still be observed. This comes in almost all such cases from the decomposition of sugar, which is split up by the protoplasm into alcohol and carbon dioxide. Such decompositions are now generally spoken of as anaërobic respiration. The decomposition of the complex molecule of the sugar liberates a certain amount of energy, as can be seen from the study of the fermentation set up by yeast, which is a process of this kind, in that it is intensified by the absence of oxygen. The liberated energy takes the form of heat, which raises the temperature of the fermenting wort. It has been ascertained that in many cases this decomposition is effected by the secretion of an enzyme, which has been termed zymase. This body has been prepared from active yeast, and from fruits and other parts which have been kept for some time in the absence of oxygen. The protoplasm appears to be able also to bring about the change without secreting any enzyme.
Expenditure of Energy by Plants.—The energy of the plant is, as we have seen, derived originally from the kinetic radiant energy of the sun. In such cells as are capable of absorbing it, by virtue of their chlorophyll apparatus, the greater part of it is converted into the potential form, and by the transport from cell to cell of the compounds constructed every part of the plant is put into possession of the energy it needs. The store of energy thus accumulated and distributed has to subserve various purposes in the economy of the plant. A certain part of it is devoted to the maintenance of the framework of the fabric of the cell, and the construction of a continuously increasing skeleton; part is used in maintaining the normal temperature of the plant, part in constructing various substances which are met with in the interior, which serve various purposes in the working of the vital mechanism. A great part again is utilized in that increase of the body of the plant which we call growth.
Growth, as usually spoken of, includes two essentially different processes. The first of these, which may be regarded as growth proper, is the manufacture of additional quantities of living substance. The second, which is usually included in the term, is the increase of such accessories of living substance as are necessary for its well-being. These include cell walls and the various stored products found in growing cells. There is clearly a difference between these two categories. The formation of living substance is a process of building up from simple or relatively simple materials; the construction of its cellulose framework and supporting substance is done by the living substance after its own formation is completed, and is attended by a partial decomposition of such living substance.
Growth is always going on in plants while they are alive. Even the oldest trees put out continually new leaves and twigs. It does not, of course, follow that increase of bulk is always conspicuous; in such trees death is present side by side with life, and the one often counterbalances the other. As, however, we can easily see that the constructive processes are much greater than those which lead to the disappearance of material from the plant-body, there is generally to be seen a conspicuous increase in the substance of the plant. This is, in nearly all cases, attended by a permanent change in form. This is not perhaps so evident in the case of axial organs as it is in that of leaves and their modifications, but even in them it can be detected to a certain extent.
In the lowliest plants growth may be co-extensive with the plant-body; In all plants of any considerable size, however, it is localized in particular regions, and in them it is associated with the formation of new protoplasts or cells. These regions have been called growing points. In such stems and roots as increase in thickness there are other growing regions, which consist of cylindrical sheaths known as cambium layers or phellogens. By the multiplication of the protoplasts in these merismatic areas the substance of the plant is increased. In other words, as these growing regions consist of cells, the growth of the entire organ or plant will depend upon the behaviour of the cells or protoplasts of which the merismatic tissues are composed.
The growth of such a cell will be found to depend mainly upon five conditions: (1) There must be a supply of nutritive or plastic materials, at the expense of which the increase of its living substance can take place and which supply the needed potential energy. (2) There must be a supply of water to such an extent as to set up a certain hydrostatic pressure in the cell, for only turgid cells can grow. (3) The supply of water must be associated with the formation of osmotic substances in the cell, or it cannot be made to enter it. (4) The cell must have a certain temperature, for the activity of a protoplast is only possible within certain limits, which differ in the case of different plants. (5) There must be a supply of oxygen to the growing cell, for the protoplast is dependent upon this gas for the performance of its vital functions, and particularly for the liberation of the energy which is demanded in the constructive processes. This is evident from the consideration that the growth of the cells is attended by the growth in surface of the cell wall, and as the latter is a secretion from the protoplasm, such a decomposition cannot readily take place unless oxygen is admitted to it.
When these conditions are present, the course of the growth of a cell appears to be the following: The young cell, immediately it is cut off from its fellow, absorbs water, in consequence of the presence in it of osmotically active substances. With the water it takes in the various nutritive substances which the former contains in solution. There is set up at once a certain hydrostatic pressure, due to the turgidity which ensues upon such absorption, and the extensible cell wall stretches, at first in all directions. The growth or increase of the protoplasm at the expense of the nutritive matter for a time keeps pace with the increased size of the cell, but by and by it becomes vacuolated as more and more water is attracted into the interior. Eventually the protoplasm usually forms only a lining to the cell wall, and a large vacuole filled with cell sap occupies the centre. The growth of the protoplasm, though considerable, is therefore not commensurate with the increase in the size of the cell. The stretching of the cell wall by the hydrostatic pressure is fixed by a secretion of new particles and their deposition upon the original wall, which as it becomes slightly thicker is capable of still greater extension, much in the same way as a thick band of india-rubber is capable of undergoing greater stretching than a thin one. The increase in surface of the cell wall is thus due—firstly to the stretching caused by turgidity, and secondly to the formation and deposition of new substance upon the old. When the limit of extensibility is reached the cell wall increases in thickness from the continuation of the latter of the two processes.
The rate of growth of a cell varies gradually throughout its course; it begins slowly, increases to a maximum, and then becomes slower till it stops. The time during which these regular changes in the rate can be observed is generally spoken of as the grand period of growth.
If we consider the behaviour of a growing organ such as a root, we find that, like a cell, it shows a grand period of growth. Just behind its apex the cells are found to be all in process of active division. Growth is small, and consists mainly in an increase of the quantity of protoplasm, for the cells divide again as soon as they have reached a certain size. As new cells are continually formed in the merismatic mass those which are farthest from the apex gradually cease to divide and a different process of growth takes place in them, which is associated more particularly with the formation of the vacuoles, consequent upon the establishment of considerable hydrostatic pressure in them, thus causing the bulk of the cells to be greatly enlarged. Here it is that the actual extension in length of the root takes place, and the cells reach the maximum point of the grand period. They then gradually lose the power of growth, the oldest ones or those farthest from the apex parting with it first, and they pass gradually over into the condition of the permanent tissues.
The same order of events may be ascertained to take place in the stem; but in this region it is complicated by the occurrence of nodes and internodes, growth in length being confined to the latter, many of which may be growing simultaneously. The region of growth in the stem is, as a rule, much longer than that of the root. The growth of the leaf is at first apical, but this is not very prolonged, and the subsequent enlargement is due to an intercalary growing region near the base.
The turgidity in the cells of a growing member is not uniform, but shows a fairly rhythmical variation in its different parts. If the member is one which shows a difference of structure on two sides, such as a leaf, the two sides frequently show a difference of degree of turgidity, and consequently of rate of growth. If we consider a leaf of the common fern we find that in its young condition it is closely rolled up, the upper or ventral surface being quite concealed. As it gets older it gradually unfolds and expands into the adult form. This is due to the fact that while young the turgidity and consequent growth are greater in the dorsal side of the leaf, so that it becomes rolled up. As it develops the maximum turgidity and growth change to its upper side, and so it becomes unfolded or expanded. These two conditions are generally described under the names of hyponasty and epinasty respectively.
Cylindrical organs may exhibit similar phenomena. One side of a stem may be more turgid than the opposite one, and the maximum turgidity, with its consequent growth, may alternate between two opposite sides. The growing apex of such a stem will alternately incline, first to one side and then to the other, exhibiting a kind of nodding movement in the two directions. More frequently the region of maximum turgidity passes gradually round the growing zone. The apex in this case will describe a circle, or rather a spiral, as it is elongating all the time, pointing to all points of the compass in succession. This continuous change of position has been called circumnutation, and is held to be universal in all growing cylindrical organs. The passage of the maximum turgidity round the stem may vary in rapidity in different places, causing the circle to be replaced by an ellipse. The bending to two sides alternately, described above, often called simple nutation, may be regarded as only an extreme instance of the latter.
Nervous System of Plants.—So far we have considered the plant almost exclusively as an individual organism, carrying out its own vital processes, and unaffected by its surroundings except in so far as these supply it with the materials for its well-being. When we consider, however, the great variability in those surroundings and the consequent changes a plant must encounter, it appears obvious that interaction and adjustment between the plant and its environment must be constant and well balanced. That such adjustment shall take place postulates on the part of the plant a kind of perception or appreciation of the changing conditions which affect it.
Careful examination soon shows an observer that such perceptions exist, and that they are followed by certain purposeful changes in the plant, sometimes mechanical, sometimes chemical, the object being evidently to secure some advantage for the plant, to ward off some danger, or to extricate it from some difficulty. We may speak, indeed, of the plant as possessed of a rudimentary nervous system, by the aid of which necessary adjustments are brought about. The most constantly occurring changes that beset a plant are connected with illumination, temperature, moisture, and contact with foreign bodies. Setting aside other susceptibilities, we have evidence that most plants are sensitive to all these.
If a growing stem receives stronger illumination on one side than another, its apex slowly turns from the vertical in the direction of the light source, continuing its change of position until it is in a direct line with the incident rays. If a root is similarly illuminated, a similar change of direction of growth follows, but in this case the organ grows away from the light. These movements are spoken of as heliotropic and apheliotropic curvatures. The purpose of the movements bears out the contention that the plant is trying to adjust itself to its environment. The stem, by pointing directly to the light source, secures the best illumination possible for all of its leaves, the latter being distributed symmetrically around it. The root is made to press its way into the darker cracks and crannies of the soil, so bringing its root-hairs into better contact with the particles round which the hygroscopic water hangs. Leaves respond in another way to the same influence, placing themselves across the path of the beam of light.
Similar sensitivenesses can be demonstrated in other cases. When a root comes in contact at its tip with some hard body, such as might impede its progress, a curvature of the growing part is set up, which takes the young tip away from the stone, or what-not, with which it is in contact. When a sensitive tendril comes into contact with a foreign body, its growth becomes so modified that it twines round it. Many instances might be given of appreciation of and response to other changes in the environment by the growing parts of plants; among them we may mention the opening and closing of flowers during the days of their expansion. One somewhat similar phenomenon, differing in a few respects, marks the relation of the plant to the attraction of gravity. Observation of germinating seedlings makes it clear that somehow they have a perception of direction. The young roots grow vertically downwards, the young stems vertically upwards. Any attempt to interfere with these directions, by placing the seedlings in abnormal positions, is frustrated by the seedlings themselves, which change their direction of growth by bringing about curvatures of the different parts of their axes, so that the root soon grows vertically downward again and the stem in the opposite direction. Other and older plants give evidence of the same perception, though they do not respond all in the same way. Speaking generally, stems grow upwards and roots downwards. But some stems grow parallel to the surface of the soil, while the branches both of stems and roots tend to grow at a definite angle to the main axis from which they come. These movements are spoken of as different kinds of geotropic curvatures. This power of perception and response is not by any means confined to the growing organs, though in these it is especially striking, and plays a very evident part in the disposition of the growing organs in advantageous positions. It can, however, be seen in adult organs, though instances are less numerous.
When the pinnate leaf of a Mimosa pudica, the so-called sensitive plant, is pinched or struck, the leaf droops rapidly and the leaflets become approximated together, so that their upper surfaces are in contact. The extent to which the disturbance spreads depends on the violence of the stimulation—it may be confined to a few leaflets or it may extend to all the leaves of the plant.
The leaves and leaflets of many plants, e.g. the telegraph plant, Desmodium gyrans, behave in a similar way under the stimulus of approaching darkness.
A peculiar sensitiveness is manifested by the leaves of the so-called insectivorous plants. In the case of Dionaea muscipula we find a two-lobed lamina, the two lobes being connected by a midrib, which can play the part of a kind of hinge. Six sensitive hairs spring from the upper surface of the lobes, three from each; when one of these is touched the two lobes rapidly close, bringing their upper surfaces into contact and imprisoning anything which for the moment is between them. The mechanism is applied to the capture of insects alighting on the leaf.
Drosera, another of this insectivorous group, has leaves which are furnished with long glandular tentacles. When these are excited by the settling of an insect on the leaf they slowly bend over and imprison the intruder, which is detained there meanwhile by a sticky excretion poured out by the glands.
In both these cases the stimulation is followed, not only by movement, but by the secretion of an acid liquid containing a digestive juice, by virtue of which the insect is digested after being killed.
The purposeful character of all these movements or changes of position indicates that they are of nervous origin. We have in them evidence of two factors, a perception of some features of the environment and following this, after a longer or shorter interval, a response calculated to secure some advantage to the responding organ. We find on further investigation that these two conditions are traceable to different parts of the organs concerned. The perception of the changes, or, in other words the reception of the stimulus, is associated for example, with the tips of roots and the apices of stems. The first recognition of a specially receptive part was made by Charles Darwin, who identified the perception of stimulation with the tip of the young growing root. Amputation of this part involved the cessation of the response, even when the conditions normally causing the stimulation were maintained. Francis Darwin later demonstrated that the tips of the plumules of grasses were sensitive parts. The responding part is situated some little distance farther back, being in fact the region where growth is active. This bending part has been proved to be insensitive to the stimuli. There is consequently a transmission of the stimulus from the sensitive organ to a kind of motor mechanism situated some little way off. We find thus three factors of a nervous mechanism present, a receptive, a conducting, and a responding part. The differentiation of the plant's substance so indicated is, however, physiological only; there is no histological difference between the cells of these regions that can be associated with the several properties they possess. Even the root tip, which shows a certain differentiation into root cap and root apex, cannot be said to be a definite sense organ in the same way as the sense organs of an animal. The root is continually growing and so the sensitive part is continually changing its composition, cells being formed, growing and becoming permanent tissue. The cells of the tip at any given moment may be sensitive, but in a few days the power of receiving the stimulus has passed to other and younger cells which then constitute the tip. The power of appreciating the environment is therefore to be associated with the protoplasm only at a particular stage of its development and is transitory in its character.
What the nature of the stimulation is we are not able to say. The protoplasm is sensitive to particular influences, perhaps of vibration, or of contact or of chemical action. We can imagine though perhaps only vaguely, the way in which light, temperature, moisture, contact, &c., can affect it. The perception of direction or the influence of gravity presents greater difficulty, as we have no clear idea of the form which the force of gravity takes. Recently some investigations by Haberlandt, Noll, Darwin and others have suggested an explanation which has much to recommend it. The sensitive cells must clearly be influenced in some way by weight—not the weight of external organs but of some weight within them. This may possibly be the cell sap in their interior, which must exercise a slightly different hydrostatic pressure on the basal and the lateral walls of the cells. Or more probably it may be the weight of definite particulate structures in their vacuoles. Many experiments point to certain small grains of starch which are capable of displacement as the position of the cell is altered. Such small granules have been observed in the sensitive cells, and there is an evident correlation between these and the power of receiving the geotropic stimulus. It has been shown that if the organ containing them is shaken for some time, so that the contact between them and the protoplasm of the cells is emphasized, the stimulus becomes more efficient in producing movement. This reduces the stimulus to one of contact, which is in harmony with the observations made upon roots similarly stimulated from the exterior. The stimulating particles, whether starch grains in all cases, or other particles as well, have been termed statoliths.
We have spoken of the absence of structural differentiation in the sense organs. There is a similar difficulty in tracing the paths by which the impulses are transmitted to the growing and curving regions. The conduction of such stimulation to parts removed some distance from the sense organ suggests paths of transmission comparable to those which transmit nervous impulses in animals. Again, the degree of differentiation is very slight anatomically, but delicate protoplasmic threads have been shown to extend through all cell-walls, connecting together all the protoplasts of a plant. These may well serve as conductors of nervous impulses. The nervous mechanism thus formed is very rudimentary, but in an organism the conditions of whose life render locomotion impossible great elaboration would seem superfluous. There is, however, very great delicacy of perception or appreciation on the part of the sense organ, stimuli being responded to which are quite incapable of impressing themselves upon the most highly differentiated animal.
The power of response is seen most easily in the case of young growing organs, and the parts which show the motor mechanism are mainly the young growing cells. We do not find their behaviour like that of the motor mechanism of an animal. The active contraction of muscular tissue has no counterpart in the plant. The peculiarity of the protoplasm in almost every cell is that it is especially active in the regulation of its permeability by water. Under different conditions it can retain it more strongly or allow it to escape more freely. This regulation of turgor is as characteristic of vegetable protoplasm as contraction is of muscle. The response to the stimulus takes the form of increasing the permeability of particular cells of the growing structures, and so modifying the degree of the turgidity that is the precursor of growth in them. The extent of the area affected and of the variation in the turgor depends upon many circumstances, but we have no doubt that in the process of modifying its own permeability by some molecular change we have the counterpart of muscular contractibility.
The response made by the adult parts of plants, to which reference has been made, is brought about by a mechanism similar in nature though rather differently applied. If the leaf of Mimosa or Desmodium be examined, it will be seen that at the base of each leaflet and each leaf, just at the junction with the respective axes, is a swelling known as a pulvinus. This has a relatively large development of succulent parenchyma on its upper and lower sides. In the erect position of the leaf the lower side has its cells extremely turgid, and the pulvinus thus forms a cushion, holding up the petiole. On stimulation these cells part with their water, the lower side of the organ becomes flaccid and the weight of the leaf causes it to fall. The small pulvini of the leaflets, by similar changes of the distribution of turgidity, take up their respective positions after receiving the stimulus. In some cases the two sides of the pulvini vary their turgidity in turns, in others only the lower side becomes modified.
Similar turgescence changes, taking place with similar rapidity in the midrib of the leaf of Dionaea, explain the closing of the lobes upon their hinge. More slowly, but yet in the same way, we may note the change in turgidity of certain cells of the Drosera tentacles, as they close over the imprisoned insect.
Organic Rhythm.—It is a remarkable fact that during the process of growth we meet with rhythmic variation of such turgidity. The existence of rhythm of this kind has been observed and studied with some completeness. It is the immediate cause of the phenomena of circumnutation, each cell of the circumnutating organ showing a rhythmic enlargement and decrease of its dimensions, due to the admission of more and less water into its interior. The restraint of the protoplasm changes gradually and rhythmically. The sequence of the phases of the rhythm of the various cells are co-ordinated to produce the movement. Nor is it only in growing organs that the rhythm can be observed, for many plants exhibit it during a much longer period than that of growth. It is easy to realize how such a rhythm can be modified by the reception of stimuli, and can consequently serve as the basis for the movement of the stimulated organ. This rhythmic affection of vegetable protoplasm can be observed in very many of its functions. What have been described as “periodicities,” such as the daily variations of root-pressure, afford familiar instances of it. It reminds us of a similar property of animal protoplasm which finds its expression in the rhythmic beat of the heart and other phenomena.
Authorities.—Sachs, Lectures on the Physiology of Plants, translated by Marshall Ward; Vines, Lectures on the Physiology of Plants; Pfeffer, The Physiology of Plants, trans. by Ewart; Reynolds Green, Introduction to Vegetable Physiology; The Soluble Ferments and Fermentation; Detmer, Practical Plant Physiology, trans. by Moor; Darwin and Acton, Practical Physiology of Plants; Davenport, C.B., Experimental Morphology, vols. i. and ii.; Verworn, General Physiology, trans. by Lee; Butschli, Investigation on Microscopic Forms and on Protoplasm, trans. by Minchin. (J. R. Gr.)
Pathology of Plants
“Phytopathology” or plant pathology (Gr. φυτόν, plant), comprises our knowledge of the symptoms, course, causes and remedies of the maladies which threaten the life of plants, or which result in abnormalities of structure that are regarded, whether directly injurious or not to life, as unsightly or undesirable. In its systematized form, as a branch of botanical study, it is of recent date, and, as now understood, the subject first received special attention about 1850, when the nature of parasitism began to be intelligible; but many disjointed references to diseased conditions of plants had appeared long before this. The existence of blights and mildews of cereals had been observed and recorded in very ancient times, as witness the Bible, where half a dozen references to such scourges occur in the Old Testament alone. The epidemic nature of wheat-rust was known to Aristotle about 350 B.C., and the Greeks and Romans knew these epidemics well, their philosophers having shrewd speculations as to causes, while the people held characteristic superstitions regarding them, which found vent in the dedication of special festivals and deities to the pests. Pliny knew that flies emerge from galls. The few records during the middle ages are borne out by what is known of famines and pestilence. Shakespeare's reference in King Lear (Act III., sc. iv.) may be quoted as evincing acquaintance with mildew in the 17th century, as also the interesting Rouen law of Loverdo (1660). Malpighi in 1679 gave excellent figures and accounts of leaf-rolling and gall insects, and Grew in 1682 equally good descriptions of a leaf-mining caterpillar. During the 18th century more academic treatment of the subject began to replace the scattered notes. Hales (1727-1733) discussed the rotting of wounds, cankers, &c., but much had to be done with the microscope before any real progress was possible, and it is easily intelligible that until the theory of nutrition of the higher plants had been founded by the work of Ingenhouss, Priestley and De Saussure, the way was not even prepared for accurate knowledge of cryptogamic parasites and the diseases they induce. It was not till De Bary (1866) made known the true nature of parasitic Fungi, based on his researches between 1853-1863, that the vast domain of epidemic diseases of plants was opened up to fruitful investigation, and such modern treatises as those of Frank (1880 and 1895), Sorauer (1886), Kirchner (1890), were gradually made possible.
Plant pathology embraces several branches of study, and may be conveniently divided as follows:—
1. The observation and accurate description of symptoms (Diagnosis).
2. The study of causes or agencies inducing disease (Aetiology).
3. The practise of preventive and remedial measures (Therapeutics).
In plants, however, the symptoms of disease are apt to exhibit themselves in a very general manner. Our perceptions differentiate but imperfectly symptoms which are due to very different causes and reactions, probably because the organization of the plant is so much less highly specialized than that of higher animals. The yellowing and subsequent casting of leaves, for instance, is a very general symptom of disease in plants, and may be induced by drought, extremes of temperature, insufficient or excessive illumination, excess of water at the roots, the action of parasitic Fungi, insects, worms, &c., or of poisonous gases, and so forth; and extreme caution is necessary in dealing with amateur descriptions of such symptoms, especially when the untrained eye has taken no cognisance of, or has only vaguely observed, the numerous collateral circumstances of the case.
The causes of disease may be provisionally classified somewhat as follows, but it may be remarked at the outset that no one of these proximal causes, or agents, is ever solely responsible; and it is very easy to err in attributing a diseased condition to any of them, unless the relative importance of primary and subordinate agencies is discoverable. For instance, a Fungus epidemic is impossible unless the climatic conditions are such as to favour the dispersal and germination of the spores; and when plants are killed off owing to the supersaturation of the soil with water, it is by no means obvious whether the excess of water and dissolved materials, or the exclusion of oxygen from the root-hairs, or the lowering of the temperature, or the accumulation of foul products of decomposition should be put into the foreground. In every case there are chains of causation concerned, and the same factors will be differently grouped in different cases.
Bearing in mind these precautions, we may classify the proximal causal agents of disease as—
- I.—External agencies.
- A. Non-living
- a. Material
- 1. Physical—
- 2. Chemical—
- 1. Physical—
- b. Non-material.
- 1. Temperature.
- 2. Illumination.
- 3. Other agencies.
- a. Material
- B. Living.
- a. Animals.
- 1. Vertebrata.
- 2. Invertebrata.
- b. Plants.
- 1. Phanerogams.
- 2. Cryptogams.
- a. Animals.
- A. Non-living
- II.—Internal agencies.
While such a classification may serve its purpose as a sort of index, it must be confessed that the limits of its usefulness are soon reached. In the first place, the so-called “internal causes” of disease is probably a mere phrase covering our ignorance of the factors at work, and although a certain convenience attaches to the distinction between those cases where tender breeds of plants apparently exhibit internal predisposition to suffer more readily than others from parasites, low temperatures, excessive growth, &c.—as is the case with some grafted plants, cultivated hybrids, &c.—the mystery involved in the phrase “internal causes” only exists until we find what action of the living or nonliving environment of the essential mechanism of the plant has upset its equilibrium.
I.—Passing to the recognized external agencies, the physical condition of the soil is a fruitful source of disease. If too closely packed, the soil particles present mechanical obstacles to growth; if too retentive of moisture, the root-hairs suffer, as already hinted; if too open or over-drained, the plant succumbs to drought. All those properties of soil known as texture, porosity, depth, inclination to the horizon, &c., are concerned here. Many maladies of plants are traceable to the chemical composition of soils—e.g. deficiency of nutritive salts, especially nitrates and phosphates, the presence of poisonous salts of iron, copper, &c., or (in the soil about the roots of trees in towns) of coal-gas and so forth. But it is worthy of special attention that the mere chemical composition of agricultural and garden soils is, as a rule, the least important feature about them, popular opinion to the contrary notwithstanding. Ordinary soils will almost always provide the necessary chemical ingredients if of proper physical texture, depth, &c. (see Fungi and Bacteriology).
As regards water, its deficiency or excess is a relative matter, and although many of the minor maladies of pot-plants in windows and greenhouses controlled by amateurs depend on its misuse, water alone is probably never a primary cause of disease. Its over-supply is, however, a frequent cause of predisposition to the attacks of parasitic Fungi—e.g. the damping off of seedlings—and in saturated soils not only are the roots and root-hairs killed by asphyxiation, but the whole course of soil fermentation is altered, and it takes time to “sweeten” such by draining, because not only must the noxious bodies be gradually washed out and the lost salts restored, but the balance of suitable bacterial and fungal life must be restored.
The atmosphere is a cause of disease in the neighbourhood of chemical works, large towns, volcanoes, &c., in so far as it carries acid gases and poisons to the leaves and roots; but it is usual to associate with it the action of excessive humidity which brings about those tender watery and more or less etiolated conditions which favour parasitic Fungi, and diminish transpiration and therefore nutrition. It is customary to speak of the disastrous effects of cold winds, snow, hail and frost, lightning, &c., under the heading of atmospheric influences, which only shows once more how impossible it is to separate causes individually.
Turning to the non-material external agents, probably no factors are more responsible for ill health in plants than temperature and light. Every plant is constrained to carry out its functions of germination, growth, nutrition, reproduction, &c., between certain limits of temperature, and somewhere between the extremes of these limits each function finds an optimum temperature at which the working of the living machinery is at its best, and, other things being equal, any great departure from this may induce pathological conditions; and many disasters are due to the failure to provide such suitable temperatures—e.g. in greenhouses where plants requiring very different optimum temperatures and illumination are kept together. Equally disastrous are those climatic or seasonal changes which involve temperatures in themselves not excessive but in wrong sequence; how many more useful plants could be grown in the open in the United Kingdom if the deceptively mild springs were not so often followed by frosts in May and June! The indirect effects of temperature are also important. Trees, of which the young buds are “nipped” by frost, would frequently not suffer material injury, were it not that the small frost-cracks serve as points of entry for Fungi; and numerous cases are known where even high temperatures can be endured on rich, deep, retentive soils by plants which at once succumb to drought on shallow or non-retentive soils.
All chlorophyll plants require light, but in very different degrees, as exemplified even in the United Kingdom by the shade-bearing beech and yew contrasted with the light-demanding larch and birch; and as with temperature so with light, every plant and even every organ has its optimum of illumination. The “drawn” or etiolated condition of over-shaded plants is a case in point, though here again the soft, watery plant often really succumbs to other disease agents—e.g. parasitic Fungi—supervening on its non-resistant condition.
Animals and plants as agents of disease or injury form part of the larger subject of the struggle for existence between living organisms, as is recognized even by those who do not so readily apprehend that diseased conditions in general are always signs of defeat in the struggle for existence between the suffering organism and its environment, living and non-living.
The Vertebrata come within the scope of our subject, chiefly as destructive agents which cause wounds or devour young shoots and foliage, &c. Rabbits and other burrowing animals injure roots, squirrels and birds snip off buds, horned cattle strip off bark, and so forth. It is among the Invertebrata that epidemics of destruction are referred to, though we should bear in mind that it is only the difference in numerical proportion that prevents our speaking of an epidemic of elephants or of rabbits, though we use the term when speaking of blight insects; there is little consistency in the matter, as it is usual to speak of an invasion or scourge of locusts, caterpillars, &c. Insect injuries are very varied in degree and in kind. Locusts devour all before them; caterpillars defoliate the plant, and necessitate the premature utilization of its reserves; other insects (e.g. Grapholitha) eat the buds or the roots (e.g. wire-worms), and so maim the plant that its foliage suffers from want of water and assimilation is diminished, or actual withering follows. Many aphides, &c., puncture the leaves, suck out the sap, and induce various local deformations, arrest of growth, pustular swellings, &c., and if numerous all the evils of defoliation may follow. Others (e.g. miners) tunnel into the leaf parenchyma, and so put the assimilating areas out of action in another way. It should be remembered that a single complete defoliation of a herbaceous annual may so incapacitate the assimilation that no stores are available for seeds, tubers, &c., for another year, or at most so little that feeble plants only come up. In the case of a tree matters run somewhat differently; most large trees in full foliage have far more assimilatory surface than is immediately necessary, and if the injury is confined to a single year it may be a small event in the life of the tree, but if repeated the cambium, bud-stores and fruiting may all suffer. Many larvae of beetles, moths, &c., bore into bark, and injure the cambium, or even the wood and pith; in addition to direct injury, the interference with the transpiration current and the access of other parasites through the wounds are also to be feared in proportion to the numbers of insects at work. Various local hypertrophies, including galls, result from the increased growth of young tissues irritated by the punctures of insects, or by the presence of eggs or larvae left behind. They may occur on all parts, buds, leaves, stems or roots, as shown by the numerous species of Cynips on oak, Phylloxera on vines, &c. The local damage is small, but the general injury to assimilation, absorption and other functions, may be important if the numbers increase. In addition to insects, various kinds of worms, molluscs, &c., are sometimes of importance as pests. The so-called eel-worms (Nematodes) may do immense damage on roots and in the grains of cereals, and every one knows how predatory slugs and snails are. (See Economic Entomology.)
Plants as agents of damage and disease may be divided into those larger forms which as weeds, epiphytes and so forth, do injury by dominating and shading more delicate species, or by gradually exhausting the soil, &c., and true parasites which actually live on and in the tissues of the plants. It must be remembered that phanerogams also include parasitic species—e.g. Cuscuta, Loranthus, Viscum, Thesium, Rhinanthus, &c.—with various capacities for injury. These enemies are as a rule so conspicuous that we do not look on their depredations as diseases, though the gradual deteriorat1on of hay under the exhausting effects of root-parasites like Rhinanthus, and the onslaught of Cuscuta when unduly abundant, should teach us how unimportant to the definition the question of size may be.
It is, however, among the Fungi that we find the most disastrous and elusive agents of disease. Parasitic Fungi may be, as regards their direct action, purely local—e.g. Schinzia, which forms gall-like swellings on the roots of rushes; Gymnosporangium, causing excrescences on juniper stems; numerous leaf Fungi such as Puccinia, Aecidium, Septoria, &c., causing yellow, brown or black spots on leaves; or Ustilago in the anthers of certain flowers. In such cases the immediate damage done may be slight; but the effects of prolonged action and the summation of numerous attacks at numerous points are often enormous, certain of these leaf diseases costing millions sterling annually to some planting and agricultural communities. In other cases the Fungus is virulent and rampant, and, instead of a local effect, exerts a general destructive action throughout the plant—e.g. Pythium, which causes the “damping off” of seedlings, reducing them to a putrid mass in a few hours, and Phytophthora, the agent of the potato disease. Many Fungi, in themselves not very aggressive, slowly bring about important and far-reaching secondary effects. Thus, many Hymenomycetes (Agarics, Polyporei, &c.) live on the wood of trees. This wood is in great part already dead substance, but the mycelium gradually invades the vessels occupied with the transmission of water up the trunk, cuts off the current, and so kills the tree; in other cases such Fungi attack the roots, and so induce rot and starvation of oxygen, resulting in “fouling.” Numerous Fungi, though conspicuous as parasites, cannot be said to do much individual injury to the host. The extraordinary malformations known as “Witches' Brooms” caused by the repeated branching and tufting of twigs in which the mycelium of Exoascus (on birch) or Aecidium (on silver fir) are living, may be borne in considerable numbers for years without any very extensive apparent injury to the tree. Again, the curious distortions on the stems of nettles attacked by the Aecidium form of the heteroecious Puccina Caricis (see Fungi for Heteroecism), or on maize stems and leaves attacked by Ustilago Maydis, or on the inflorescence of crucifers infested with Cystopus, &c., are not individually very destructive; it is the cumulative effects of numerous attacks or of extensive epidemics which eventually tell. Some very curious details are observable in these cases of malformation. For instance, the Aecidium elatinum first referred to causes the new shoots to differ in direction, duration and arrangement, and even shape of foliage leaves from the normal; and the shoots of Euphorbia infected with the aecidia of Uromyces Pisi depart so much from the normal in appearance that the attacked plants have been taken for a different species. Similarly with Anemone infested with Puccinia and Vaccinium with Calyptospora, and many other cases of deformations due to hypertrophy or atrophy. Instances of what we may term tolerated parasitism, where the host plant seems to accommodate itself very well to the presence of the Fungus, paying the tax it extorts and nevertheless not succumbing but managing to provide itself with sufficient material to go on with, are not rare; and these seem to lead to those cases where the mutual accommodation between host and guest has been carried so far that each derives some benefit from the association—symbiosis (see Fungi).
II. The kinds of disease due to these various agencies are very different. A plant may be diseased as a whole, because nearly all its tissues are in a morbid or pathological condition, owing to some Fungus pervading the whole—e.g. Pythium in seedlings—or to a poison diffusing from cell to cell; in the case of unicellular plants—e.g. an alga infested with a Chytridium—indeed, matters can hardly bc otherwise. But the case is obviously different where a plant dies because some essential organ or tissue tract has been destroyed, and other parts have suffered because supplies are cut off—e.g. when the upper parts of a tree die off owing to destruction of the roots, or to the ringing of the stem lower down, and consequent interference with the transpiration current. In a large number of cases, however, the disease is purely local, and does not itself extend far into the organ or tissue affected.
If a mass of living plant-tissue is cut, the first change observed is one of colour: the white “flesh” of a potato or an apple turns brown as the air enters, and closer examination shows that cell walls and contents are alike affected. The cut cells die, and oxidized products are concerned in the change of colour, the brown juices exuding and soaking into the cell-walls. The next change observable after some hours is that the untouched cells below the cut grow larger, push up the dead surface, and divide by walls tangential to it, with the formation of tabloid cork-cells. The layer of cork thus formed cuts out the dead débris and serves to protect the uninjured cells below. Such healing by cork formation is accompanied by a rise of temperature: the active growth of the dividing cells is accompanied by vigorous metabolism and respiration, and a state of “wound fever” supervenes until the healing is completed. The phenomena described occur in all cases of cicatrization of wounds in nature—e.g. leaf tissue, young stems, roots, &c., when cut or pierced by insects, thorns and so forth They are concerned in the occlusion of broken twigs and of falling leaves, and it is from the actively growing “callus” developed at the surface of the wounded tissues of cuttings, buddings, prunings, &c., that the healing and renewal of tissues occur of which advantage is taken in the practice of what might well be termed plant surgery. A third phenomenon observable in such healing tissues is the increased flow and accumulation of plastic materials at the seat of injury. The enhanced metabolism creates a current of draught on the supplies of available food-stuffs around. The phenomenon of irritability here concerned is well shown in certain cases where a parasitic organism gains access to a cell—e.g. Pleotrachelus causes the invaded Pilobolus to swell up, and changes the whole course of its cell metabolism, and similarly with Plasmodiophora in the roots of turnips, and many other cases.
Irritation and hypertrophy of cells are common signs of the presence of parasites, as evinced by the numerous malformations, galls, witches'-brooms, &c., on diseased plants. The now well-known fact that small doses of poisonous substances may act as stimuli to living protoplasm, and that respiratory activity and growth may be accelerated by chloroform, ether and even powerful mineral poisons, such as mercuric chloride, in minimal doses, offers some explanation of these phenomena of hypertrophy, “wound fever,” and other responses to the presence of irritating agents. Still further insight is afforded by our increasing knowledge of the enzymes, and it is to be remarked that both poisons and enzymes are very common in just such parasitic Fungi as induce discolorations, hypertrophies and the death of cells—e.g. Botrytis, Ergot, &c. Now it is clear that if an organism gains access to all parts of a plant, and stimulates all or most of its cells to hypertrophy, we may have the latter behaving abnormally—i.e. it may be diseased throughout; and such actually occurs in the case of Euphorbia pervaded with Uromyces Pisi, the presence of which alters the whole aspect of the host-plant. If such a general parasite carries its activities farther, every cell may be killed and the plant forthwith destroyed—e.g. Phytophthora in potatoes. If, on the other hand, the irritating agent is local in its action, causing only a few cells to react, we have the various pimples, excrescences, outgrowths, &c., exhibited in such cases as Ustilago Maydis on the maize, various galls, witches'-brooms, &c.
It must not be overlooked that the living cells of the plant react upon the parasite as well as to all external agencies, and the nature of disease becomes intelligible only if we bear in mind that it consists in such altered metabolism—deflected physiology—as is here implied. The reaction of the cells may be in two directions, moreover. For instance, suppose the effect of a falling temperature is to so modify the metabolism of the cells that they fill up more and more with watery sap; as the freezing-point is reached this may result in destructive changes, and death from cold may result. If, on the contrary, the gradual cooling is met by a corresponding depletion of the cells of water, even intense cold may be sustained without injury.
Or, take another case. If the attack of a parasite is met by the formation of some substance in the protoplasm which is chemotactically repulsive to the invader, it may be totally incapable of penetrating the cell, even though equipped with a whole armoury of cytases, diastatic and other enzymes, and poisons which would easily overcome the more passive resistances offered by mere cell-walls and cell-contents of other plants, the protoplasm of which forms bodies chemotactically attractive to the Fungus.
The various degrees of parasitism are to a certain extent explained by the foregoing. In order that a Fungus may enter a plant, it must be able to overcome not merely the resistance of cell-walls, but that of the living protoplasm; if it cannot do this, it must remain outside as a mere epiphyte, e.g. Fumago, Herpotrichia, &c., or, at most, vegetate in the intercellular spaces and anchor itself to the cell-walls, e.g. Trichosphaeria. The inability to enter the cells may be due to the lack of chemotactic bodies, to incapacity to form cellulose-dissolving enzymes, to the existence in the host-cells of antagonistic bodies which neutralize or destroy the acids, enzymes or poisons formed by the hyphae, or even to the formation and excretion of bodies which poison the Fungus. But even when inside it does not follow that the Fungus can kill the cell, and many cases are known where the Fungus can break through the cell's first lines of defence (cell wall and protoplasmic lining); but the struggle goes on at close quarters, and various degrees of hypertrophy, accumulation of plastic bodies or secretions, discolorations, &c., indicate the suffering of the still living cell. Finally, cases occur where the invaded cell so adapts itself to the presence of the intruder that life in common—symbiosis—results.
The dissemination of plant parasites is favoured by many circumstances not always obvious, whence an air of mystery regarding epidemics was easily created in earlier times. The spores of Rusts, Erysipheae and other Fungi may be conveyed from plant to plant by snails; those of tree-killing polyporei, &c., by mice, rabbits, rats, &c., which rub their fur against the hymenophores. Bees carry the spores of Sclerotinia as they do the pollen of the bilberries, and flies convey the conidia of ergot from grain to grain. Insects, indeed, are largely concerned in disseminating Fungi, either on their bodies or via the alimentary canal. Worms bring spores to the surface of soil, ducks and other birds convey them on their muddy feet, and, as is well-known, wind and other physical agencies are very efficient in dissemination. The part played by man also counts for much. Gardeners and farm labourers convey spores from one bed or field to another, carted soil, manure, &c., may abound in spores of Smuts, Fusarium, Polypores and in sclerotia; and articles through the post and so forth often carry infective spores. Every time a carpenter saws fresh timber with a saw recently put through wood attacked with dry-rot, he risks infecting it with the Fungus; and similarly in pruning, in propagating by cuttings, &c.
The annual losses due to epidemic plant diseases attain proportions not easily estimated. As regards money value alone the following figures may serve in illustration. In 1882 the United States was calculated to have lost £40,000,000 to £60,000,000 from insect and other pests. The wheat-rust costs Australia £2,000,000 to £3,000,000 annually, and in 1891 alone the loss which Prussia suffered from grain-rusts was estimated at £20,000,000 sterling.
The terrible losses sustained by whole communities of farmers, planters, foresters, &c., from plant diseases have naturally stimulated the search for remedies, but even now the search is too often conducted in the spirit of the believer in quack medicines, although the agricultural world is awakening to the fact that before any measures likely to be successful can be attempted, the whole chain of causation of the disease must be investigated. Experience with epidemics, dearly bought in the past, has shown that one fruitful cause is the laying open to the inroads of some Fungus or insect, hitherto leading a quiet endemic life in the fields and forests, large tracts of its special food, along which it may range rampant without check to its dispersal, nutrition and reproduction. Numerous wild hypotheses as to changes in the constitution of the host-plant, leading to supposed vulnerability previously non-existent, would probably never have seen the light had the full significance of the truth been grasped that an epidemic results when the external factors favour a parasite somewhat more than they do the host. It may be that in particular cases particular modes of cultivation disfavour the host, or that the soil, climate or seasons do so; but overwhelming evidence exists to show that the principal causes of epidemics reside in circumstances which favour the spread, nutrition and reproduction of the pest, and the lesson to be learnt is that precautions against the establishment of such favouring conditions must be sought. Nevertheless, epidemics occur, and practical measures are devised to meet the various cases and to check the ravages already begun. The procedure consists in most cases in spraying the affected plants with poisonous solutions or emulsions, or in dusting them with fungicidal or insecticidal powders, or applying the fumes of lethal gases. For the composition of the numerous liquids and powders special works must be consulted, but the following principles apply generally. The poison must not be strong enough to injure the roots, leaves, &c., of the host-plant, or allowed to act long enough to bring about such injury. Care and intelligence are especially needful with certain insecticides such as poisonous gases, or the operators may suffer. It is worse than useless to apply drastic remedies if the main facts of the life-history of the pest are not known; e.g. the application of ordinary antiseptic powders to leaves inside which a Fungus, such as a Uredo or Ustilago, is growing can only result in failure, and similarly if tobacco fumes, for instance, are applied when the insects concerned are hibernating in the ground beneath. Such applications at the moment when spores are germinating on the leaves, e.g. Peronospora, or to the young mycelia of epiphytic parasites, e.g. Erysiphe, or the steeping in hot water of thoroughly ripe hard grains to which spores are attached, e.g. Ustilago, and filling a greenhouse with hydrocyanic acid gas when young insects are commencing their ravages, e.g. Red-spider—all these and similar procedures timed to catch the pest at a vulnerable stage are intelligent and profitable prophylactic measures, as has been repeatedly shown. Numerous special methods of preventing the spread of Fungi, or the migrations of insects, or of trapping various animals; of leaving infested ground fallow, or of growing another crop useless to the pest, &c., are also to be found in the practical treatises. More indirect methods, such as the grafting of less resistant scions on more vigorous stocks, of raising special late or early varieties by crossing or selection, and so on, have also met with success; but it must be understood that “resistant” in such cases usually means that some peculiarity of quick growth, early ripening or other life-feature in the plant is for the time being taken advantage of. Among the most interesting modern means of waging war against epidemic pests is that of introducing other epidemics among the pests themselves—e.g. the infection of rats and mice with disease bacilli, or of locusts with insect-killing Fungi, and signs of the successful carrying out of such measures are not wanting. That the encouragement of insectivorous birds has been profitable is well established, and it is equally well-known that their destruction may lead to disastrous insect plagues.
Diseases and Symptoms—The symptoms of plant diseases are, as already said, apt to be very general in their nature, and are sometimes so vaguely defined that little can be learned from them as to the causes at work. We may often distinguish between primary symptoms and secondary or subordinate symptoms, but for the purposes of classification in an article of this scope we shall only attempt to group the various cases under the more obvious signs of disease exhibited.
1. Discolorations are among the commonest of all signs that a plant is “sickly” or diseased. The principal symptom may show itself in general pallor, including all cases where the normal healthy green hue is replaced by a sickly yellowish hue indicating that the chlorophyll apparatus is deficient. It may be due to insufficient illumination (Etiolation), as seen in geraniums kept in too shaded a situation, and is then accompanied by soft tissues, elongation of internodes, leaves usually reduced in size, &c. The laying of wheat is a particular case. False etiolation may occur from too low a temperature, often seen in young wheat in cold springs. Cases of pallor due to too intense illumination and destruction of chlorophyll must also be distinguished. Chlorosis is a form of pallor where the chlorophyll remains in abeyance owing to a want of iron, and can be cured by adding ferrous salts. Lack of other ingredients may also induce chlorotic conditions. Yellowing is a common sign of water-logged roots, and if accompanied by wilting may be due to drought. Over-transpiration in bright wintry weather, when the roots are not absorbing, often results in yellowing. In other cases the presence of insects, Fungi or poisons at the roots may be looked for. Albinism, with which variegated foliage may be considered, concerns a different set of causes, still obscure, and usually regarded as internal, though experiments go to show that some variegation's are infectious.
2. Spotted Leaves, &c.—Discoloured spots or patches on leaves and other herbaceous parts are common symptoms of disease, and often furnish clues to identification of causes, though it must be remembered that no sharp line divides this class of symptoms from the preceding. By far the greater number of spot-diseases are due to Fungi, as indicated by the numerous “leaf-diseases” described, but such is by no means always the case. The spot or patch is an area of injury; on (or in) it the cell-contents are suffering destruction from shading, blocking of stomata, loss of substance or direct mechanical injury, and the plant suffers in proportion to the area of leaf surface put out of action. It is somewhat artificial to classify these diseases according to the colour of the spots, and often impossible, because the colour may differ according to the age of the part attacked and the stage of injury attained; many Fungi, for instance, induce yellow spots which become red, brown or black as they get older, and so on. White or grey spots may be due to Peronospora, Erysiphe, Cystopus, Entyloma and other Fungi, the mycelium of which will be detected in the discoloured area; or they may be scale insects, or the results of punctures by Red-spider, &c. Yellow spots, and especially bright orange spots, commonly indicate Rust Fungi or other Uredineae; but Phyllosticta, Exoascus, Clasterosporium, Synchytrium, &c., also induce similar symptoms. Certain Aphides, Red-spider, Phylloxera and other insects also betray their presence by such spots. It is a very common event to find the early stages of injury indicated by pale yellow spots, which turn darker, brown, red, black, &c., later, e.g. Dilophia, Rhytisma, &c. Moreover, variegations deceptively like these disease spots are known, e.g. Senecio Kaempferi. Red spots may indicate the presence of Fungi, e.g. Polystigma, or insects, e.g. Phytoptus. Brown spots are characteristic of Phytophthora, Puccinia, &c., and black ones of Fusicladium, Ustilago, Rhytisma, &c. Both are common as advanced symptoms of destruction by Fungi and insects. Brilliantly coloured spots and patches follow the action of acid fumes on the vegetation near towns and factories, and such parti-coloured leaves often present striking resemblance to autumn foliage. Symptoms of scorching owing to abnormal insolation—e.g. in greenhouses where the sun's rays are concentrated on particular spots—and a certain class of obscure diseases, such as “silver-leaf” in plums, “foxy leaves” in various plants, may also be placed here.
3. Wounds.—The principal phenomena resulting from a simple wound, and the response of the irritated cells in healing by cork and in the formation of callus, have been indicated above. Any clean cut, fracture or bruise which injures the cambium over a limited area is met with the same response. The injured cells die and turn brown; the living cells beneath grow out, and form cork, and under the released pressure bulge outwards and repeatedly divide, forming a mass of succulent regenerative tissue known as callus. Living cells of the pith, phloem, cortex, &c., may also co-operate in this formation of regenerative tissue, and if the wound is a mere knife-cut in the “bark,” the protruding lips of callus formed at the edges of the wound soon meet, and the slit is healed over—occluded. If a piece of bark and cortex are torn off, the occlusion takes longer, because the tissues have to creep over the exposed area of wood; and the same is true of a transverse cut severing the branch, as may be seen in an properly pruned tree. Wounds may be artificially grouped under such heads as the following: Burrows and excavations in bark and wood due to boring insects, especially beetles. Breakages and abrasions due to wind, snow, lightning, and other climatic agents. Cuts, breakages, &c., due to man and other vertebrate animals. Erosions of leaves and herbaceous parts by caterpillars, slugs, earwigs and so forth. Frost-cracks, scorching of bark by sun and fire, &c., and wounds due to plants which entwine, pierce or otherwise materially injure trees, &c., on a large scale.
4. Excrescences.—Outgrowths, more or less abnormal in character, are frequent signs of diseased organs. They are due to hypertrophy of young tissues, which may undergo profound alterations subsequently, and occur on all parts of the plants. The injury which initiates them may be very slight in the first place—a mere abrasion, puncture or Fungus infection—but the minute wound or other disturbance, instead of healing over normally, is frequently maintained as a perennial source of irritation, and the regenerative tissues grow on month after month or year after year, resulting in extraordinary outgrowths often of large size and remarkable shape. Excrescences may be divided into those occurring on herbaceous tissues, of which Galls are well-known examples, and those found on the wood stem, branches, &c., and themselves eventually woody, of which Burrs of various kinds afford common illustrations. Among the simplest examples of the former are the hairs which follow the irritation of the cells by mites. These hairs often occur in tufts, and are so coloured and arranged that they were long taken for Fungi and placed in the “genus” Erineum.
Cecidia or galls arise by the hypertrophy of the subepidermal cells of a leaf, cortex, &c., which has been pierced by the ovipositor of an insect, and in which the egg is deposited. The irritation set up by the hatching egg and its resulting larva appears to be the stimulus to development, and not a poison or enzyme injected by the insect. The extraordinary forms, colours and textures of the true galls have always formed some of the most interesting of biological questions, for not only is there definite co-operation between a given species of insect and of plant, as shown by the facts that the same insect may induce galls of different kinds on different plants or organs, while different insects induce different galls on the same plant—e.g. the numerous galls on the oak—but the gall itself furnishes well adapted protection and abundant stores of nutriment to its particular larva, and often appears to be borne without injury to the plant. This latter fact is no doubt due to the production of an excess of plastic materials over and above what the tree requires for its immediate needs. Galls in the wide sense—technically Cecidia—are not always due to insects. The nodules on the roots of leguminous plants are induced by the presence of a minute organism now known to do no injury to the plant. Those on turnips and other Cruciferae are due to the infection of Plasmodiophora, a dangerously parasitic Myxomycete. Nodules due to “eel-worms” (Nematodes) are produced on numerous classes of plants, and frequently result in great losses—e.g. tomatoes, cucumbers, &c.; and the only too well known Phylloxera, which cost France and other vine-growing countries many millions sterling, is another case in point. Fungus-galls on leaves and stems are exemplified by the “pocket-plums” caused by the Exoasceae, the black blistering swellings of Ustilago Maydis, the yellow swellings on nettles due to Aecidium, &c.
In many cases the swellings on leaves are minute, and may be termed pustules—e.g. those due to Synchytrium, Protomyces, Cystopus, many Ustilagineae, &c. These cases are not easily distinguished superficially from the pustular outgrowth of actual mycelia and spores (stromata) of such Fungi as Nectria, Puccinia, &c. The cylindrical stem-swellings due to Calyptospora, Epichloe, &c., may also be mentioned here, and the tyro may easily confound with these the layers and cushions of eggs laid on similar organs by moths. There is a class of gall-like or pustular outgrowths for which no external cause has as yet been determined, and which are therefore often ascribed to internal causes of disease. Such are the cork-warts on elms, maples, &c., and the class of outgrowths known as Intumescences. Recent researches point to definite external conditions of moisture, affecting the processes of respiration and transpiration, &c., as being responsible for some of those. The “scab” of potatoes is another case in point. Frost blisters are pustular swellings due to the up-growth of callus-tissue into cavities caused by the uprising of the superficial cortex under the action of intense cold.
Turning now to outgrowths of a woody nature, the well-known burrs or “knaurs,” so common on elms and other trees are cases in point. They are due to some injury—e.g. bruising by a cartwheel, insects—having started a callus on which adventitious buds arise, or to the destruction of buds at an early stage. Then, stores of food-material being accumulated at the injured place, other buds arise at the base of or around the injured one. If matters are propitious to the development of these buds, then a tuft of twigs is formed and no burr; but if the incipient twigs are also destroyed at an early stage, new buds are again formed, and in larger numbers than before, and the continued repetition of these processes leads to a sort of conglomerate woody mass of fused bud-bases, not dead, but unable to grow out, and thus each contributing a crowded portion of woody material as it slowly grows. There are many varieties of burrs, though all woody outgrowths of old trees are not to be confounded with them, e.g. the “knees” of Taxodium, &c. Many typical burrs might be described as witches'-brooms, with all the twigs arrested to extremely short outgrowths. Witches'-brooms are the tufted bunches of twigs found on silver firs, birches and other trees, and often present resemblances to birds' nests or clumps of mistletoe if only seen from a distance. They are branches in whicha perennial Fungus (Aecidium, Exoascus, &c.) has obtained a hold. This Fungus stimulates the main twig to shoot out more twigs than usual; the mycelium then enters each incipient twig and stimulates it to a repetition of the process, and so in the course of years large broom-like tufts result, often markedly different from the normal.
But undoubtedly the most important of the woody excrescences on trees are cankers. A canker is the result of repeated frustrated attempts on the part of the callus to heal up a wound. If a clean cut remains clean, the cambium and cortical tissues soon form callus over it, and in this callus—regenerative tissue—new wood, &c., soon forms, and if the wound was a small one, no trace is visible after a few years. But the occluding callus is a mass of delicate succulent cells, and offers a dainty morsel to certain insects—e.g. Aphides—and may be easily penetrated by certain Fungi such as Peziza, Nectria; and when thus attacked, the repeated conflicts between the cambium and callus, on the one hand, trying to heal over the wound, and the insect or Fungus, on the other, destroying the new tissues as they are formed, results in irregular growths; the still uninjured cambium area goes on thickening the branch, the dead parts, of course, remain unthickened, and the portion in which the Fungus is at work may for the time being grow more rapidly. Such cankers often commence in mere insect punctures, frosted buds, cracks in the cortex, &c., into which a germinating spore sends its hypha. The seriousness of the damage done is illustrated by the ravages of the larch disease, apple canker, &c.
5. Exudations and Rotting.—The outward symptoms of many diseases consist in excessive discharges of moisture, often accompanied by bursting of over-turgid cells, and eventually by putrefactive changes. Conditions of hyper-turgescence are common in herbaceous plants in wet seasons, or when overcrowded and in situations too moist for them. This unhealthy state is frequently combined with etiolation: what is termed rankness is a particular case, and if the factors concerned are removed by drainage, weeding out, free transpiration, &c., no permanent harm may result. With seedlings and tender plants, however, matters are frequently complicated by the onslaughts of Fungi—e.g. Pythium, Peronospora, Completoria, Volutella, Botrytis, &c. That such overturgescence should lead to the bursting of fleshy fruits, such as gooseberries, tomatoes and grapes, is not surprising, nor can we wonder that fermentation and mould Fungi rapidly spread in such fruits; and the same is true for bulbs and herbaceous organs generally. The rotting of rhizomes, roots, &c., also comes into this category; but while it is extremely difficult in given cases to explain the course of events in detail, certain Fungi and bacteria have been so definitely associated with these roots—e.g. beet-rot, turnip disease, wet-rot of potatoes—that we have to consider each case separately. It is, of course, impossible to do this here, but I will briefly discuss one or two groups of cases.
Honey-dew.—The sticky condition of leaves of trees—e.g. lime—in hot weather is owing to exudation's of sugar. In many cases the punctures of Aphides and Coccideae are shown to be responsible for such exudations, and at least one instance is known where a Fungus—Claviceps—causes it. But it also appears that honey-dew may be excreted by ordinary processes of over-turgescence pressing the liquid through water-pores, as in the tropical Caesalpinia, Calliandra, &c. That these exudations on leaves should afterwards serve as pabulum for Fungi—e.g. Fumago, Antennaria—is not surprising, and the leaves of limes are often black with them.
Flux.—A common event in the exudation of turbid, frothing liquids from wounds in the bark of trees, and the odours of putrefaction and even alcoholic fermentation in these are sufficiently explained by the coexistence of albuminous and saccharine matters with fungi, yeasts and bacteria in such fluxes. It is clear that in these cases the obvious symptom—the flux—is not the primary one. Some wound in the succulent tissues has become infected by the organisms referred to, and their continued action prevents healing. At certain seasons the wound “bleeds,” and the organisms—some of which, by the bye, are remarkable and interesting forms—multiply in the nutritious sap and ferment it. The phenomenon is, in fact, very like that of the fermentation of palm wine and pulque, where the juices are obtained from artificial cuts.
Comparable with these cases is that of Cuckoo-spit, due to the juices sucked out by Aphrophthora, on herbaceous plants of all kinds. Outflows of resin—Resinosis—also come under this general heading; but although some resin-fluxes are traced to the destructive action of Agaricus melleus in Conifers, others, as well as certain forms of Gummosis, are still in need of explanation.
Bacteriosis.—Many of the plant diseases involving rot have been ascribed to the action of bacteria, and in some cases—e.g. cabbage-rot, bulb-rot of hyacinths, &c., carnation disease—there is evidence that bacteria are causally connected with the disease. It is not sufficient to find bacteria in the rotting tissues, however, nor even to be successful in infecting the plant through an artificial wound, unless very special and critical precautions are taken, and in many of the alleged cases of bacteriosis the saprophytic bacteria in the tissues are to be regarded as merely secondary agents.
6. Necrosis—A number of diseases the obvious symptoms of which are the local drying up and death of tissues, in many cases with secondary results on organs or parts of organs, may be brought together under this heading. No sharp line can be drawn between these diseases and some of the preceding, inasmuch as it often depends on the external conditions whether necrosis is a dry-rot, in the sense I employ the term here, or a wet-rot, when it would come under the preceding category. The “dying back” of the twigs of trees and shrubs is a frequent case. The cortical tissues gradually shrink and dry up, turning brown and black in patches or all over, and when at length the cambium and medullar ray tissues dry up the whole twig dies off. This may be due to frost, especially in “thin-barked” trees, and often occurs in beeches, pears, &c., or it may result from bruising by wind, hailstones, gun-shot wounds in coverts, &c., the latter of course very local. It is the common result of fires passing along too rapidly to burn the trees; and “thin-barked” trees—hornbeam, beech, firs, &c.—may exhibit it as the results of sunburn, especially when exposed to the south-west after the removal of shelter. The effects of frost and of sunburn are frequently quite local. The usual necrosis of the injured cortex occurs—drying up, shrivelling, and consequent stretching and cracking of the dead cortex on the wood beneath. Such frost-cracks, sun-cracks, &c., may then be slowly healed over by callus, but if the conditions for necrosis recur the crack may be again opened, or if Fungi, &c., interfere with occlusion, the healing is prevented, in such cases the local necrosis may give rise to cankers. The dying back of twigs may be brought about by many causes. General attacks of leaf-diseases invariably lead to starvation and necrosis of twigs, and similarly with the ravages of caterpillars and other insects. Drought and consequent defoliation result in the same, and these considerations help us to understand how old-established trees in parks, &c., apparently in good general health, become “stag-headed” by the necrosis of their upper twigs and smaller branches: the roots have here penetrated into subsoil or other unsuitable medium, or some drainage scheme has deprived them of water, &c., and a dry summer just turns the scale. Such phenomena are not uncommon in towns, where trees with their roots under pavement or other impervious covering do well for a time, but suddenly fail to supply the crown sufficiently with water during some hot summer.
7. Monstrosities.—A large class of cases of departure from the normal form, depending on different and often obscure causes, may be grouped together under this heading, most of them are of the kind termed Teratological, and it is difficult to decide how far they should be regarded as pathological if we insist that a disease threatens the existence of the plant, since many of these malformations—e.g. double flowers, phyllody of floral parts, contortions and fascinations, dwarfing, malformed leaves, &c.—can not only be transmitted in cultivation, but occur in nature without evident injury to the variety. In many cases, however, monstrosities of flowers have been shown to be due to the irritating action of minute insects or Fungi, and others are known which, although induced by causes unknown to us, and regarded as internal, would not be likely to survive in the wild condition. This subject brings the domain of pathology, however, into touch with that of variation, and we are profoundly ignorant as to the complex of external conditions which would decide in any given case how far a variation in form would be prejudicial or otherwise to the continued existence of a species. Under the head of malformations we place cases of atrophy of parts or general dwarfing, due to starvation, the attacks of Fungi or minute insects, the presence of unsuitable food-materials and so on, as well as cases of transformation of stamens into petals, carpels into leaves, and so forth. Roots are often flattened, twisted and otherwise distorted by mechanical obstacles, stems by excess of food in rich soils, the attacks of minute parasites, overgrowth by climbing plants, &c. Leaves are especially apt to vary, and although the formation of crests, pitchers, puckers, &c., must be put down to the results of abnormal development, it is often difficult to draw the line between teratological and merely varietal phenomena. For instance, the difference between the long-stalked and finely-cut leaves of Anemone attacked with rust and the normal leaves with broad segments, or between the urceolate leaves occasionally found on cabbages and the ordinary form—in these cases undoubtedly pathological and teratological respectively—is nothing like so great as between the upper and lower normal leaves of many Umbelliferae or the submerged and floating leaves of an aquatic Ranunculus or Cabomba. When we come to phenomena such as proliferations, vivipary, the development of “Lammas shoots,” adventitious buds, epicormic branches, and to those malformations of flowers known as peloria, phyllody, virescence, &c. while assured that definite, and in man cases recognizable, physiological disturbances are at work, we find yourselves on the borderland between pathological and physiological variation, where each case must be examined with due regard to all the circumstances, and no generalization seems possible beyond what has been sketched. This is equally true of the phenomena of apogamy and apospory in the light of recent researches into the effects of external conditions on reproduction.
This sketch of an enormous subject shows us that the pathology of plants is a special department of the study of variations which threaten injury to the plant, and passes imperceptibly into the study of variations in general. Moreover, we have good reasons for inferring that different constellations of external causes may determine whether the internal physiological disturbances induced by a given agent shall lead to pathological and dangerous variations, or to changes which may be harmless or even advantageous to the plant concerned.
Authorities.—General and Historical.—Berkeley, “Vegetable Pathology,” Gardener's Chronicle (1854) p. 4; Plowright, British Uredineae and Ustilagineae (1889); Eriksson and Henning, Die Getreideroste (Stockholm, 1896); De Bary, Comparative Morph. and Biol. of the Fungi, &c. (1887); Frank, Die Krankheiten der Pflanzen (1895-1896); Sorauer, Handbuch der Pflanzenkrankheiten (1906); Ward, Disease in Plants (1901). Causes of Disease, &c.—Pfeffer, Physiology of Plants (Oxford, 1900); Sorauer, Treatise on the Physiology of Plants (1895); Bailey, The Principles of Agriculture (1898); Lafar, Technical Mycology (1898); Hartig, Diseases of Trees (1894); Marshall Ward, Proc. Roy. Soc. (1890) xlvii. 394; and Timber and some of its Diseases (London, 1889). Fungi.—See Fungi and Bacteria; also Marshall Ward, Diseases of Plants (Romance of Science Series), S.P.C.K.; Massee, Text-Book of Plant Diseases (1899); Tubeuf, Diseases of Plants (London, 1897). Insects.—Ormerod, Manual of Injurious Insects (1890); C. V. Riley, Insect Life, U.S. Department of Agriculture (1888-1895); Judeich and Mitsche, Lehrbuch der mitteleuropaischen Forstinsektenkunde (Vienna, 1889). Healing of Wounds, &c.—Shattock, “On the Reparatory Processes which occur in Vegetable Tissues,” Journ. Linn. Soc. (1882) xix. 1; Richards, “The Respiration of Wounded Plants,” Ann. of Bot. (1896), x. 531; and “The Evolution of Heat by Wounded Plants,” Ann. of Bot. (1897), xi. 29. Enzymes.—Green, The Soluble Ferments and Fermentation (1899). Chemotaxis, &c.—Miyoshi, “Die Durchbohrung von Membranen durch Pilzfaden,” Pringsh. Jahrb., B. (1895), xxviii. 269, and literature. Parasitism, &c.—Marshall Ward, “On some Relations between Host and Parasite, &c.,” Proc. Roy. Soc. xlvii. 393; and “Symbiosis,” Ann. of Bot. (1899), xiii. 549, with literature. Specialization of Parasitism—Salmon, in Massee's Text-Book of Fungi (1906), pp. 146-157. Statistics.—See Wyatt, Agricultural Ledger (Calcutta, 1895), p. 71; Balfour, The Agricultural Pests of India (1887), p. 13; Eriksson and Henning, Die Getreideroste; the publications of the U.S. Agricultural Department; the Kew Bulletin; Zeitschrift für Pflanzenkrankheiten, and elsewhere. Spraying, &c.—See Lodeman, The Spraying of Plants (1896), and numerous references in the publications of U.S. Agricultural Department, Zeitschr. f. Pflanzenkrankheiten, the Gardener's Chronicle, &c. Etiolation, &c.—Pfeffer, Physiology of Plants, and other works on physiology. Albinism, &c.—Church, “A Chemical Study of Vegetable Albinism,” Journ. Chem. Soc. (1879, 1880 and 1886); Beijerinck, “Ueber ein Contagium,” &c., in Verhandl. d. kön. Acad. v. Wet. (Amsterdam, 1898); Koning in Zeitschr. f. Pflanzenkrankh. (1899), ix. 65; Baur, Ber. deutschen bot. Ges. (1904), xxii. 453; Sitzungsber. berlin. Akad. (Jan. 6, 1906); Hunger, Zeitschr. f. Pflanzenkrankheiten (1905) xv. Heft 5. Wounds, &c.—Marshall Ward, Timber and some of its Diseases, p. 210; Hartig, Diseases of Trees (London, 1894). Cecidia and Galls.—Küster, “Beiträge zur Kenntniss der Gallenanatomie,” Flora (1900), p. 117; Pathologische Pflanzenanatomie (1903); Molliard, Revue générale de bot. (1900), p. 157. Canker.—Frank, Krankheiten der Pflanzen, and papers in Zeitschr. f. Pflanzenkrankh. Rotting, &c.—Migula, Krit. Uebersicht derjenigen Pflanzenkrankheiten, welche angeblich durch Bakterien verursacht werden (1892); Smith, “Pseudomonas campestris,” Cent. f. Bakt. B. iii. 284 (1897); Arthur and Bolley, Bacteriosis of Carnations, Purdue Univ. Agr. Station (1896), vii. 17; A. F. Woods, “Stigmonose, a Disease of Carnations,” Vegetable, Physiol. and Pathol. Bull. 19 U.S. Department of Agriculture (1900); Sorauer, Handbuch der Pflanzenkrankheiten (1905), 18-93. Frost, Drought, &c.—Hartig, Lehrbuch der Anat. und Phys. der Pflanzen; Fischer, Forest Protection, iv. of Schlich's Manual of Forestry. Teratology, &c.—Masters, Vegetable Teratology, Ray Society (1869); Molliard, “Cécidies florales,” Ann. Sci. Nat. sér. 8, i. (bot.) p. 67 (1895). (H. M. W.)
Ecology of Plants
Introduction.—The word ecology is derived from οῖκος, a house (habitat), and λόγος, a discourse. As a botanical term, ecology denotes that branch of botany which comprises the study of the relations of the individual plant, or the species, or the plant community with the habitat. Following Schröter (Flahault and Schröter, 1910: 24), the term autecology may be used for the study of the habitat conditions in relation to the single species, and the term synecology for this study in relation to plant communities.
From the phytogeographical standpoint, ecology is frequently termed ecological plant geography. Thus Warming (1901: 1 and 2) subdivided plant geography into floristic plant geography and ecological plant geography. The former is concerned with the division of the earth's surface into major districts characterized by particular plants or taxonomic groups of plants, with the subdivision of these floristic districts, and with the geographical distribution (both past and present) of the various taxonomic units, such as species, genera, and families. On the other hand, ecological plant geography seeks to ascertain the distribution of plant communities, such as associations and formations, and enquires into the nature of the factors of the habitat which are related to the distribution of plants—plant forms, species, and communities. In a general way, floristic plant geography is concerned with species, ecological plant geography with vegetation. The study of the distribution of species dates back to the time of the early systematists, the study of vegetation to the time of the early botanical travellers. Humboldt, for example, defined his view of the scope of plant geography as follows: “C'est cette science qui considère les vegetaux sous les rapports de leur association locale dans les différents climats” (1807: 14).
The Habitat.—The term habitat, in its widest sense, includes all the factors of the environment which affect a plant or a plant community, though the term is frequently used to signify only some of these factors. The factors of the habitat may be grouped as follows: geographical, physical, and biological.
Geographical Factors.—Geographical position determines the particular species of plants which grow in any particular locality. This matter is bound up with the centres of origin and with the past migrations of species, and such questions are usually treated as a part of floristic plant geography. Here, therefore, floristics and ecology meet. Flahault and Schröter, in defining the term habitat, appear to exclude all geographical factors. They state that “the term habitat is understood to include everything relating to the factors operative in a geographically defined locality, so far as these factors influence plants” (1910: 24); but the exclusion of geographical and historical factors from the concept of the habitat does not appear to be either desirable or logical.
Physical Factors.—These are frequently classified as edaphic or soil factors and climatic factors; but there is no sharp line of demarcation between them. Edaphic factors include all those relating to the soil. The water content of the soil, its mineral content, its humus content, its temperature, and its physical characteristics, such as its depth and the size of its component particles are all edaphic factors. Climatic factors include all those relating to atmospheric temperature, rainfall, atmospheric humidity, and light and shade. Factors connected with altitude, aspect, and exposure to winds are also climatic such are often spoken of as physiographical factors. The difficulty of sharply delimiting edaphic and climatic factors is seen in the case of temperature. Soil temperature is partly dependent on the direct rays of the sun, partly on the colour and constitution of the soil, and partly on the water content of the soil. Again, the temperature of the air is affected by radiation from the soil; and radiation differs in various soils.
Biological Factors.—These include the reactions of plants and animals on the habitat. Here again, no sharp boundary-line can be drawn. In one sense, the accumulation of humus and peat is a biological factor, as it is related to the work of organisms in the soil; but the occurrence or otherwise of these organisms in the soil is probably related to definite edaphic and climatic conditions. Again, the well-known action of earthworms may be said to be a biological work, but the resulting aeration of the soil causes edaphic differences, and earthworms are absent from certain soils, such as peat. The pollination of flowers and the dispersal of seeds by various animals are biological factors; but pollination and dispersal by the wind cannot be so regarded. The influence of man on plants and vegetation is also a biological factor, which is frequently ignored as such, and treated as if it were a thing apart.
When the nature and effect of ecological factors have become more fully understood, it will be possible to dispense with the above artificial classification of factors, and to frame one depending on the action of the various factors; but such a classification is not possible in the present state of knowledge.
Ecology and Physiology.—Whilst our knowledge of the nature and effect of habitat is still in a very rudimentary condition, much progress has been made in recent years in the study of plant communities, but even here the questions involved in relating the facts of the distribution of plant communities to the factors of the habitat are very imperfectly understood. This is due to a lack of precise knowledge of the various habitat factors and also of the responses made by plants to these factors. Until much more advance has been made by ecologists in the investigation of the nature of habitat factors, and until the effect of the factors on the plants has been more closely investigated by physiologists, it will remain impossible to place ecology on a physiological basis: all that is possible at present is to give a physiological bias to certain aspects of ecological research. Obviously no more than this is possible until physiologists are able to state much more precisely than at present what is the influence of common salt on the plants of salt-marshes, of the action of calcium carbonate on plants of calcareous soils, and of the action of humous compounds on plants of fens and peat moors.
Ecological Classes.—Many attempts have been made to divide plants and plant communities into classes depending on habitat factors. One of the best known classifications on these lines is that by Warming. Warming recognized and defined four ecological classes as follows:—
Hydrophytes.—These live in a watery or wet substratum, with at least 80% of water. Warming included plants of peat-bogs among his hydrophytes.
Xerophytes.—These are plants which live in very dry places, where the substratum has less than 10% of water
Halophytes.—These are plants living in situations where the substratum contains a high proportion of sodium chloride.
Mesophytes.—These are plants which live in localities which are neither specially dry nor specially wet nor specially salty.
Such terms as hydrophytes, xerophytes, and halophytes had been used by plant geographers before Warming's time e.g., by Schouw; and the terms evidently supply a want felt by botanists as they have come into general use. However, the terms are incapable of exact definition, and are only useful when used in a very general way. The above classification by Warming, although it was without doubt the best ecological classification which had, at the time, been put forward, has not escaped criticism. The criticisms were directed chiefly to the inclusion of sand dune plants among halophytes, to the exclusion of halophytes from xerophytes, to the inclusion of “bog xerophytes” among hydrophytes, to the inclusion of all conifers among xerophytes and of all deciduous trees among mesophytes, and to the group of mesophytes in general.
Schimper made a distinct advance when he distinguished between physical and physiological dryness or wetness of the soil. A soil may be physically wet, but if the plants absorb the water only with difficulty, as in a salt marsh, then the soil is, as regards plants, physiologically dry. All soils which are physically dry are also physiologically dry, and hence only the physiological dryness or wetness of soils need be considered in ecology.
Schimper used the term xerophytes to include plants which live in soils which are physiologically dry, and the term hygrophytes those which live in soils which are physiologically wet or damp. Schimper recognized that the two classes are connected by transitional forms, and that it is useless to attempt to give the matter a statistical basis. It is only in a general sense like Schimper's that such ecological terms as xerophytes have any value, and it is not possible, at least at present, to frame ecological classes, which shall have a high scientific value, on a basis of this nature. Whilst Schimper objected to the constitution of a special category, such as mesophytes, to include all plants which are neither pronounced xerophytes nor pronounced hygrophytes, he recognized the necessity of a third class in which to place those plants which, like deciduous trees and bulbous plants, are hygrophytes during one season of the year and xerophytes during another season of the year. Such plants, which comprise the great majority of the species of the central European flora, Schimper termed tropophytes.
Recently, Warming (1909: 136), assisted by Vahl, has modified his earlier classification, and adopted the following:—
A. The soil (in the widest sense) is very wet, and the abundant water is available to the plant (at least in hydrophytes).
1. Hydrophytes.—These include plants of the plankton, or microphytes that float free on water, of the pleuston, or macrophytes which float on or are suspended in water, and of the benthos, or all aquatic plants which are fixed to the substratum.
2. Helophytes.—These are marsh plants which normally have their roots in soaking soil but whose branches and foliage are more or less aerial. Warming admits there is no sharp limit between marsh plants and land plants; and it seems equally obvious that there is no sharp limit between some of his helophytes and some of his hydrophytes. For example, the difference between aquatic plants with floating leaves, such as the yellow water-lily (Nymphaea lutea) and those with erect leaves, such as Typha angustifolia, is probably more apparent than real. Among helophytes, Warming places plants of the reed swamp, and includes such trees as the alder (Alnus rotundifolia), willows (e.g., Salix alba, S. fragilis, S. cinerea, S. pentandra), birch, and pine, when these grow in marshy places.
B. The soil is physiologically dry.
3. Oxylophytes.—These plants, sometimes spoken of as “bog xerophytes,” grow in soils which contain an abundance of free humous compounds, and include plants which grow on fens and moors.
4. Psychrophytes.—These include the plants which grow on the cold soils of subniveal and polar districts.
5. Halophytes.—These are plants which grow on saline soils.
C. The soil is physically dry.
6. Lithophytes.—These are plants which grow on “true rock,” but not “on the loose soil covering rock, even though this may entertain species that are very intimately associated with the rock. Still to this limitation an exception must be made in favour of the vegetation growing in clefts and niches” (Warming, 1909: 240). Many Algae, lichens, and mosses are included among lithophytes and also Saxifraga Aizoon, S. oppositifolia, Silene acaulis, and Gnaphalium luteo-album.
7. Psammophytes.—These are plants which grow on sand and gravel. Plants of sand-dunes, whether in maritime or inland localities, are psammophytes, as well as plants (such as Calluna vulgaris) of dune heaths, dune “bushland” or scrub, and dune forest.
8. Chersophytes.—Here are placed certain “xerophytic perennial herbs” which occur on “particular dry kinds of soil, such as limestone rocks, stiff clay, and so forth” (Warming, 1909: 289).
D. The climate is very dry, and the properties of the soil are decided by climate.
9. Eremophytes.—Under this term, are placed plants of deserts and steppes.
10. Psilophytes.—Here are placed plants found in “savannah-vegetation,” viz. (i.) “thorny savannah-vegetation, including: (a) orchard-scrub, (b) thorn-bushland and thorn-forest; (ii.) true savannah: tropical and sub-tropical savannah; (iii.) savannah-forest, including bush-forest in Africa and ‘campos serrados’ in Brazil” (Warming, 1909: 293 et seq.).
11. Sclerophyllous formations, e.g., garigues, mäquis, and forests of evergreen oaks (Q. Ilex, Q. Ballota, Q. Suber), and of Eucalyptus spp.
E. The soil is physically or physiologically dry.
12. Coniferous forest formations, e.g., of Pinus sylvestris, Picea excelsa, Abies pectinata, Larix sibirica, L. decidua.
F. “Soil and climate favour the development of mesophilous formations.”
13. Mesophytes.—Warming defines mesophytes as “plants that show a preference for soil and air of moderate humidity, and avoid soil with standing water or containing a great abundance of salts” (1909: 317). Under mesophytes, Warming places plants occurring in “Arctic and Alpine mat-grassland and mat-herbage,” in “mat-vegetation of the Alps,” in meadows, in pasture on cultivated soil, in “mesophytic bushland,” in deciduous dicotyledonous forests, and in evergreen dicotyledonous forests.
This new system of Warming's, whilst probably too involved ever to come into general use, must be taken as superseding his older one; and perhaps the best course open to botanists is to select such terms as appear to be helpful, and to use the selected terms in a general kind of way and without demanding any precise definitions of them: it must also be borne in mind that the various classes are neither mutually exclusive nor of equivalent rank. From this point of view, the following terms will perhaps be found the most serviceable:—
Hydrophytes (submerged aquatic plants).—Plants whose vegetative organs live wholly in water; e.g., most Algae, many mosses, such as Fontinalis spp., and liverworts, such as Jungermannia spp.; a few Pteridophytes, such as Pilularia spp., Isoëtes spp.; several flowering plants, such as Potamogeton pectinatus, Ceratophyllum spp., Hottonia palustris, Utricularia spp., Littorella lacustris.
Hemi-hydrophytes (swamp plants, marsh plants &c.).—Plants whose vegetative organs are partly submerged and partly aerial; Vaucheria terrestris, Philonotis fontana, Scapania undulata, Marsilia spp., Salvinia natans, Azolla spp., Equisetum limosum, Typha angustgolia, Phragmites communis, Scirpus lacustris, Nymphaea lutea, Oenanthe fistulosa, Bidens cernua.
Hygrophytes.—Plants which are sub-evergreen or evergreen but not sclerophyllous, and which live in moist soils; e.g., Lastraea Felix-mas, Poa pratensis, Carex ovalis, Plantago lanceolata, and Achillaea Millefolium.
Xerophytes.—Plants which grow in very dry soils; e.g., most lichens, Ammophila (Psamma) arenaria, Elymus arenarius, Anabasis aretioides, Zilla macroptera, Sedum acre, Bupleurum spinosum, Artemisia herba-alba, Zollikofferia arborescens.
Halophytes.—Plants which grow in very saline soils; e.g., Triglochin maritimum, Salicornia spp., Zygophyllum cornutum, Aster Tripolium, Artemisia maritima. It should be recognized, however, that “a halophyte, in fact, is one form of xerophyte” (Warming, 1909: 219).
Sclerophyllous Plants.—These are plants with evergreen leathery leaves, an typical of tropical, sub-tropical, and warm temperate regions; e.g., Quercus Suber, Ilex Aquifolium, Hedera Helix, Eucalyptus Globulus, Rosmarinus officinalis. Sclerophyllous leaves are usually characterized by entire or sub-entire margins, a thick cuticle, small but rarely sunken stomata, a well-developed and close-set palisade tissue and a feeble system of air-spaces.
Hydro-xerophytes (“bog xerophytes”).—Plants which live in wet, peaty soils, an which possess aeration channels and xerophilous leaves; e.g., Cladium Mariscus, Eriophorum angustifolium, Rubus Chamaemorus, and Vaccinium Vitis-Idaea. The term “oxylophyte” is open to the objection that some peaty waters are alkaline, and not acidic as the term implies. Many plants of peaty soils are sclerophyllous.
Tropophytes.—Plants which are hygrophytes during some favourable part of the year and xerophytes during the rest of the year; e.g., deciduous trees and shrubs, deciduous herbaceous plants with underground perennating organs, and annuals and ephemerals.
Plant Communities.—The study of plant communities (Formationslehre or synecology) has made much progress in recent years. Even here, however, general agreement has not been reached; and the questions involved in relating the facts of the distribution of plant communities to the factors of the habitat are very imperfectly understood. Plant communities may be classified as follows:—
A plant association is a community of definite floristic composition: it may be characterized by a single dominant species; or, on the other hand, it may be characterized by a number of prominent species, one of which is abundant here, another there, whilst elsewhere two or more species may share dominance. The former are pure associations, and are well illustrated by a heather moor, whereCalluna vulgaris is the dominant plant. The latter are mixed associations, such as fens, where different facies are produced by the varying abundance of characteristic plants, such as Cladium Mariscus, Phragmites communis, Molinia coerulea, Calamagrostis lanceolata, and Juncus obtusiflorus. The different facies are possibly related to slight differences in a generally uniform habitat: it is unscientific to regard them as due to chance; still, in the majority of cases, the causes of the different facies have not been demonstrated. A local aggregation of a species other than the dominant one in an association brings about a plant society; for example, societies of Erica Tetralix, of Scirpus caespitosus, of Molinia coerulea, of Carex curta, of Narthecium ossifragum, and others may occur within an association of Calluna vulgaris. The plant societies are also doubtless due to slight variations of the habitat.
The plant association is sometimes referred to in technical language; the termination -etum is added to the stem of the generic name, and the specific name is put in the genitive. Thus an association of Quercus sessiliflora may be referred to as a Quercetum sessiliflorae.
A plant formation is a group of associations occupying habitats which are in essentials identical with each other. Thus, associations of Agropyrum (Triticum) junceum, of Carex arenaria, of Ammophila (Psamma) arenaria, and of other plants occur on sand dunes: the associations are related by the general identity of the habitat conditions, namely, the physiological dryness and the loose soil; but they are separated by differences in floristic composition, especially by different dominant species, and by minor differences of the common habitat. The whole set of associations on the sand dunes constitutes a plant formation.
The plant formation may be designated in technical language by the termination -ion added to a stem denoting the habitat. Thus, a sand dune formation may be termed an Arenarion. The associational term, in the genitive, may be added to the formational term to indicate the relationship of the formation and the association; thus, a plant association of Ammophila arenaria belonging to the plant formation of the sand dunes may be designated an Arenarion Ammophilae-arenariae (cf. Moss, op. cit. 1910: 43).
The question of universal names for vegetation units is bound up with that of the universality or otherwise of particular formations. “Remote regions which are floristically distinct . . . may possess areas physically almost identical and yet be covered by different formations” (Clements, 1905: 203). For example, the sand dunes of North America and those of western Europe are widely separated in geographical position and therefore in floristic composition, yet they are related by common physical factors. This relationship may be indicated by the addition of some prefix to the formational name. For example, an Arenarion in one climatic or geographical region might be termed an α-Arenarion and one in a different region a β-Arenarion, and so on (Moss, loc. cit.).
It is, however, frequently desirable to consider such allied formations as a single group. Such a group of formations may be designated a plant federation: and this term may be defined as a group of formations, which are characterized by common edaphic factors of the habitat, and which occur in any geographical region. Thus, different geographical or climatic regions are characterized by salt marshes. The latter all agree in their edaphic characteristics; but they differ climatically and in floristic composition. The salt marshes of a given region constitute a single plant formation: the salt marsh formations of the whole world constitute a plant federation.
Again, it is possible to arrange plant associations into groups related by a common plant form. Thus woodland associations may be classified as deciduous forests, coniferous forests, sclerophyllous forests, &c. These, in a general way, are the “formations” of Warming, and (in part) the “climatic formations” of Schimper. Thus the various reed-swamps of the whole world constitute a “formation” in Warming's sense (1909: 187).
There is much difference of opinion among ecologists and plant geographers as to which of these points of view is the most fundamental. Among British authorities, it is now customary to adopt the position of Clements, who states (1905: 292) that “the connexion between formation and habitat is so close that any application of the term to a division greater or smaller than the habitat is both illogical and unfortunate,” and that (1905: 18) “habitats are inseparable from the formations which they bear” (cf. Moss, 1910).
From the standpoint of plant communities, it is convenient to divide the earth's surface into (1) tropical districts; sub-tropical and warm temperate districts; (3) temperate districts; (4) cold temperate and frigid districts.
1. Tropical Districts.—The vegetation of tropical districts hu been subdivided by Schimper (1903:260, et seq.) as follows:— (i.) Tropical woodland: (a) rain forest, (b) monsoon forest, (c) savana forest, (d) thorn forest. (ii) Tropical grassland: (a) savana, (b) steppe. (iii.) Tropical desert: (a) scrub, (b) succulent plants, (c) perennial herbs.
Schimper regards the minor divisions as groups of “climatic formations”; and he also distinguishes certain tropical “edaphic formations,” such as mangrove swamps. He states that rain forests and high monsoon forests in the tropics occur when the average rainfall is over 70 in. (178 cm.) per annum, and that tropical thorn forest may prevail when the mean annual rainfall is below 35 in.
A tropical rain forest exhibits great variety both of species of plant and of plant forms. There is great diversity in the trees and masses of tangled lianes, and a wealth of flowers in the leafy forest crown. Humboldt points out that whilst temperate forests frequently furnish pure associations, such uniformity of association is usually absent from the tropics. Some tropical forests exhibit dense foliage from the forest floor to the topmost leafy layer; and the traveler finds the mass of foliage almost impenetrable. Other tropical forests afford a free passage and a clear outlook. It is obvious that tropical forests will eventually be subdivided into plant associations; but the difficulties of determining the relative abundance of the species of plants in the upper layers of tropical rain and monsoon forests are very great. One of the best known results of the great struggle for light which takes place in tropical forests is the number of epiphytic plants which grow on the high branches of the trees.
The leaves of the trees are frequently of leathery consistency, very glossy, usually evergreen, entire or nearly so, and seldom hairy; and thus they agree closely with the leaves of sclerophyllous forest generally.
Monsoon forests are characteristic of localities with a seasonal rainfall. The trees usually lose their foliage during the dry season and renew it during the monsoon rains. With a less abundant rainfall, savana forest and thorn forest occur. Less precipitation induces tropical grassland, which, according to Schimper (1903: 346) is of the savana type; but Warming (1909: 327) thinks that all grassland in the tropics is artificial. Still greater drought induces desert vegetation; but, as deserts are more characteristic of subtropical districts, they are discussed later on.
Mangrove swamps, or tropical tidal forests, occur in saline or brackish swamps on flat, muddy shores in the tropics; and, being almost independent of atmospheric precipitations, Schimper regards them as “edaphic formations.” However, they are climatic communities in the sense that they occur only in hot districts. Cases such as this illustrate the difficulty of regarding the distinction between “climatic formations” and “edaphic formations” as absolute. The plants exhibit markedly xerophilous structures; and many of the fruits and seeds of the mangrove trees and shrubs are provided with devices to enable them to float and with curious pneumatophores or “prop roots.” The latter serve as supports and also as a means of supplying air to the parts buried in the mud. The seedlings of characteristic species of Rhizophoraceae germinate on the trees, and probably perform some assimilatory work by means of the hypocotyl.
Other tropical “edaphic formations” occur on sandy shores, where the creeping Ipomoea biloba (Pes-caprae) and trees of Barringtonia form characteristic plant associations.
The succession of associations on new soils of a tropical shore has recently been described by Ernst.
2. Warm Temperate and Subtropical Districts.—In subtropical and warm temperate districts, characterized by mild and rainy winters and hot and dry summers, we find two types of forests. First, there are forests of evergreen trees, with thick, leathery leaves. Such forests are known as sclerophyllous forests, and they occur in the Mediterranean region, in south-west Africa, in south and south-west Australia, in central Chile, and in western California. In the Mediterranean district, forests of this type are sometimes dominated by the Cork Oak (Quercus Suber), sometimes by the Holm Oak (Q. Ilex). When these forests become degenerate, maquis and garigues respectively are produced. Maquis and garigues are characterized by the abundance of shrubs and undershrubs, especially by shrubby Leguminous plants, and by species Cistus and Lavandula. Secondly, there are forests of coniferous trees. In the Mediterranean region, even at comparatively low altitudes, forests occur of the maritime pine (Pinus maritima) and of the Aleppo pine (P. halepensis); and these forests are also related to maquis and garigues respectively in the same way as the evergreen oaks. The occurrence of forests of this type in the Mediterranean and in Arctic regions, whose dominant species belong to the same genus (Pinus) and to the same plant form, renders it difficult to regard “coniferous forests” as a natural ecological group. At much higher altitudes, in the south-west of the Mediterranean region, forests occur of the Atlantic cedar (Cedrus atlantica). These occur from about 4000 ft. (1219 m.) to about 7000 ft. (2133 m.) on the Atlas Mountains. Some sclerophyllous forests of the eastern Atlas Mountains are, owing to a comparatively high rainfall, characterized by many deciduous trees, such as Fraxinus oxyphylla, Ulmus campestris (auct. alg.), Alnus rotundifolia, Salix pedicellata, Prunus avium, &c.; and thus they have some elements in common with the deciduous forests of central Europe.
The forests of these subtropical and warm temperate regions are situated near the sea or in mountainous regions, and (as already stated) are characterized by winter rains. In inland localities, where the rainfall is much lower, steppes occur. For example, in southern Algeria, a region of steppes is situated on a flat plateau, about 3000 ft (914 metres) high, between the southern slopes of the Tell Atlas and the northern slopes of the Saharan Atlas. The rainfall, which occurs chiefly in winter, only averages about 10 in. (254 mm) per annum. Here we find open plant associations of Halfa or Esparto Grass (Stipa tenacissima) alternating with steppes of Chih (Artemisia herba-alba); and each plant association extends for several scores of miles. In the hollows of this steppe region, salt water lakes occur, known as Chotts; and on the saline soils surrounding the Chotts, a salt marsh formation occurs, with species of Salicornia, some of which are undershrubs.
Where the rainfall is still lower, deserts occur. At Ghardaia, in south-eastern Algeria, the mean annual rainfall, from 1887 to 1892, was about 4½ in. (114 mm.). In 1890, it fell as low as 2 in. (53 mm.) (Schimper, 1903: 606). At Beni Ounif and Colomb Béchar, in south-western Algeria, I was informed, in March 1910, that there had been no rain for about three years. Here the gravelly desert is characterized by “cushion plants,” such as Anabasis aretioides; by “switch plants,” such as Retama Retam; and specially by spiny pants, such as Zizyphus Lotus and Zilla macropteris; whereas succulent plants are rare. Both in the steppe and in the desert, small ephemeral species occur on the bare ground away from the large plants and especially in the wadis. Steppe and desert formations are of the open type.
3. Temperate Districts.—Temperate districts are characterized by forests of deciduous trees and of coniferous trees, the latter being of different species from those of the warm temperate districts, but frequently of the same plant form. The identity of plant form of many of the conifers of both temperate and of warm temperate districts is probably a matter of phylogenetic and not of ecological importance.
Britain is fairly typical of the west European district. In these islands, we find forests or woods of oak (Quercus Robur and Q. sessiliflora), of birch (Betula tomentosa), of ash (Fraxinus excelsior, and of beech (Fagus sylvatica). In central Scotland, forests occur of Pinus sylvestris, and, in south-eastern England, extensive plantations and self-sown woods occur of the same species.
Just as in the Mediterranean region, the degeneration of forests has given rise to maquis and garigues, so in western Europe, the degeneration of forests has brought about different types of grassland, heaths, and moors.
4. Cold Temperate and Frigid Districts.—In the coldest portion of the north temperate zone, forests of dwarfed trees occur, and these occasionally spread into the Arctic region itself (Schimiper, 1904: 685). Schimper distinguishes moss tundra, Polytrichum tundra, and lichen tundra; and the lichen tundra is subdivided into Cladonia tundra, Platysma tundra, and Alectoria heath. Where the climate is most rigorous, rock tundra occurs (p. 685).
The types of vegetation (tropical forests, sclerophyllous forest, temperate forests, tundra, &c.) thus briefly outlined are groups of Schimper's “climatic formations.” Such groups are interesting in that they are vegetation units whose physiognomy is, in a broad sense, related more to climatic than to edaphic conditions. For example, Schimper, after describing the sclerophyllous woodland of the Mediterranean district and of the Cape district, says: “The scrub of West and South Australia in its ecological aspect resembles so completely the other sclerophyllous formations that a description of it must seem a repetition.” This resemblance, however, only has reference to the general aspect or physiognomy of the vegetation and to the plant forms: the floristic composition of the various sclerophyllous—and other physiognomically allied—associations in the various geographical districts is very different; and indeed it is true that, just as the general physiognomy of plant associations is related to climate, so their floristic composition is related to geographical position. Hence, in any cosmopolitan treatment of vegetation, it is necessary to consider the groups of plant communities from the standpoint of the climatic or geographical district in which they occur; and this indeed is consistently done by Schimper. Finally, within any district of constant or fairly constant climatic conditions, it is possible to distinguish plant communities which are related chiefly to edaphic or soil conditions; and the vegetation units of these definite edaphic areas are the plant formations of some writers, and, in part, the “edaphic formations” of Schimper.
When a district like England is divided into edaphic areas, a general classification such as the following may be obtained:—
1. Physically and physiologically wet habitats, with the accompanying plant communities of lakes, reed swamps, and marshes.
2. Physically wet but physiologically dry habitats, with the accompanying plant communities of fens, moors, and salt marshes.
3. Physically and physiologically dry habitats, with the accompanying plant communities of sand dunes and sandy heaths with little humus in the soil.
4. Habitats of medium wetness, with the accompanying plant communities of woodlands and grasslands. This class may be subdivided as follows:—
a. Habitats poor in mineral salts, especially calcium carbonate, often rich in acidic humous compounds, and characterized by oak and birch woods, siliceous pasture, and heaths with much acidic humus in the sandy soil.
b. Habitats rich in mineral salts, especially calcium carbonate, poor in acidic humous compounds, and characterized by ash woods, beech woods, and calcareous pasture.
Ecological Adaptations.—It is now possible to consider the ecological adaptations which the members of plant communities show in a given geographical district such as western Europe, of which England of course forms a part. In the present state of knowledge, however, this can only be done in a very meagre fashion; as the effect of habitat factors on plants is but little understood as yet either by physiologists or ecologists.
Hydrophytes and hemi-hydrophytes (aquatic plants).—Of marine hydrophytes, there are, in this country, only the grass-wracks (Zostera marina and Z. nana) among the higher plants. Even these species are sometimes left stranded by low spring tides, though the mud in which the are rooted remains saturated with sea-water. Although many plants typical of fresh water are able to grow also in brackish water, there are only a few species which appear to be quite confined to the latter habitats in this country. Such species perhaps include Ruppia maritima, R. spiralis, Zannichellia maritima, Z. polycarpa, Potamogeton interruptus ( = P. flabellatus), and Naias marina.
In freshwater lakes and ponds, especially if the water is stagnant, aquatic plants are abundant. Aquatic vegetation may be conveniently classified as follows:—
Aquatic plants with submerged leaves: Chara spp., Naias spp., Potamogeton pectinatus, Ceratophyllum spp., Myriophyllum spp., Hottonia palustris, Utricularia spp.
Aquatic plants with submerged and floating leaves: Glyceria luitans, Ranunculus peltatus, Nymphaea (Nuphar) lutea, Callitriche stagnalis, Potamogeton polygonifolius.
Aquatic plants with floating leaves: Lemna spp., Hydrocharis Morsus-ranae, Castalia (Nymphaea) alba.
Aquatic plants with submerged leaves and erect leaves or stems: Sagittaria sagittifolia, Scirpus lacustris, Hippuris vulgaris, Sium latifolium.
Aquatic plants with erect leaves or stems (reed swamp plants): Equisetum palustre, Phragmites communis, Glyceria aquatica, Carex riparia, Iris Pseudacorus, Rumex Hydrolapathum, Oenanthe fistulosa, Bidens spp.
Marsh plants: Alopecurus geniculatus, Carex disticha, Juncus spp., Caltha palustris, Nasturtium palustre.
In many aquatic plants, the endosperm of the seed is absent or very scanty. The root-system is usually small. Root-hairs are frequently missing. The submerged stems are slender or hollow. Strengthening tissue of all kinds (and sometimes even the phloem) is more or less rudimentary. The stems are frequently characterized by aeration channels, which connect the aerial parts with the parts which are buried in practically airless mud or silt. Submerged leaves are usually filamentous or narrowly ribbon shaped, thus exposing a large amount of surface to the water, some of the dissolved gases of which they must absorb, and into which they must also excrete certain gases. Stomata are often absent, absorption and excretion of gases in solution being carried on through the epidermal layer. Chloroplastids are frequently present in the epidermal cells, as in some shade plants. Very few aquatic plants are pollinated under water, but this is well-known to occur in species of Zostera and of Naias. In such plants, the pollen grains are sometimes filiform and not spherical in shape. In the case of aquatic plants with aerial flowers, the latter obey the ordinary laws of pollination. Heterophylly is rather common among aquatic plants, and is well seen in several aquatic species of Ranunculus, many species of Potamogeton, Sagittaria sagittifolia, Scirpus lacustris, Castalia (Nymphaea) alba, Hippuris vulgaris, Callitriche spp., Sium latifolium.
Insectivorous species occur among aquatic plants; e.g. Utricularia spp., which are locally abundant in peaty waters, are insectivorous.
Xerophytes.—These plants have devices (a) for procuring water, (b) or for storing water, (c) or for limiting transpiration; and these adaptations are obviously related to the physically or physiologically dry habitats in which the plants live. Plants of physically dry habitats, such as deserts and sand dunes, have frequently long tap-roots which doubtless, in some cases, reach down to a subterranean water supply. The same plants have sometimes a superficial root system in addition, and are thus able to utilize immediately the water from rain showers and perhaps also from dew, as Volkens maintains. Root-hairs give an enlarged superficial area to the roots of plants, and thus are related to the procuring of water.
The stems of some xerophytes, e.g. Cactaceous and Crassulaceous plants, may be succulent, i.e. they have tissues in which water is stored. Some deserts, like those of Central America, are specially characterized by succulents; in other deserts, such as the Sahara, succulents are not a prominent feature. Other xerophytes again are spinous. “Switch plants,” such as Retama Retam and broom (Cytisus scoparius), have reduced leaves and some assimilating tissue in their stems; and stomata occur in grooves on the stem.
The transpiring surface of xerophytes is frequently reduced. The ordinary leaves may be small, absent, or spinous. In “cushion plants” the leaves are very small, very close together, and the low habit is protective against winds. The latter, of course, greatly increase transpiration. A “cushion plant” (Anabasis aretioides) of the north-western Sahara, frequently shows dead leaves on the exposed side whilst the plant is in full vigour on the sheltered side. The buds and leaves on the exposed side are probably killed by sand blasts. Many xerophytes are hairy or have sunken stomata which may be further protected by partial plugs of wax: the stomata are frequently in grooves: the leaves are frequently rolled—sometimes permanently so, whilst sometimes the leaves roll up only during unfavourable weather. These adaptations tend to lessen the amount of transpiration by protecting the stomata from the movements of the air. In species of Eucalyptus, the leaves are placed edge-wise to the incident rays of light and heat. The coriaceous leaves of “sclerophyllous plants” also, to some extent, are similarly protective. In such leaves, there are a well-marked cuticle, a thick epidermis a thick hypodermis at least on the upper side of the leaf, well-developed palisade tissue, and a poorly developed system of air-spaces. Such adaptations are well seen in the leaf of the holly (Ilex aquifolium). Warming, however, states that “Ilex aquifolium is undubitably a mesophyte” (1909: 135).
Halophytes, or plants which live in saline soils, have xerophytic adaptations. A considerable proportion of halophytes are succulents, i.e. their leaves and, to some extent, their stems have much water-storing tissue and few intercellular spaces. Some halophytes tend to lose their succulence when cultivated in a non-saline soil, and some non-halophytes tend to become succulent when cultivated in a salty soil; there is, it need scarcely be stated, little or no evidence that such characters are transmitted. British salt marshes furnish few instances of spiny plants, though such occur occasionally on the inland salt marshes of continental districts. Salsola Kali is British, and a hemi-halophyte at least; and it is rather spiny. Warming states that “the stomata of true, succulent, littoral halophytic herbs, in cases so far investigated, are not sunken” (1909: 221). It is possible, however, that the absence of sunken stomata, and the occurrence of some other halophytic features, are related merely to the succulent habit and not to halophytism, for succulent species often occur on non-saline soils. Similarly, the small amount of cuticular and of epidermal protection, and of lignification in succulent halophytes may also be related to the same circumstance. Forms of “stone cells” or “stereids” occur in some of the more suffruticose halophytes, as in Arthrocnemum glaucum. The interesting occurrence of certain halophytes and hemi-halophytes on sea-shores and also on mountains is probably to be explained by the past distribution of the species in question. At one time, such plants were probably of more general occurrence; now they have been extirpated in the intermediate localities, chiefly owing to the cultivation of the land in these places by man. In the west of Ireland and in the Faröes, where certain inland and lowland localities are still uncultivated, Plantago maritima and other halophytes occur in quantity and side by side with some “Alpine species,” such as Dryas octopetala.
The effect of common salt on the metabolism of plants is not understood. Lesage has shown that the height of certain plants is decreased by cultivation in a saline soil, and that the leaves of plants under such conditions become smaller and more succulent He showed further, that the increase of common salt in the soil is correlated with a reduction in the number and size of the chloroplastids, and therefore in the amount of chlorophyll. On the other hand, some plants did not respond to the action of common salt, whilst others were killed. Warming (1909; 220) quotes Griffon (1898), to the effect that “the assimilatory activity is less in the halophytic form than in the ordinary form of the same species.” Schimper had previously maintained that the action of common salt in the cell-sap is detrimental as regards assimilation. Many marine Algae appear to be able to regulate their osmotic capacity to the surrounding medium; and T. G. Hill has shown that the root hairs of Salicornia possess this property. There has, however, been performed upon halophytes very little physiologically experimental work which commands general acceptance.
Bog Xerophytes live in the peaty soil of fens and moors which are physically wet, but which are said to be physiologically dry. Related to the physiological drought, such plants possess some xerophytic characters; and, related to the physical wetness, the plants possess the aeration channels which characterize many hydrophytes and hemi-hydrophytes. The occurrence of xerophytic characters in plants of this type has given rise to much difference of opinion. It is sometimes maintained, for example, by Schimper, that their xerophytic characters are related to the physiological dryness of the habitat: this, however, is denied by others who maintain (Clements, 1905: 127) that the xerophytism is due to the persistence of ancestral structures. It is possible, of course, that each explanation is correct in particular cases, as the views are by no means mutually exclusive. With regard to the occurrence of plants, such as Juncus effusus, which possess xerophytic characters and yet live in situations which are not ordinarily of marked physiological dryness, it should be remembered that such habitats are liable to occasional physical drought; and a plant must eventually succumb if it is not adapted to the extreme conditions of its habitat. The xerophytic characters being present, it is not surprising that many marsh plants, like Juncus effusus and Iris pseudacorus, are able to survive in dry situations, such as banks and even garden rockeries.
Tropophytes.—These plants are characterized by being xerophytic during the unfavourable season. For example, deciduous trees shed their leaves in winter: geophytes go through a period of dormancy by means of bulbs, rhizomes, or other underground organs with buds; whilst annuals and ephemerals similarly protect themselves by means of the seed habit. All such plants agree in reducing transpiration to zero during the unfavourable season, although few or no xerophytic characters may be demonstrable during the period favourable to growth.
Hygrophytes.—Living, as these plants do, under medium conditions as regards soil, moisture and climate, they exhibit no characters which are markedly xerophytic or hydrophytic. Hence, such plants are frequently termed mesophytes. Assimilation goes on during the whole year, except during periods of frost or when the plants are buried by snow. An interesting special case of hygrophytes is seen with regard to plants which live in the shade of forests. Such plants have been termed sciophytes. Their stomata are frequently not limited to the underside of the leaves, but may occur scattered all over the epidermal surface. The epidermal cells may contain chlorophyll. Strengthening tissue is feebly developed. Many sciophytes are herbaceous tropophytes, and are dormant for more than half the year, usually during late summer, autumn and early winter. It may be that this is a hereditary character (cf. “bog xerophytes”), or that the physical drought of summer is unfavourable to shade-loving plants. In this connexion, it is interesting that in the east of England with the lowest summer rainfall of this country, many common sciophytes are absent or rare in the woods, such, for example, as Melica uniflora, Allium ursinum, Lychnis dioica, Oxalis Acetosella, and Asperula odorata. However, the cause of the absence or presence of a given species from a given locality is a department of ecology which has been studied with little or no thoroughness.
Calcicole and Calcifuge Species.—Plants which invariably inhabit calcareous soils are sometimes termed calcicoles; calcifuge species are those which are found rarely or never on such soils. The effect of lime on plants is less understood even than the effect of common salt. Doubtless, the excess of any soluble mineral salt or salts interferes with the osmotic absorption of the roots; and although calcium carbonate is insoluble in pure water, it is slightly soluble in water containing carbon dioxide. In England, the following species are confined or almost confined to calcareous soils: Asplenium Ruta-muraria, Melica nutans, Carex digitata, Aceras anthropophora, Ophrys apifera, Thalictrum minus, Helianthemum Chamaecistus, Viola hirta, Linum perenne, Geranium lucidum, Hippocrepis comosa, Potentilla verna, Viburnum Lantana, Galium asperum ( = G. sylvestre), Asperula cynanchica, Senecio campestris. The following plants, in England, are calcifuge: Lastraea Oreopteris, Holcus mollis, Carex echinata, Spergula arvensis, Polygala serpyllacea, Cytisus scoparius, Potentilla procumbens, Galium hercynicum ( = G. saxatile), Gnaphalium sylvaticum, Digitalis purpurea. Other plants occur indifferently both on calcareous and on non-calcareous soils.
It is sometimes said that lime acts as a poison on some plants and not on others, and sometimes that it is the physiological dryness of calcareous soils that is the important factor. In relation to the latter theory, it is pointed out that some markedly calcicole species occur on sand dunes; but this may be due to the lime which is frequently present in dune sand as well as to the physical dryness of the soil. Further, no theory of calciolous and calcifugous plants can be regarded as satisfactory which fails to account for the fact that both kinds of plants occur among aquatic as well as among terrestrial plants. Schimper (1903: 102) thinks that in the case of aquatic plants, the difference must depend on the amount of lime in the water, for the physical nature of the substratum is the same in each case. Again, acidic humus does not form in calcareous soils; and hence one does not expect to find plants characteristic of acidic peat or humus on calcareous soils. Some such species are Blechnum boreale, Aira flexuosa, Calluna vulgaris, Vaccinium, Myrtillus, Rubus, Chamaemorus, Empetrum nigrum, Drosera spp. Some, at least, of these species possess mycorhiza in their roots, and are perhaps unable to live in soils where such organisms are absent.
In England, the number of calcicole species is greater than the number of silicolous species. It would therefore be curious if it were proved that lime acts on plants as a poison. It is said that some plants may be calcicoles in one geographical district and not in another. However, until more is known of the exact chemical composition of natural—as contrasted with agricultural—soils, and until more is known of the physiological effects of lime, it is impossible to decide the vexed question of the relation of lime-loving and lime-shunning plants to the presence or absence of calcium carbonate in the soil. From such points of view as this, it is indeed true, as Warming has recently stated, “that ecology is only in its infancy.” (C. E. M.)
Cytology of Plants
The elementary unit of plant structure, as of animal structure, is the cell. Within it or its modifications all the vital phenomena of which living organisms are capable have their origin. Upon our knowledge of its minute structure or cytology, combined with a study of its physiological activities, depends the ultimate solution of all the important problems of nutrition and growth, reception and conduction of stimuli, heredity, variation, sex and reproduction.
The Cell Theory.—For a general and historical account of the cell theory see Cytology. It is sufficient to note here that cells were first of all discovered in various vegetable tissues by Robert Hooke in 1665 (Micrographia); Malpighi and Grew (1674-1682) gave the first clear indications of the importance of cells in the building up of plant tissues, but it was not until the beginning of the 19th century that any insight into the real nature of the cell and its functions was obtained. Hugo von Mohl (1846) was the first to recognize that the essential vital constituent of the plant cell is the slimy mass—protoplasm—inside it, and not the cell wall as was formerly supposed. The nucleus was definitely recognized in the plant cell by Robert Brown in 1831, but its presence had been previously indicated by various observers and it had been seen by Fontana in some animal cells as early as 1781. The cell theory so far as it relates to plants was established by Schleiden in 1838. He showed that all the organs of plants are built up of cells, that the plant embryo originates from a single cell, and that the physiological activities of the plant are dependent upon the individual activities of these vital units. This conception of the plant as an aggregate or colony of independent vital units governing the nutrition, growth and reproduction of the whole cannot, however, be maintained. It is true that in the unicellular plants all the vital activities are performed by a single cell, but in the multicellular plants there is a more or less highly developed differentiation of physiological activity giving rise to different tissues or groups of cells, each with a special function. The cell in such a division of labour cannot therefore be regarded as an independent unit. It is an integral part of an individual organization and as such the exercise of its functions must be governed by the organism as a whole.
General Structure and Differentiation of the Vegetable Cell.—The simplest cell forms are found in embryonic tissues, in reproductive cells and in the parenchymatous cells, found in various parts of the plant. The epidermal, conducting and strengthening tissues show on the other hand considerable modifications both in form and structure.
The protoplasm of a living cell consists of a semifluid granular substance, called the cytoplasm, one or more nuclei, and sometimes centrosomes and plastids. Cells from different parts of a plant differ very much in their cell-contents. Young cells are full of cytoplasm, old cells generally contain a large vacuole or vacuoles, containing cell-sap, and with only a thin, almost invisible layer of cytoplasm on their walls. Chlorophyll grains, chromatophores, starch-grains and oil-globules, all of which can be distinguished either by their appearance or by chemical reagents, may also be present. Very little is known of the finer structure of the cytoplasm of a vegetable cell. It is sometimes differentiated into a clearer outer layer, of hyaloplasm, commonly called the ectoplasm, and an inner granular endoplasm. In some cases it shows, when submitted to a careful examination under the highest powers of the microscope, and especially when treated with reagents of various kinds, traces of a more or less definite structure in the form of a meshwork consisting of a clear homogeneous substance containing numerous minute bodies known as microsomes, the spaces being filled by a more fluid ground-substance. This structure, which is visible both in living cells and in cells treated by reagents, has been interpreted by many observers as a network of threads embedded in a homogeneous ground-substance. Bütschli, on the other hand, interprets it as a finely vacuolated foam-structure or emulsion, comparable to that which is observed when small drops of a mixture of finely powdered potash and oil are placed in water, the vacuoles or alveoli being spaces filled with liquid, the more solid portion representing the mesh-work in which the microsomes are placed. Evidence is not wanting, however, that the cytoplasm must be regarded as, fundamentally, a semifluid, homogeneous substance in which by its own activity, granules, vacuoles, fibrils, &c., can be formed as secondary structures. The cytoplasm is largely concerned in the formation of spindle fibres and centrosomes, and such structures as the cell membrane, cilia, or fiagella, the coenocentrum, nematoplasts or vibrioids and physodes are also products of its activity.
Protoplasmic Movements.—In the cells of many plants the cytoplasm frequently exhibits movements of circulation or rotation. The cells of the staminal hairs of Tradescantia virginica contain a large sap-cavity across which run, in all directions, numerous protoplasmic threads or bridges. In these, under favourable conditions, streaming movements of the cytoplasm in various directions can be observed. In other forms such as Elodea, Nitella, Chara, &c., where the cytoplasm is mainly restricted to the periphery of the sap vacuole and lining the cell wall, the streaming movement is exhibited in one direction only. In some cases both the nucleus and the chromatophores may be carried along in the rotating stream, but in others, such as Nitella, the chloroplasts may remain motionless in a non-motile layer of the cytoplasm in direct contact with the cell wall.
Desmids, Diatoms and Oscillaria show creeping movements probably due to the secretion of slime by the cells; the swarm-spores and plasmodium of the Myxomycetes exhibit amoeboid movements; and the motile spores of Fungi and Algae, the spermatozoids of mosses, ferns, &c., move by means of delicate prolongations, cilia or flagella of the protoplast.
Chromatophores.—The chromatophores or plastids are protoplasmic structures, denser than the cytoplasm, and easily distinguishable from it by their colour or greater refractive power. They are spherical, oval, fusiform, or rod-like, and are always found in the cytoplasm, never in the cell-sap. They appear to be permanent organs of the cell, and are transmitted from one cell to another by division. In young cells the chromatophores are small, colourless, highly refractive bodies, principally located around the nucleus. As the cell grows they may become converted into leucoplasts (starch-formers), chloroplasts (chlorophyll-bodies), or chromoplasts (colour-bodies). And all three structures may be converted one into the other (Schimper). The chloroplasts are generally distinguished by their green colour, which is due to the presence of chlorophyll; but in many Algae this is masked by another colouring matter—Phycoerythrin in the Florideae, Phycophaein in the Phaeophyceae, and Phycocyanin in the Cyanophyceae. These substances can, however, be dissolved out in water, and the green colouring matter of the chloroplast then becomes visible. The chloroplast consists of two parts, a colourless ground substance, and a green colouring matter, which is contained either in the form of fibrils, or in more or less regular spherical masses, in the colourless ground-mass. The chloroplasts increase in number by division, which takes place in higher plants when they have attained a certain size, independent of the division of the cell. In Spirogyra and allied forms the chloroplast grows as the cell grows, and only divides when this divides. The division in all cases takes place by constriction, or by a simultaneous splitting along an equatorial plane. Chloroplasts are very sensitive to light and are capable in some plants of changing their position in the cell under the stimulus of a variation in the intensity of the light rays which fall upon them. In the chromatophores of many Algae and in the Liverwort Anthoceros there are present homogeneous, highly refractive, crystal-like bodies, called pyrenoids or starch-centres, which are composed of proteid substances and surrounded by an envelope of starch-grains. In Spirogyra the pyrenoids are distinctly connected by cytoplasmic strands to the central mass of cytoplasm, which surrounds the nucleus, and according to some observers, they increase exclusively by division, followed by a splitting of the cytoplasmic strands. Those chromatophores which remain colourless, and serve simply as starch-formers in parts of the plant not exposed to the light, are called leucoplasts or amyloplasts. They are composed of a homogeneous proteid substance, and often contain albuminoid or proteid crystals of the same kind as those which form the pyrenoid. If exposed to light they may become converted into chloroplasts. The formation of starch may take place in any part of the leucoplast. When formed inside it, the starch-grains exhibit a concentric stratification; when formed externally in the outer layers, the stratification is excentric, and the hilum occurs on that side farthest removed from the leucoplast. As the starch-grains grow, the leucoplasts gradually disappear.
Chromoplasts are the yellow, orange or red colour-bodies found in some flowers and fruits. They arise either from the leucoplasts or chloroplasts The fundamental substance or stroma is colourless and homogeneous. The colour is due to the presence of xanthophyll, or carotin or both. The colouring matters are not dissolved in the stroma of the chromoplast, but exist as amorphous granules, with or without the presence of a protein crystal, or in the form of fine crystalline needles, frequently curved and sometimes present in large numbers, which are grouped together in various ways in bundles and give the plastids their fusiform or triangular crystalline shape. Such crystalline plastids occur in many fruits and flowers (e.g. Tamus communis, Asparagus, Lonicera, berries of Solaneae, flowers of Cacalia coccinea, Tropaeolum, bracts of Strelitzia, &c.), and in the root of the carrot. In some cases the plastid disappears and the crystalline pigment only is left. In the red variety of Cucurbita pepo these crystals may consist of rods, thin plates, flat ribbons or spirals. Starch grains may often be seen in contact with the pigment crystals. The crystalline form appears to be due entirely to the carotin, which can be artificially crystallized from an alcohol or ether solution. In addition to the plastids, there are found in some plant-cells, e.g. in the epidermal cells of the leaf of species of Vanilla (Wakker), and in the epidermis of different parts of the flower of Funkia, Ornithogalum, &c. (Zimmermann), highly refractive bodies of globular form, elaioplasts, which consist of a granular protein ground-substance containing drops of oil. They are stained deep red in dilute solution of alkanin.
Substances contained in the Protoplasm.—Starch may be found in the chlorophyll bodies in the form of minute granules as the first visible product of the assimilation of carbon dioxide, and it occurs in large quantities as a reserve food material in the cells of various parts of plants. It is highly probable that starch is only produced as the result of the activity of chromatophores, either in connexion with chromoplasts, chloroplasts or leucoplasts. Starch exists, in the majority of cases, in the form of grains, which are composed of stratified layers arranged around a nucleus or hilum. The stratification, which may be concentric or excentric, appears to be due to a difference in density of the various layers. The outer layers are denser than the inner, the density decreasing more or less uniformly from the outside layers to the centre of hilum. The outermost, newly formed layer is composed of a more homogeneous, denser substance than the inner one, and can be distinguished in all starch-grains that are in process of development. The separate layers of the starch-grain are deposited on it by the activity of the chromatophore, and according to Meyer the grain is always surrounded by a thin layer of the chromatophore which completely separates it from the cytoplasm. The layers appear to be made up of elements which are arranged radially. These are, according to Meyer, acicular crystals, which he calls trichites. The starch grain may thus be regarded as a crystalline structure of the nature of a sphere crystal, as has been suggested by many observers.
Whether the formation of the starch grain is due to a secretion from the plastid (Meyer, 1895) or to a direct transformation of the proteid of the plastid (Timberlake, 1901) has not been definitely established.
Aleurone—Aleurone is a proteid substance which occurs in seeds especially those containing oil, in the form of minute granules or large grains. It may be in the form of an albumen crystal sometimes associated with a more or less spherical body—globoid—composed of a combination of an organic substance with a double phosphate of magnesium and calcium. Albumen crystals are also to be found in the cytoplasm, in leucoplasts and rarely in the nucleus.
Glycogen, a substance related to starch and sugar, is found in the Fungi and Cyanophyceae as a food reserve. It gives a characteristic red-brown reaction with iodine solution. In the yeast cell it accumulates and disappears very rapidly according to the conditions of nutrition and is sometimes so abundant as to fill the cell almost entirely (Errera, 1882, 1895: Wager and Peniston, 1910).
Volutin occurs in the cytoplasm of various Fungi, Bacteria, Cyanophyceae, diatoms, &c., in the form of minute granules which have a characteristic reaction towards methylene blue (Meyer). It appears to have some of the characteristics of nucleic acid, and according to Meyer may be a combination of nucleic acid with an unknown organic base.
Numerous other substances are also found in the cytoplasm, such as tannin, fats and oil, resins, mucilage, caoutchouc, guttapercha, sulphur and calcium oxalate crystals. The cell sap contains various substances in solution such as sugars, inulin, alkaloids, glucosides, organic acids and various inorganic salts. The colours of flowers are due to colouring matters contained in the sap of which the chief is anthocyanin.
Reference must also be made here to the enzymes or unorganized ferments which occur so largely in the cytoplasm. It is probable that most, if not all, the metabolic changes which take place in a cell, such as the transformation of starch, proteids, sugar, cellulose; and the decomposition of numerous other organic substances which would otherwise require a high temperature or powerful reagents is also due to their activity. Their mode of action is similar to that of ordinary mechanical catalytic agents, such as finely divided platinum (see Bayliss, The Nature of Enzyme Action, and J. R. Green, The Soluble Ferments).
The Nucleus.—The nucleus has been demonstrated in all plants with the exception of the Cyanophyceae and Bacteria, and even here structures have been observed which resemble nuclei in some of their characteristics. The nucleus is regarded as a controlling centre of cell-activity, upon which the growth and development of the cell in large measure depends, and as the agent by which the transmission of specific qualities from one generation to another is brought about. If it is absent, the cell loses its power of assimilation and growth, and soon dies. Haberlandt has shown that in plant cells, when any new formation of membrane is to take place in a given spot, the nucleus is found in its immediate vicinity; and Klebs found that only that portion of the protoplasm of a cell which contains the nucleus is capable of forming a cell-wall; whilst Townsend has further shown that if the non-nucleated mass is connected by strands of protoplasm to the nucleated mass, either of the same cell or of a neighbouring cell, it retains the power of forming a cell-membrane.
The Structure of the Nucleus.—In the living condition the resting nucleus appears to consist of a homogeneous ground substance containing a large number of small chromatin granules and one or more large spherical granules—nucleoli—the whole being surrounded by a limiting membrane which separates it from the cytoplasm. When fixed and stained this granular mass is resolved into a more or less distinct granular network which consists of a substance called Linin, only slightly stained by the ordinary nuclear stains, and, embedded in it, a more deeply stainable substance called Chromatin. The nucleolus appears to form a part of the Linin network, but has usually also a strong affinity for nuclear stains. The staining reactions of the various parts of the nucleus depend to some extent upon their chemical constitution. The chromatin is practically identical with nuclein. This has a strong attraction for basic aniline dyes, and can usually be distinguished from other parts of the cell which are more easily coloured by acid anilines. But the staining reactions of nuclei may vary at different stages of their development; and it is probable that there is no method of staining which differentiates with certainty the various morphological constituents of the nucleus.
Our knowledge of the chemical constitutions of the nucleus is due to the pioneer researches of Sir Lauder Brunton, Plosz, Miescher, Kossel and a host of more recent investigators. Nuclein is a complex albuminoid substance containing phosphorus and iron in organic combination (Macallum). It appears to be a combination of a protein with nucleic acid. Recent researches have shown that the nucleic acid can be broken up by chemical means into a number of different compounds or bases. The results at first obtained were very confusing and seemed to show that nucleic acid is very variable in constitution, but thanks to the work of Schmiedeberg and Stendel (Germany), Ivar Bang (Sweden) and Walter Jones and Levene (America), the confusion has been reduced to some sort of order, and it now seems probable that all ordinary nucleic acids yield two purine bases, adenine and guanine; two pyrimidine bases, cytosine and thymine and a hexose carbohydrate, the identity of which is uncertain.
The Nucleolus.—In the majority of plant-nuclei, both in the higher and lower plants, there is found, in addition to the chromatin network, a deeply stained spherical or slightly irregular body (sometimes more than one) called the nucleolus (fig. 2, A to D). It is often vacuolar, sometimes granular, and in other cases it is a homogeneous body with no visible structure or differentiation. The special function of this organ has been a source of controversy during the past few years, and much uncertainty still exists as to its true nature. It forms a part of the linin or plastin network of the nucleus and may become impregnated with varying quantities of chromatin stored up for use in the formation of the chromosomes and other nuclear activities. The relation of the nucleolus to the chromosomes is clearly seen in the reconstruction of the daughter nuclei after division in the cells of the root-apex of Phaseolus (fig. 1, A to F). The chromosomes (fig. 1, A) unite to form an irregular mass (fig. 1, B) out of which is evolved the nucleolus and nuclear network (fig
s. 1, E, F) by a fusion of the chromosomes (fig. 1, C, D).
Centrosome.—The centrosome is a minute homogeneous granule found in the cytoplasm of some cells in the neighbourhood of the nucleus. It is generally surrounded by a granular or radiating cytoplasmic substance. In plant cells its presence has been demonstrated in the Thallophytes and Bryophytes. In the higher plants the structures which have been often described as centrosomes are too indefinite in their constitution to allow of this interpretation being placed upon them, and many of them are probably nothing more than granules of the fragmented nucleolus. The centrosomes in plants do not appear to be permanent organs of the cell. They are prominent during cell-division, but many disappear in the resting stage. They are more easily seen, when the nucleus is about to undergo mitosis, at the ends of the spindle, where they form the centres towards which the radiating fibres in the cytoplasm converge (see fig. 7, E G). The centrosome or centrosphere is usually regarded as the dynamic centre of the cell and a special organ of division; but its absence in many groups of plants does not lend support to this view so far as plant-cells are concerned.
Nuclear Division.—The formation of new cells is, in the case of uninucleate cells, preceded by or accompanied by the division of the nucleus. In multinucleate cells the division of the nucleus is independent of the division of the cell. Nuclear division may be indirect or direct, that is to say it may either be accompanied by a series of complicated changes in the nuclear structures called mitosis or karyokinesis (fig. 2), or it may take place by simple direct division, amitosis, or fragmentation. Direct division is a much less common phenomenon than was formerly supposed to be the case. It occurs most frequently in old cells, or in cells which are placed under abnormal conditions. It may also take place where rapid proliferation of the cell is going on, as in the budding of the Yeast plant. It takes place in the internodal cells of Characeae; in the old inter nodalcells of Tradescantia; and in various other cells which have lost their power of division. It has been shown that, in cells of Spirogyra placed under special conditions, amitotic division can be induced, and that normal mitosis is resumed when they are placed again under normal conditions. Amitosis is probably connected by a series of intermediate gradations with karyokinesis.
Mitosis.—In indirect nuclear division the nucleus undergoes a series of complicated changes, which result in an equal division of the chromatic substance between the two daughter nuclei. Four stages can be recognized. (1) Prophase.—The nucleus increases in size; the network disappears, and a much convoluted thread takes its place (fig. 2, B). The chromatin substance increases in amount; the thread stains more deeply, and in most cases presents a homogeneous appearance. This is commonly called the spirem-figure. The chromatin thread next becomes shorter and thicker, the nucleoli begin to disappear, and the thread breaks up into a number of segments—chromosomes—which vary in number in different species, but are fairly constant in the same species (fig. 2, C, D). Coincident with these changes the nuclear membrane disappears and a spindle-shaped or barrel shaped group of threads makes its appearance in the midst of the chromosomes, the longitudinal axis of which is at right angles to the plane of the division (fig. 2, F). At each pole of this spindle figure there often occur fibres radiating in all directions into the cytoplasm, and sometimes a minute granular body, the centrosome, is also found there. (2) Metaphase.—The chromosomes pass to the equator of the spindle and become attached to the spindle-fibres in such a way that they form a radiating star-shaped figure—Aster—when seen from the pole of the spindle. This is called the nuclear plate (fig. 2, E, F, G, H). As they pass into this position they undergo a longitudinal splitting by which the chromatin in each chromosome becomes divided into equal halves. (3) Anaphase.—The longitudinal division of the chromosomes is completed by the time they have taken up their position in the nuclear plate, and the halves of the chromosomes then begin to move along the spindle-fibres to opposite poles of the spindle (fig. 2, I, J). Many observers hold the view that the chromosomes are pulled apart by the contraction of the fibres to which they are attached. (4) Telophase.—When they reach the poles the chromosomes group themselves again in the form of stars—Diaster—with spindle-fibres extending between them (fig. 2, K). The chromosomes then fuse together again to form a single thread (fig. 2, L), a nucleolus appears, a nuclear membrane is formed, and daughter nuclei are thus constituted which possess the same structure and staining reactions as the mother nucleus.
The spindle figure is probably the expression of forces which are set up in the cell for the purpose of causing the separation of the daughter chromosomes. Hartog has endeavoured to show that it can only be formed by a dual force, analogous to that of magnetism, the spindle-fibres being comparable to the lines of force in a magnetic field and possibly due to electrical differences in the cell. The spindle arises partly from the cytoplasm, partly from the nucleus, or it may be derived entirely from the nucleus—intranuclear spindle—as occurs in many of the lower plants (Fungi, &c.). The formation of the spindle begins in the prophases of division. A layer of delicate filamentous cytoplasm—kinoplasm—may collect around the nucleus, or at its poles, out of which the spindle is formed. As division proceeds, the filamentous nature of this cytoplasm becomes more prominent and the threads begin either to converge towards the poles of the nucleus, to form a bipolar spindle, or may converge towards, or radiate from, several different points, to form a multipolar spindle. The wall of the nucleus breaks down, and the cytoplasmic spindle-fibres become mixed with those derived from the nuclear network. The formation of the spindle differs in details in different plants.
Fig. 2.—Various Stages in the Nuclear Division of the Pollen Mother-cells of Lilium.
The significance of this complex series of changes is very largely hypothetical. It is clear, however, that an equal quantitative division and distribution of the chromatin to the daughter cells is brought about; and if, as has been suggested, the chromatin consists of minute particles or units which are the carriers of the hereditary characteristics, the nuclear division also probably results in the equal division and distribution of one half of each of these units to each daughter cell.
Reduction Divisions (Meiosis).—The divisions which take place leading to the formation of the sexual cells show a reduction in the number of chromosomes to one-half. This is a necessary consequence of the fusion of two nuclei in fertilization, unless the chromosomes are to be doubled at each generation. In the vascular cryptograms and phanerogams it takes place in the spore mother cells and the reduced number is found in all the cells of the gametophyte, the full number in those of the sporophyte. We know very little of the details of reduction in the lower plants, but it probably occurs at some stage in the life history of all plants in which sexual nuclear fusion takes place. The reduction is brought about simply by the segmentation of the spirem thread into half the number of segments instead of the normal number. In order to effect this the individual chromosomes must become associated in some way, for there is no diminution in the actual amount of nuclear substance, and this leads to certain modifications in the division which are not seen in the vegetative nuclei. The two divisions of the spore mother cell in which the reduction takes place, follow each other very rapidly and are known as Heterotype and Homotype (Flemming), or according to the terminology of Farmer and Moore (1905) as the meiotic phase. In the heterotype division the spirem thread is divided longitudinally before the segmentation occurs (fig. 2, B), and this is preceded by a peculiar contraction of the thread around the nucleolus which has been termed synapsis (fig. 1, A). A second contraction may take place later, immediately preceding the segmentation of the thread. It has been suggested that synapsis may be connected with the early longitudinal splitting of the thread or with the pairing of the chromosomes, but it is possible that it may be connected with the transference of nucleolar substance to the nuclear thread. The segments of each chromosome are usually twisted upon each other and may be much contorted (fig. 2, C, D), and appearances are observed which suggest a second longitudinal division, but which are more probably due to a folding of the segment by which the two halves come to lie more or less parallel to each other, and form variously shaped figures of greater or less regularity (fig. 2, E). The chromosomes now become attached to the spindle-fibres (fig. 2, F, G) and as the daughter chromosomes become pulled asunder they often appear more or less V-shaped so that each pair appears as a closed ring of irregular shape, the ends of the V's being in contact thus—<> (fig. 2, H. I, J, K). This V has been variously interpreted. Some observers consider that it represents a longitudinal half of the original segment of the spireme, others that it is a half of the segment produced by transverse division by means of which a true qualitative separation of the chromatin is brought about. The problem is a very difficult one and cannot be regarded as definitely settled, but it is difficult to understand why all this additional complexity in the division of the nucleus should be necessary if the final result is only a quantitative separation of the chromatin. It seems to be fairly well established that in the meiotic phase there is a true qualitative division brought about by the pairing of the chromosomes during synapsis, and the subsequent separation of whole chromosomes to the daughter nuclei. The method by which this is brought about is, however, the subject of much controversy. There are two main theories: (1) that the chromosomes which finally separate are at first paired side by side (Allen, Grégoire, Berghs, Strasburger and others), and (2) that they are joined together or paired end to end (Farmer and Moore, Gregory, Mottier and others). Good cytological evidence has been adduced in favour of both theories, but further investigation is necessary before any definite conclusion can be arrived at. The second or homotype division which immediately follows reverts to the normal type except that the already split chromosomes at once separate to form the daughter nuclei without the intervention of a resting stage.
Cell Division.—With the exception of a few plants among the Thallophytes, which consist of a single multinucleate cell, Caulerpa, Vaucheria, &c., the division of the nucleus is followed by the division of the cell either at once, in uninucleate cells, or after a certain number of nuclear divisions, in multinucleate cells. This may take place in various ways. In the higher plants, after the separation of the daughter nuclei, minute granular swellings appear, in the equatorial region, on the connecting fibres which still persist between the two nuclei, to form what is called the cell-plate. These fuse together to form a membrane (fig. 1, C, D) which splits into two layers between which the new cell-wall is laid down. In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi. Cell budding takes place in yeast and in the formation of the conidia of Fungi.
In a few cases both among the higher and the lower plants, of which the formation of spores in the ascus is a typical example, new cells are formed by the aggregation of portions of the cytoplasm around the nuclei which become delimited from the rest of the cell contents by a membrane. This is known as free cell formation.
In Fucus and allied forms the spindle-fibres between the daughter nuclei disappear early and the new cell-wall is formed in the cytoplasm.
Cell Membrane.—The membrane which surrounds the protoplasts in the majority of plants is typically composed of cellulose, together with a number of other substances which are known as pectic compounds. Some of these have a neutral reaction, others react as feeble acids. They can be distinguished by their insolubility in cuprammonia, which dissolves cellulose, and by their behaviour towards stains, some of which stain pectic substances but not cellulose. Cellulose has an affinity for acid stains, pectic substances for basic stains. The cell-membrane may become modified by the process of lignification, suberization, cuticularization or gelatinization. In the Fungi it is usually composed of a modified form of cellulose known as fungus cellulose, which, according to Mangin, consists of callose in combination either with cellulose or pectic compounds. The growth of the cell-wall takes place by the addition of new layers to those already formed. These layers are secreted by the protoplasm by the direct apposition of substances on those already in existence; and they may go on increasing in thickness, both by apposition and by the intussusception of particles probably carried in through the protoplasmic fibres, which penetrate the cell-wall as long as the cell lives. The growth of the cell-wall is very rarely uniform. It is thickened more in some places than in others, and thus are formed the spiral, annular and other markings, as well as the pits which occur on various cells and vessels. Besides the internal or centripetal growth, some cell-walls are thickened on the outside, such as pollen grains, oospores of Fungi, cells of Peridineae, &c. This centrifugal growth must apparently take place by the activity of protoplasm external to the cell. The outer protective walls of the oospores of some Fungi are formed out of protoplasm containing numerous nuclei, which is at an early stage separated from the protoplasm of the oospore. In the Peridineae, Diatoms and Desmids, according to recent researches, the thickenings on the outer walls of the cells are due to the passage of protoplasm from the interior of the cell to the outside, through pores which are found perforating the wall on all sides.
Cell-walls may become modified by the impregnation of various substances. Woody or lignified cell-walls appear to contain substances called coniferin and vanillin, in addition to various other compounds which are imperfectly known. Lignified tissues are coloured yellow by aniline sulphate or aniline chloride, violet with phloroglucin and hydrochloric acid, and characteristic reactions are also given by mixtures containing phenol, indol, skatol, thallin, sulphate, &c. (see Zimmermann's Microtechnique). Staining reagents can also be used to differentiate lignified cell-walls. Cuticularized or suberized cell-walls occur especially in those cells which perform a protective function. They are impervious to water and gases. Both cuticularized and suberized membranes are insoluble in cuprammonia, and are coloured yellow or brown in a solution of chlor-iodide of zinc. It is probable that the corky or suberized cells do not contain any cellulose (Gilson, Wisselingh); whilst cuticularized cells are only modified in their outer layers, cellulose inner layers being still recognizable. The suberized and cuticularized cell-walls appear to contain a fatty body called suberin, and such cell-walls can be stained red by a solution of alcanin, the lignified and cellulose membranes remaining unstained.
Fertilization.—The formation of the zygote or egg-cell takes place usually by the fusion of the contents of two cells, and always includes, as an essential feature, the fusion of two germ nuclei. In many of the lower plants the fusing cells—gametes—are precisely similar so far as size and general appearance are concerned; and the whole contents of the two cells fuse together, cytoplasm with cytoplasm, nucleus with nucleus, nucleolus with nucleolus and plastid with plastid. The gametes may be motile (some Algae) or non-motile, as in Spirogyra, Mucor, Basidiobolus, &c. In many of the lower plants and in all higher plants there is a difference in size in the fusing cells, the male cell being the smaller. The reduction in size is due to the absence of cytoplasm, which is in some cases so small in amount that the cell consists mainly of a nucleus. In all cases of complete sexual differentiation the egg-cell is quiescent; the male cell may be motile or non-motile. In many of the Fungi the non-motile male cell or nucleus is carried by means of a fertilizing tube actually into the interior of the egg-cell, and is extruded through the apex in closing proximity to the egg nucleus. In the Florideae, Lichens and Laboulbeniaceae the male cell is a non-motile spermatium, which is carried to the female organ by movements in the water. In Monoblepharis, one of the lower Fungi, in some Algae, in the Vascular Cryptograms, in Cycads (Zamia and Cycas), and in Ginkgo, an isolated genus of Gymnosperms, the male cell is a motile spermatozoid with two or more cilia. In the Algae, such as Fucus, Volvox, Oedogonium, Bulbochaete, and in the Fungus Monoblepharis, the spermatozoid is a small oval or elongate cell containing nucleus, cytoplasm and sometimes plastids. In the Characeae, the Vascular Cryptogams, in Zamia and Cycas, and in Ginkgo, the spermatozoids are more or less highly modified cells with two or more cilia, and resemble in many respects, both in their structure and mode of formation, the spermatozoids of animals. In Characeae and Muscineae they are of elongate spiral form, and consist of an elongate dense nucleus and a small quantity of cytoplasm. At the anterior end are attached two cilia or flagella. In the Vascular Cryptogams the structure is much the same, but a more or less spherical mass of cytoplasm remains attached to the posterior spirals, and a large number of cilia are grouped along the cytoplasmic anterior portion of the spiral. In Zamia (fig. 4, A), Cycas and Ginkgo they consist of large spherical or oval cells with a coiled band of cilia at one end, and a large nucleus which nearly fills the cell. They are carried by the pollen tube to the apex of the prothallus, where they are extruded, and by means of their cilia swim through a small quantity of liquid, contained in a slight depression to the oosphere. In the other Phanerogams the male cell, which is non-motile, is carried to the oosphere by means of a pollen tube. In the spermatozoids of Chara, Vascular Cryptogams, and in those of Cycas, Zamia and Ginkgo, the cilia arise from a centrosome-like body which is found on one side of the nucleus of the spermatozoid mother-cell. This body has been called a blepharoplast, and in the Pteridophytes, Cycads and Ginkgo it gives rise to the spiral band on which the cilia are formed. Belajeff regards it as a true centrosome; but this is doubtful, for while in some cases it appears to be connected with the division of the cell, in others it is independent of it. The egg-cell or oosphere is a large cell containing a single large nucleus, and in the green plants the rudiments of plastids. In plants with multinucleate cells, such as Albugo, Peronospora and Vaucheria, it is usually a uninucleate cell differentiated by separation of the nuclei from a multinucleate cell, but in Albugo bliti it is multinucleate, and in Sphaeroplea it may contain more than one nucleus. In some cases the region where the penetration of the male organ takes place is indicated on the oosphere by a hyaline receptive spot (Oedogonium, Vaucheria, &c.), or by a receptive papilla consisting of hyaline cytoplasm (Peronosporeae). Fertilization is effected by the union of two nuclei in all those cases which have been carefully investigated. Even in the multinucleate oosphere of Albugo bliti the nuclei fuse in pairs; and in the oospheres of Sphaeroplea, which may contain more than one nucleus, the egg nucleus is formed by the fusion of one only of these with the spermatozoid nucleus (Klebahn). In the higher Fungi nuclear fusions take place in basidia or asci which involve the union of two (fig. 7, A) nuclei, which may be regarded as physiologically equivalent to a sexual fusion. The union of the germ nuclei has now been observed in all the main groups of Angiosperms, Gymnosperms, Ferns, Mosses, Algae and Fungi, and presents a striking resemblance in all. In nearly all cases the nuclei appear to fuse in the resting stage (fig. 3, C). In many Gymnosperms the male nucleus penetrates the female nucleus before fusing with it (Blackman, Ikeno). In other cases the two nuclei place themselves side by side, the nuclear membrane between them disappears, and the contents fuse together—nuclear thread with nuclear thread, and nucleolus with nucleolus—so completely that the separate constituents of the nuclei are not visible. It was at one time thought that the centrosomes played an important part in the fertilization of plants, but recent researches seem to indicate that this is not so. Even in those cases where the cilia band, which is the product of the centrosome-like body or blepharoplast, enters the ovum, as in Zamia (c in fig. 4, B, C, D), it appears to take no part in the fertilization phenomena, nor in the subsequent division of the nucleus. During the process of fertilization in the Angiosperms it has been shown by the researches of Nawaschin and Guignard that in Lilium and Fritillaria both generative nuclei enter the embryo sac, one fusing with the oosphere nucleus, the other with the polar nuclei (fig. 3, A, B). A double fertilization thus takes place. Both nuclei are elongated vermiform structures, and as they enter the embryo sac present a twisted appearance like a spermatozoid without cilia (fig. 3, A, B). It has since been shown by other observers that this double fertilization occurs in many other Angiosperms, both Dicotyledons and Monocctyledons, so that it is probably of general occurrence throughout the group (see Angiosperms).
Fig. 4.—Spermatozoid and Fertilization in Zamia.
The Nucleus in Relation to Heredity.—There is a certain amount of cytological evidence to show that the nucleus is largely concerned with the transmission of hereditary characters. Whether this is entirely confined to the nucleus is, however, not certain. The strongest direct evidence seems to be that the nuclear substances are the only parts of the cells which are always equivalent in quantity, and that in the higher plants and animals the male organ or spermatozoid is composed almost entirely of the nucleus, and that the male nucleus is carried into the female cell without a particle of cytoplasm.
Since, however, the nucleus of the female cell is always accompanied by a larger or smaller quantity of cytoplasm, and that in a large majority of the power plants and animals the male cell also contains cytoplasm, it cannot yet be definitely stated that the cytoplasm does not play some part in the process. On the other hand, the complex structure of the nucleus with its separate units, the chromosomes, and possibly even smaller units represented by the chromatin granules, and the means taken through the complex phenomena of mitosis to ensure that an exact and equal division of the chromosomes shall take place, emphasizes the importance of the nucleus in heredity. Further, it is only in the nucleus and in its chromosomes that we have any visible evidence to account for the Mendelian segregation of characters in hybrids which are known to occur. Visible differences in the chromosomes have even been observed, especially in insects, which are due apparently to an unequal division by which an additional or accessory chromosome is produced, or in some cases one or two extra chromosomes which differ in size from the others. These differences indicate a separation of different elements in the formation of the chromosomes and have been definitely associated with the determination of sex. It is possible, however, that the segregation of characters in the gametes may depend upon something far more subtle and elusive than the chromosomes or even of possible combinations of units within the chromosomes, but so far as we can see at present these are the only structures in the cell with which it can be satisfactorily associated. Boveri in fact has put forward the view that the chromosomes are elementary units which maintain an organic continuity and independent existence in the cell. The cytological evidence for this appears to be made stronger for animal than for plant cells. From numerous investigations which have been made to trace the chromosomes through the various stages of the nuclear ontogeny of plant cells, it appears that the individuality and continuity of the chromosomes can only be conceived as possible if we assume the existence of something like chromosome centres in the resting nucleus around which the chromosomes become organized for purposes of division. Rosenberg (1909) adduces evidence for the existence of chromosomes or “prochromosomes” in resting nuclei in a large number of plants, but most observers consider that the chromosomes during the resting stage become completely resolved into a nuclear network in which no trace of the original chromosomes can be seen.
Special Cell-Modifications for the Reception of Stimuli.—In studying the physiology of movement in plants certain modifications of cell-structure have been observed which appear to have been developed for the reception of the stimuli by which the response to light, gravity and contact are brought about. Our knowledge of these structures is due mainly to Haberlandt.
Organs which respond to the mechanical stimulus of contact are found to possess special contrivances in certain of their cells—(1) sensitive spots, consisting of places here and there on the epidermal cells where the wall is thin and in close contact with protoplasmic projections. These occur on the tips of tendrils and on the tentacles of Drosera; (2) sensitive papillae found on the irritable filaments of certain stamens; and (3) sensitive hairs or bristles on the leaves of Dionaea muscipula and Mimosa pudica—all of which are so constructed that any pressure exerted on them at once reacts on the protoplasm.
Response to the action of gravity appears to be associated with the movements of starch grains in certain cells—statolith cells—by which pressure is exerted on the cytoplasm and a stimulus set up which results in the geotropic response.
The response to the action of light in diatropic leaves is, according to Haberlandt, due to the presence of epidermal cells which are shaped like a lens, or with lens-shaped thickenings of the cuticle, through which convergence of the light rays takes place and causes a differential illumination of the lining layer of protoplasm on the basal walls of the epidermal cells, by which the stimulus resulting in the orientation of the leaf is brought about. Fig. 5, A, shows the convergence of the light to a bright spot on the basal walls of the epidermal cells of Saxifraga hirsutum and Fig. 5, B, shows a photograph taken from life through the epidermal cells of Tradescantia fluminensis. Notwithstanding the fact, however, that these cells are capable of acting as very efficient lenses the explanation given by Haberlandt has not been widely accepted and evidence both morphological and physiological has been brought forward against it.
The presence of an eye-spot in many motile unicellular Algae and swarm spores is also probably concerned with the active response to light exhibited by these organisms. In Euglena viridis, which has been most carefully studied in this respect, the flagellum which brings about the movement bears near its base a minute spherical or oval refractive granule or swelling which is located just in the hollow of the red pigment-spot (fig. 6); and it has been suggested that the association of these two is analogous to the association of the rods and cones of the animal eye with their pigment layer, the light absorbed by the red pigment-spot setting up changes which react upon the refractive granule and being transmitted to the flagellum bring about those modifications in its vibrations by which the direction of movement of the organism is regulated.
|(From the Journal of the Linnean Society, “Zoology” vol. xxvii.)|
Fig. 6.—A, Eye-spots of Euglena viridis. B, Anterior end of Euglena showing the flagellum with its swelling just in the hollow of the eye-spot.
The Nuclei of the Lower Plants.—It is only in comparatively recent times that it has been possible to determine with any degree of certainty that the minute deeply stainable bodies described more especially by Schmitz (1879) in many Algae and Fungi could be regarded as true nuclei. The researches of the last twenty years have shown that the structure of the nucleus and the phenomena of nuclear division in these lower forms conforms in all essential details to those in the higher plants. Thus in the Basidiomycetes (fig. 7) the nuclei possess all the structures found in the higher plants, nuclear membrane, chromatin network and nucleolus (fig. 7, B), and in the process of division, chromosomes, nuclear spindle and centrosomes are to be seen (fig. 7, C-G). The investigations of Dangeard, Harper, Blackman, Miss Fraser and many others have also shown that in the Ascomycetes, Rust Fungi, &c., the same structure obtains so far as all essential details are concerned.
The only groups of plants in which typical nuclei have not been found are the Cyanophyceae, Bacteria and Yeast Fungi. In the Cyanophyceae the contents of the cell are differentiated into a central colourless region, and a peripheral layer containing the chlorophyll and other colouring matters together with granules of a reserve substance called cyanophycin. Chromatin is contained in the central part together with granules known as volutin, the function of which is unknown. The central body probably plays the part of a nucleus and some observers consider that it has the characters of a typical nucleus with mitotic division. But this is very doubtful. The central body seems to consist merely of a spongy mass of slightly stainable substance, more or less impregnated with chromatin, which divides by constriction. At a certain stage in the division figures are produced resembling a mitotic phase (fig. 8, 1), which are not, in the opinion of the writer, to be interpreted as a true mitosis. It is interesting to note that in many species the formation of new cell-walls is initiated before any indication of nuclear division is to be seen.
|(From Proc. Roy. Soc., vol. lxxix.)|
|Fig. 8.—Cell Structure of the Cyanophyceae.|
A and B, Tolypothrix lanata. (1) Young, (2) Old cells. C, Oscillaria limosa: transverse microtome section.
The bacteria, in most cases, have no definite nucleus or central body. The chromatin is distributed throughout the cytoplasm in the form of granules which may be regarded as a distributed nucleus corresponding to what Hertwig has designated, in protozoa, chromidia.
In the yeast cell the nucleus is represented by a homogeneous granule, probably of a nucleolar nature, surrounded and perhaps to some extent impregnated by chromatin and closely connected with a vacuole which often has chromatin at its periphery, and contains one or more volutin granules which appear to consist of nucleic acid in combination with an unknown base. Some observers consider that the yeast nucleus possesses a typical nuclear structure, and exhibits division by mitosis, but the evidence for this is not very satisfactory.
Tissues.—The component parts of the tissues of which plants are composed may consist of but slightly modified cells with copious protoplasmic contents, or of cells which have been modified in various ways to perform their several functions. In some the protoplasmic contents may persist, in others they disappear. The formation of the conducting tubes or secretory sacs which occur in all parts of the higher plants is due either to the elongation of single cells or to the fusion of cells together in rows by the absorption of the cell-walls separating them. Such cell-fusions may be partial or complete. Cases of complete fusion occur in the formation of laticiferous vessels, and in the spiral, annular and reticulated vessels of the xylem. Incomplete fusion occurs in sieve tubes. Tubes formed by the elongation of single cells are found in bast fibres, tracheides, and especially in laticiferous cells.
Laticiferous Tissue.—The laticiferous tissue consists of a network of branching or anastomosing tubes which contain a coagulable fluid known as latex. These tubes penetrate to all parts of the plant and occur in all parts of the root, stem and leaves. A protoplasmic lining is found on their walls which contains nuclei. The walls are pitted, and protoplasmic connexions between the laticiferous tubes and neighbouring parenchyma-cells have been seen. There are two types of laticiferous tissue—non-articulate and articulate. The non-articulate tissue which occurs in Euphorbiaceae, Apocynaceae, Urticaceae, Asclepiadaceae, consists of long tubes, equivalent to single multinucleate cells, which ramify in all directions throughout the plant. Laticiferous vessels arise by the coalescence of originally distinct cells. The cells not only fuse together in longitudinal and transverse rows, but put out transverse projections, which fuse with others of a similar nature, and thus form an anastomosing network of tubes which extends to all parts of the plant. They are found in the Compositae (Cichoriaceae), Campanulaceae, Papaveraceae, Lobeliaceae, Papayaceae, in some Aroideae and Musaceae, and in Euphorbiaceae (Manihot, Hevea). The nuclei of the original cells persist in the protoplasmic membrane. The rows of cells from which the laticiferous vessels are formed can be distinguished in many cases in the young embryo while still in the dry seed (Scott), but the latex vessels in process of formation are more easily seen when germination has begun. In the process of cell-fusion the cell-wall swells slightly and then begins to dissolve gradually at some one point. The opening, which is at first very small, increases in size, and before the cross-wall has entirely disappeared the contents of the two cells become continuous (Scott). The absorption of the cell-walls takes place very early in the germinating seedling.
Sieve Tubes.—The sieve tubes consist of partially fused rows of cells, the transverse or lateral walls being perforated by minute openings, through which the contents of the cells are connected with each other, and which after a certain time become closed by the formation of callus on the sieve plates. The sieve tubes contain a thin lining layer of protoplasm on their walls, but no nuclei, and the cell sap contains albuminous substances which are coagulable by heat. Starch grains are sometimes present. In close contact with the segments of the sieve tubes are companion cells which communicate with the sieve tubes by delicate protoplasmic strands; they can be distinguished from ordinary parenchymatous cells by their small size and dense protoplasm. Companion cells are not found in the Pteridophyta and Gymnos erms. In the latter their place is taken by certain cells of the medullary rays and bast parenchyma. The companion cells are cut off from the same cells as those which unite to form the sieve tube. The mode of formation of the sieve plate is not certainly known; but from the fact that delicate connecting threads of protoplasm are present between the cells from their first development it is probable that it is a special case of the normal protoplasmic continuity, the sieve pores being produced by a secondary enlargement of the minute openings through which these delicate strands pass. According to Lecomte, the young wall consists partly of cellulose and partly of a substance which is not cellulose, the latter existing in the form of slight depressions, which mark the position of the future pores. As the sieve plate grows these non-cellulose regions swell and gradually become converted into the same kind of mucous substance as that contained in the tube; the two cells are thus placed in open communication. If this is correct it is easy to see that the changes which take place may be initiated by the original delicate protoplasmic strands which pass through the cell-wall (For further information regarding tissues, see the section on Anatomy above)
Fig. 9.—Continuity of protoplasm of cells of Tamus communis (A) and endosperm of Lilium Martagon (B)
Protoplasmic Continuity.—Except in the unicellular plants the cell is not an independent unit. Apart from their dependence in various ways upon neighbouring cells, the protoplasts of all plants are probably connected together by fine strands of protoplasm which pass through the cell-wall (Tangl, Russow, Gardiner, Kienitz-Gerloff and others) (fig. 9). In Pinus the presence of connecting threads has recently been demonstrated throughout all the tissues of the plant. These protoplasmic strands are, except in the case of sieve tubes, so delicate that special methods have to be employed to make them visible. The basis of these methods consists in causing a swelling of the cell-wall by means of sulphuric acid or zinc chloride, and subsequent staining with Hoffmann's blue or other aniline dyes. The results so far obtained show that the connecting threads may be either “pit-threads” which traverse the closing membrane of the pits in the cell-walls (fig. 9, B), or “wall-threads” which are present in the wall of the cell (fig. 9, A). Both pit-threads and wall-threads may occur in the same cell, but more often the threads are limited to the pits. The pit-threads are larger and stain more readily than the wall-threads. The threads vary in size in different plants. They are very thick in Viscum album, and are well seen in Phaseolus multiflorus and Lilium Martagon. They are present from the beginning of the development of the cell-wall, and arise from the spindle fibres, all of which may be continued as connecting threads (endosperm of Tamus communis), or part of them may be overlaid by cellulose lamellae (endosperm of Lilium Martagon), or they may be all overlaid as in pollen mother-cells and pollen grains of Helleborus foetidus. The presence of these threads between all the cells of the plant shows that the plant body must be regarded as a connected whole; the threads themselves probably play an important part in the growth of the cell-wall, the conduction of food and water, the process of secretion and the transmission of impulses.
Literature.—The following is a list of a few of the more important papers in which further information and a more complete list of literature will be found: Allen, “Nuclear Division in the Pollen Mother-cells of Lilium canadense,” Annals of Botany (1905), vol. xix.; Berghs, “La Formation des chromosomes hétérotypiques dans la sporogénèse végétale,” La Cellule (1904), vol. xxi.; Blackman, “On the Fertilization, Alternation of Generations, and General Cytology of the Uredineae,” Ann. of Bot. (1904), vol. xviii.; Bütschli, Untersuchungen über mikroskopische Schäume und das Protoplasma (Leipzig, 1892; Eng. trans. by Minchin, London, 1894); also Untersuchungen über Struktur (Leipzig, 1898); Courchet, “Recherches sur les Chromoleucites,” Ann. d. sci. nat. (bot.): (1888); Delage, L'Année biologique: comptes rendus annuels des travaux de biologie générale (Paris, 1895), &c.; Farmer, “Recent Advances in Vegetable Cytology,” Science Progress (1896), vol. v.; “The Cell and some of its Constituent Structures,” Science Progress (1897); Farmer and Moore, “On the Meiotic Phase in Animals and Plants,” Quart. Journ. Micr. Sci. (1905), vol. xlviii.; Farmer and Digby, “Studies in Apospory and Apogamy in Ferns,” Ann. of Bot. (1907), vol. xxi.; “On the Cytological Features exhibited by certain Varietal and Hybrid Ferns,” Ann. of Bot. (1910), vol. xxiv.; Fischer, Fixirung, Färbung und Bau des Protoplasmas (Jena, 1899); Flemming, “Morphologie der Zelle,” Ergebnisse der Anatomie und Entwickelungsgeschichte (1896); Gardiner. “The Histology of the Cell-Wall, with Special Reference to the Mode of Connexion of Cells,” Proc. Roy. Soc. (1897-1898), lxii., and his earlier papers there cited; see also Proc. Camb. Phil. Soc. (1908), vol. ix.; “The Genesis and Development of the Wall and Connecting Threads in the Plant Cell. Preliminary Communication,” Proc. Roy. Soc. (1900), lxvi.; Gates, “A Study of Reduction in Oenothera rubrinervis,” Bot. Gaz. (1908), vol. xlvi.; Green, “The Cell Membrane,” Science Progress (1897), new series, vol. i.; Grégoire, “Les Cinèses polliniques chez les Lilicacées,” La Cellule (1899), vol. xvi.; “Les Résultats acquis sur les cinéses de maturation dans les deux régnes,” La Cellule (1905), vol. xxii. and (1910) vol. xxvi.; Grégoire and Wygaerts, “La Reconstitution du noyau et la formation des chromosomes dans les cinèses somatiques,” i. La Cellule (1903), vol. xxi.; Guignard, “Sur les anthérozoides et la double copulation sexuelle chez les végétaux angiospermes,” Comptes rendus (1899), 128; Haberlandt, Physiologische Pflanzenanatomie (Leipzig, 1909); Die Lichtsinnesorgane der Laubblätter (Leipzig, 1905); R. A. Harper, Sexual Reproduction and the Organisation of the Nucleus in certain Mildews (pub. Carnegie Institution, 1905); M. Hartog, “The Dual Force of the Dividing Cell,” Proc. Roy. Soc., B. lxxvi.; Henneguy, Leçons sur la cellule, morphologie et reproduction (Paris, 1896); O. Hertwig, Die Zelle und die Gewebe (Jena, 1893 and 1898; see Eng. ed., London, 1894); Hirase, “Études sur la fécondation et l'Embryogénie du Ginkgo biloba,” Journ. Coll. Sci. Imp. Univ. (Japan, 1895); Ikeno, “Untersuchungen über die Entwickelung der Geschlechtsorgane und den Vorgang der Befruchtung bei Cycas revoluta,” Jahr. f wiss. Botanik (1898), 32; Lee, The Microtomist's Vade Mecum (London, 1900); Macallum, “On the Detection and Localization of Phosphorus in Animal and Vegetable Cells,” Proc. Roy. Soc. (1898), vol. lxiii.; “On the Distribution of Assimilated Iron Compounds other than Haemoglobin and Haematins, in Animal and Vegetable Cells,” Quart. Journ. Micr. Sci. (1896), vol. xxxviii.; Meyer, Untersuchungen über die Stärke-Körner (Jena, 1895); Montgomery, “Comparative Cytological Studies, with especial regard to the Morphology of the Nucleolus,” Journ. of Morphology, vol. xv. (Boston, 1899); D. M. Mottier, “The Development of the Heterotype Chromosomes in Pollen Mother-cells,” Ann. of Bot. (1907), vol. xxi.; “On the Prophases of the Heterotype Mitosis in the Embryo-sac Mother-cell of Lilium,” Ann. of Bot (1909), vol. xxiii.; Fecundation in Plants (Carnegie Institution, 1904}: Nawaschin, “Resultate einer Revision der Befruchtungsvorgänge bei Lilium Martagon und Fritillaria tenella,” Bull. de l'acad. des sci. de St Petersbourg (1898); “Ueber die Befruchtungsvorgänge bei einigen Dicotyledoneen,” Ber. d. deutsch. bot. Gesell. (1900}, vol. 18; Rosenberg, “Cytologische and morphologische Studien an Drosera longifolia X. rotundifolia,” Kungl. svenska vetenskapsakad. handl. (1909), vol. xliv.; Salter, “Zur näheren Kenntniss der Stärkekörner,” Pringsh. Jahrb. (1898); Sargant, “The Formation of the Sexual Nuclei in Lilium Martagon, I. and II.,” Ann. of Bot. (1896-1897), vols. x. and xi.; “Recent Work on the Results of Fertilization in Angiosperms,” Ann. of Bot. (1900), vol. xiv.; Schimper, “Sur l'Amidon et les Leucites,” Ann. des sci. nat. (bot.) (1887); Scott, “Development of Articulated Laticiferous Vessels,” Quart. Journ. Micr. Sci. (1882); “On the Laticiferous Tissue of Manihot Glaziovii (the Cearà Rubber),” Quart. Journ. Micr. Sci. (1884); Strasburger, “Chromosomenzahlen, Plasmastrukturen, Vererbungsträger und Reduktionsteilung,” Jahrb. wiss. Bot. (1908), vol. xlv.; Histologische Beiträge vols. i. to vii. (Jena); Strasburger and others, “Cytologische Studien aus dem Bonner botanischen Institut,” Jahrb. für wissensch. Botanik (1897), vol. 30; Wager, “On Nuclear Division in the Hymenomycetes,” Ann. of Bot. (1893), vol. vii.; “On the Structure and Reproduction of Cystopus candidus,” Ann. of Bot. (1896), vol. x.; “The Cell Structure of the Cyanophyceae,” Proc. Roy. Soc. (1903), vol. lxxii.; Wager and Peniston, “Cytological Observations on the Yeast Plant,” Ann. of Bot. (1910), vol. xxiv.; Webber, “The Development of the Antherozoids of Zamia,” Bot. Gaz. (1897), vol. xxiv.; Wilson, The Cell in Development and Inheritance (New York and London, 1900); Zimmermann, “Sammel-Referate aus dem Gesammtgebiete der Zellenlehre,” Beihefte zum bot. Centralbl. (1893 and 1894); Die Morphologie und Physiologie des pflanzlichen Zellkernes (Jena, 1898). (H. W.*)
Morphology of Plants
The term morphology, which was introduced into science by Goethe (1817), designates, in the first place, the study of the form and composition of the body and of the parts of which the body may consist; secondly, the relations of the parts of the same body; thirdly, the comparison of the bodies or parts of the bodies of plants of different kinds; fourthly, the study of the development of the body and of its parts (ontogeny); fifthly, the investigation of the historical origin and descent of the body and its parts (phylogeny); and, lastly, the consideration of the relation of the parts of the body to their various functions, a study that is known as organography.
It is this last department of morphology that was the first to be pursued. The earliest scientific result of the study of plants was the recognition of the fact that the various parts of the body are associated with the performance of different kinds of physiological work; that they are, in fact, organs discharging special functions. The origin of the organography of the present day may be traced back to Aristotle, who described the parts of plants as “organs, though very simple ones.” It was not until many centuries had passed that the parts began to be regarded from the point of view of their essential nature and of their mutual relations; that is, morphologically instead of organographically. Joachim Jung, in his Isagoge phytoscopica (1678), recognized that the plant-body consists of certain definite members, root, stem and leaf, and defined them by their different form and by their mutual relations. This point of view was further developed in the following century by Caspar Friedrich Wolff (Theoria generationis, 1759), who first followed the development of the members at the growing-point of the stem. He observed that the “appendicular organs,” as he called the leaves, are developed in the same way, whether they be foliage-leaves, or parts of the flower, and stated his conclusions thus: “In the entire plant, whose parts we wonder at as being, at the first glance, so extraordinarily diverse, I finally perceive and recognize nothing beyond leaves and stem (for the root may be regarded as a stem). Consequently all parts of the plant, except the stem, are modified leaves.” Similar views were arrived at by Goethe, though by the deductive rather than the inductive method, and were propounded in his famous pamphlet, Versuch die Metamorphose der Pflanzen zu erklären (1790), from which the following is a quotation: “The underlying relationship between the various external parts of the plant, such as the leaves, the calyx, the corolla, the stamens, which develop one after the other and, as it were, out of one another, has long been generally recognized by investigators, and has in fact been specially studied, and the operation by which one and the same organ presents itself to us in various forms has been termed Metamorphosis of Plants.”
Pure Morphology.—Thus it became apparent that the many and various organs of plants are, for the most part, different forms of a small number of members of the body, which have been distinguished as follows, without any reference to function. The thallus (thallome) is a plant-body which is not differentiated into the members root, stem and leaf; it is the morphologically simplest body, such as is of common occurrence in the lower plants (e.g. Thallophyta). In a differentiated body the stem (caulome) is an axis capable of bearing leaves and (directly or indirectly) the proper reproductive organs. The leaf (phyllome) is an appendicular member only borne by a stem, but differing from it more or less obviously in form and development, though co-ordinate with it in complexity of structure. The root is an axis which never bears either leaves or the proper reproductive organs (whether sexual or asexual) of the plant. The hair (trichome) is a superficial appendage of simple structure, which may be borne by any of the other members. The emergence is also an appendicular member of more complex structure than the hair (e.g. the prickles of the rose). Further, it has been found convenient to designate the leaf-bearing stem as a whole by the term shoot, so that the body may, as Sachs suggested, be primarily analysed into shoot and root.
At the present time some objection is being taken to this purely morphological conception of the body and its parts as being too abstract. It is urged that the various parts are, as a matter of fact, organs; and that it is therefore inadmissible to ignore their functions, as is done in the foregoing definitions. To this it may be replied that pure morphology and organography are not alternatives, but are two complementary and equally necessary modes of considering the composition of the plant-body. Moreover, the abstract terms “stem,” “leaf,” “root,” &c., are absolutely indispensable; and are continually used in this sense by the most ardent organographers. It has not yet been suggested that they should be replaced by organographical terms; were this accomplished, descriptive botany would become impossible.
It is also urged against these definitions that they are not of universal applicability; that there are exceptional structures which cannot be brought within the limits of any one of them. But admitting the validity of this criticism, and even going so far as to question the possibility of ever devising absolutely inclusive and, at the same time, exclusive definitions, no sufficient reason is adduced for giving up all attempt at morphological analysis.
Homology.—All members belonging to the same morphological category are said to be homologous, however diverse their functions. Thus, in a phanerogam, the sepals, petals, stamens and foliage-leaves all come under the category leaf, though some are parts of the perianth, others are spore-bearing organs (sporophylls), and others carry on nutritive processes. The homology of members was based, in the first instance, upon similarity of development and upon similar relations to the other parts of the body, that is, upon ontogeny. But since the general adoption of the theory of evolution, similarity of descent, that is of phylogeny, has come to form an essential part of this conception; in other words, in order that their homology may be established the parts compared must be proved to be homogenetic.
The introduction of the phylogenetic factor has very much increased the difficulty of determining homologies; for the data necessary for tracing phylogeny can only be obtained by the study of a series of allied, presumably ancestral, forms. One of the chief difficulties met with in this line of research, which is one of the more striking developments of modern morphology, is that of distinguishing between organs which are “reduced,” and those which are really “primitive.” The object of the phylogenetic study of any organ is to trace it back to its primitive form. But, as will be pointed out later, organs are often found to have undergone “degeneration” or “reduction,” and such reduced or degenerate structures may easily be mistaken for primitive structures, and so the investigator may be misled.
The effect of the phylogenetic factor in homology may be illustrated in the following cases. The leaves of the true mosses and those of the club-mosses (Lycopodium, Selaginella) being somewhat alike in general appearance and in ontogeny, might be, and indeed have been, regarded as homologous on that ground. However, they belong respectively to two different forms in the life-history of the plants; the leaves of the mosses are borne by the gametophyte, those of the club-mosses by the sporophyte. In accordance with the prevalent antithetic view of the alternation of generations in these plants (see Plants, Reproduction of), the forms distinguished as sporophyte and gametophyte are not homogenetic; consequently their leaves are not homologous, but are only functionally similar (homoplastic; see infra).
Another effect is that different degrees of homology have to be recognized, just as there are different degrees of relationship or affinity between individual plants. When two organs can be traced along the same line of descent to one primitive form, that is when they are found to be monophyletic, their homology is complete; when, however, they are traceable to two primitive forms, though these forms belong to the same morphological series, they are polyphyletic and therefore only incompletely homologous. For instance, all the leaves of the Bryophyta are generally homologous inasmuch as they are all developments of the gametophyte. But there is reason to believe that they have been differentiated quite independently in various groups, such as the Marchantiaceae, the Jungermanniaceae, and the mosses proper; consequently their phylogeny is not the same, they are polyphyletic, and therefore they are not completely homologous, but are parallel developments.
Analogy.—Considering the parts of the body in relation to their functions, that is as organs, they are found to present peculiarities of form and structure which are correlated with the functions that they have to discharge; in other words, the organ shows adaptation to its functions. All organs performing the same function and showing similar adaptations are said to be analogous or homoplastic, whatever their morphological nature may be; hence organs are sometimes both homologous and analogous, sometimes only analogous. The tendrils of a vetch and of a cucumber are analogous, and also homologous because they both belong to the category leaf; but they are only analogous to the tendrils of the vine and of the passion-flower, which belong to the category stem.
Metamorphosis.—It has already been pointed out that each kind of member of the body may present a variety of forms. For example, a stem may be a tree-trunk, or a twining stem, or a tendril, or a thorn, or a creeping rhizome, or a tuber; a leaf may be a green foliage-leaf, or a scale protecting a bud, or a tendril, or a pitcher, or a floral leaf, either sepal, petal, stamen or carpel (sporophyll); a root may be a fibrous root, or a swollen tap-root like that of the beet or the turnip. All these various forms are organs discharging some special function, and are examples of what Wolff called “modification,” and Goethe “metamorphosis.” It may be inquired what meaning is to be attached to these expressions, and what are the conditions and the nature of the changes assumed by them. The leaf of the higher plants will be taken as the illustrative case because it is the most “plastic” of the members, the one, that is, which presents the greatest variety of adaptations, and because it has been most thoroughly studied.
In this, as in all morphological inquiries, two lines of investigation have to be followed, the phylogenetic and the onto genetic. Beginning with its phylogeny, it appears, so far as present knowledge goes, that the differentiation of the shoot of the sporophyte into stem and leaf first occurred in the Pteridophyta; and, in accordance with the views of Bower (Origin of a Land-Flora), the primitive leaf was a reproductive leaf, a sporophyll, from which the foliage-leaf was derived by progressive sterilization. From the nature of the case, this view is not, and could not be, based upon actual observation, nor is it universally accepted; however, it seems to correspond more closely than any other to the facts of comparative morphology. It was formerly assumed, and the view is still held, that the foliage-leaf was the primitive form from which all others were derived, mainly on the ground that, in ontogeny, the foliage-leaf generally precedes the sporophyll. The phylogeny of the various floral leaves, for instance, was generally traced as follows: foliage-leaf, bract, sepal, petal, stamen and carpel (sporophylls)—in accordance with what Goethe termed “ascending metamorphosis.” Recent researches, however, more especially those of Celakovsky, tend to prove that the perianth-leaves have been derived from the stamens (i.e. from sporophylls); that is, they are the result of “descending metamorphosis.” Moreover there is the fact that the flowers of nearly all the primitive phanerogams, such as the Gymnosperms, consist solely of sporophylls, having no perianth. There is thus a considerable body of evidence to support Bower's view of the primitive nature of the sporophyll.
Accepting this view of the phylogeny of the leaf, the perianth-leaves (sepals and petals) and the foliage-leaves may be regarded as “modified” or “metamorphosed” sporophylls; that is, as leaves which are adapted to functions other than the bearing of spores. The sepals are generally organs for the protection of the flower-bud; the petals, for attracting insects by their conspicuous form and colour; the foliage-leaves, for the assimilation of carbon dioxide and other associated functions. But this phylogenetic differentiation of the organs was not what Wolff and Goethe had in mind; what they contemplated was an ontogenetic change, and there is abundant evidence that such changes actually occur. Taking first the conversion of members of one morphological category into those of another, this has been actually observed, though rarely. Goebel (Organography) gives several instances of the conversion of the root into a shoot in ferns, and a few in phanerogams (Listera ovata, Neottia nidusavis, Anthurium longifolium). Much more common is the conversion of one form of a member into another form. The most varied changes of this kind have been described, and are generally familiar as “monstrosities”; the study of them constitutes, under the name of teratology, a distinct department of biology. A simple case is that of “double” flowers, in which the number of the petals is increased by the “metamorphosis” of stamens; or again the conversion of floral leaves into green leaves, a change known as “chloranthy.” These changes may be brought about by external causes, such as the attacks of insects or of fungi, alterations in external conditions, &c., or by some unexplained internal disturbance of the morphological equilibrium. They can also be effected experimentally. Goebel has shown that if the developing foliage-leaves of the fern Onoclea struthiopteris be removed as they are formed, the subsequently developed sporophylls assume more or less completely the habit of foliage-leaves, and may be sterile. Similarly bud-scales can be caused to develop into foliage-leaves, if the buds to which they belong are caused to grow out in the year of their formation by the removal of the existing foliage-leaves.
Useful and suggestive as they often are, teratological facts played, at one time, too large a part in the framing of morphological theories; for it was thought that the “monstrous” form gave a clue to the essential nature of the organ assuming it. There is, however, no sufficient reason for regarding the monstrous form as necessarily primitive or ancestral, nor even as a stage in the ontogeny of the organ. For when the older morphologists spoke of a stamen as a “metamorphosed” leaf, it was implied that it originated as a foliage-leaf and subsequently became a stamen. As a matter of fact, a stamen is a stamen and nothing else, from the very beginning. The development of the organ is already determined at its first appearance upon the growing-point; though, as already explained, the normal course of its ontogeny may be interfered with by some abnormal external or internal condition. The word “metamorphosis” cannot, in fact, be used any longer in its original sense, for the change which it implied does not normally occur in ontogeny, and in phylogeny the idea is more accurately expressed by the term “differentiation.” However, it may still be useful in describing “monstrosities,” and perhaps also those cases in which an organ serves first one purpose and then another, as when a leafy shoot eventually becomes a thorn, or the base of a foliage-leaf becomes a bud-scale.
Differentiation.—Any account of the general morphology of living organisms is incomplete if it does not include some attempt at an explanation of its causation; though such an attempt cannot be carried far at the present time. A survey of the vegetable kingdom indicates that evolution has proceeded, on the whole, from the simple to the complex; at the same time, as has been already mentioned, evidence of reduction or degeneration in common. Thus in the series Bryophyta, Pteridophyta, Phanerogamia, whilst the sporophyte presents progressive development, the gametophyte presents continuous reduction.
Evolution means the gradual development of “highly organized” from “lowly organized” forms; that is, of forms in which the “physiological division of labour” is more complete, from those in which it is less complete; of forms possessing a variety of organs, from forms possessing but few. Differentiation means the development and the specialization as organs of various parts of the body. It presents itself in two aspects: there is morphological differentiation, which can be traced in the distinction of the members of the body, root, stem, leaf, &c.; there is physiological differentiation, which can be traced in the adaptation of these members to various functions. But, in actual operation, these two processes are simultaneous; every member is developed as an organ for the performance of some special function.
Factors in Evolution.—Evolution in the race involves progressive differentiation in the individual; hence the causes of evolution and of differentiation must be the same. The evolution of higher from lower plants, it is generally assumed, has proceeded by variation. With regard to the causation of variation Darwin says (Origin of Species, ch. v.): “In all cases there are two factors, the nature of the organism, which is much the most important of the two, and the nature of the conditions. The direct action of changed conditions leads to definite or indefinite results. In the latter case the organization seems to become plastic, and we have much fluctuating variability. In the former case the nature of the organism is such that it yields readily, when subjected to certain conditions, and all or nearly all the individuals become modified in the same way.”
In spite of the statement that the “nature of the organism” is the most important factor in variation, the tendency amongst evolutionists has been to take much more account of the influence of external conditions. Exceptions to this attitude are Lamarck, who speaks with regard to animals (but not to plants!) of “la composition croissant de l'organisation” (Philosophie zoologique, t. i.), and Nägeli, who attributes variation to causes inherent in the “idioplasm,” and has elaborately worked out the view in his Abstammungslehre.
The position assumed in this article is in agreement with the views of Lamarck and of Nägeli. All but the lowest plants visibly tend towards or actually achieve in various degrees the differentiation of the body, whether sporophyte or gametophyte, into stem, leaf, root, &c., that is, the differentiation of parts not previously present. It is inconceivable that external conditions can impart to an organism the capacity to develop something that it does not already possess: can impart to it, that is, the capacity for variation in the direction of higher complexity. The alternative, which is here accepted, is that differentiation is essentially the expression of a developmental tendency inherent in the protoplasm of plants. Just as every crystallizable chemical substance assumes a definite and constant crystalline form which cannot be accounted for otherwise than by regarding it as one of the properties of the substance, so every living organism assumes a characteristic form which is the outcome of the properties of its protoplasm. But whereas the crystalline form of a chemical substance is stable and fixed, the organized form of a living organism is unstable and subject to change.
Influence of External Conditions.—This position does not, however, exclude the influence of external conditions; that influence is undeniable. Darwin's expression “the nature of the organism” has been interpreted in the preceding paragraph to mean an inherent tendency towards higher organization; that interpretation may now be completed by adding that the organism is susceptible to, and can respond to, the action of external conditions. There is every reason to believe that plants are as “irritable” to varying external conditions as they are to light or to gravity. A change in its external conditions may act as a “stimulus,” evoking in the organism a response of the nature of a change in its form. As Darwin has pointed out, this response may be direct or indirect. In illustration of the indirect response, the evolution of the Bryophyta and of more highly organized plants may be briefly considered. It is generally admitted that life originated in water, and that the earliest plants were Algae. The study of existing Algae, that is of plants that have continued to live in water, shows that under these conditions no high degree of organization has been reached, though some of them have attained gigantic dimensions. The primitive water-plants were succeeded by land-plants, a land-flora being gradually established. With the transition from water to land came the progressive development of the sporophyte which is the characteristic feature of the morphology of the Bryophyta and of all plants above them in the scale of life (see Bower, Origin of a Land-Flora). This evolution of the sporophyte is no doubt to be correlated with the great change in the external conditions of life. There is no conclusive ground for regarding the action of this change as having been direct, it is more reasonable to regard it as indirect, having acted as a general stimulus to which the ever-increasing complexity of the sporophyte was the response.
Adaptation.—The morphological and physiological differentiation of the plant-body has, so far, been attributed to (1) “the nature of the organism,” that is to its inherent tendency towards higher organization, and (2) to the “indefinite results” of the external conditions acting as a stimulus which excites the organism to variation, but does not direct the course of variation. The “definite results” of the action of external conditions have still to be considered.
It is a familiar observation that climatic and edaphic (nature of soil) conditions exert an influence upon the form and structure of plants (see Plants: Ecology of). For instance, some xerophytes are dry and hard in structure, whilst others are succulent and fleshy. This so-called direct effect of external conditions upon the form and structure of the body differs from the indirect effect in that the resulting variations bear a relation, of the nature of adaptation, to those conditions, the effect of the conditions is not only to cause variation, but to cause variation in a particular direction. Thus all existing hygrophytes (excepting the Algae) are considered to have been derived from land-plants which have adapted themselves to a watery habitat. The effect can also be demonstrated experimentally: thus it has been observed that a xerophyte grown in moist air will lose its characteristic adaptive features, and may even assume those of a hygrophyte.
Climatic and edaphic conditions are not, however, the only ones to affect the structure and composition of the body or its parts, other conditions are of importance, particularly the relations of the plant to animals and to other plants. For instance, the “animal traps” of carnivorous plants (Drosera, Nepenthes, &c.) did not, presumably, originate as such; they began as organs of quite another kind which became adapted to their present function in consequence of animals having been accidentally caught. It is also probable that the various forms of the angiospermous flower, with its many specialized mechanisms for pollination, may be the result of insect-visits, the flowers becoming adapted to certain kinds of insects, and the insects having undergone corresponding modification. Parasites, again, were derived from normal autotrophic plants, which, as the parasitic habit became more pronounced, acquired the corresponding characteristics of form and structure; there is, in fact, the group of hemi-parasites, plants which still retain autotrophic characters though they are root-parasites.
Though adaptation to the environment seems sometimes to be considered, especially by neo-Lamarckians, as equivalent to, or at least as involving, the evolution of higher forms from lower, there does not appear to be any evidence that this is the case. The effect of external conditions is confined to the modification in various directions of members or organs already existing, and one very common direction is that of reduction or entire disappearance of parts: for instance, the foliage-leaves of certain xerophytes (e.g. Cactaceae, Euphorbiaceae), of parasites, and of saprophytes. Moreover, had the evolution of plants proceeded along the line of adaptation, the vegetable kingdom could not be subdivided, as it is, into the morphological groups Thallophyta, Bryophyta, Pteridophyta, Phanerogamia, but only into physiological groups, Xerophyta, Hygrophyta, Tropophyta, &c.
In endeavouring to trace the causation of adaptation, it is obvious that it must be due quite as much to properties inherent in the plant as to the action of external conditions; the plant must possess adaptive capacity. In other words, the plant must be irritable to the stimulus exerted from without, and be capable of responding to it by changes of form and structure. Thus there is no essential difference between the “direct” and the “indirect” action of external conditions, the difference is one of degree only. In the one case the stimulus induces indefinite variation, in the other definite; but no hard-and-fast line can be drawn between them.
Adaptive characters are often hereditary, for instance, the seed of a parasite will produce a parasite, and the same is true of a carnivorous plant. On the other hand, adaptations, especially those evoked by climatic or edaphic conditions, may only be shown by the seedling if grown under the appropriate external conditions; here what is hereditary is not the actual adaptation, but the capacity for responding in a particular way to a certain set of external conditions.
Summary.—The general theory of differentiation propounded in this article is an attempt at an analysis of the factors termed by Darwin “the nature of the organism” and “the nature of the conditions.” It is assumed, as an inevitable conclusion from the facts of evolution, that plant-protoplasm possesses (1) an inherent tendency towards higher organization, and (2) that it is irritable to external conditions, or to changes in them, and can respond to them by changes of form which may be either indefinite or definite (adaptive). Thus it is that the variations are produced upon which natural selection has to work.
Material Cause of Differentiation.—It may be inquired, in conclusion, if there are any facts which throw light upon the internal mechanism of differentiation, whether spontaneous or induced; if it is possible to refer it to any material cause. It may be replied that there are such facts, and though they are but few as yet, they suffice to suggest an hypothesis that may eventually prove to be a law. Sachs was the first to formulate the theory that morphological differences are the expression of differences in material composition. He considered, for instance, that stems, leaves, roots and flowers differ as they do because the plastic substances entering into their structure are diverse. This view he subsequently modified to this—that a relatively small proportion of diverse substance in each of these parts would suffice to account for their morphological differences. This modification is important, because it transfers the formative influence from the plastic substances to the protoplasm, suggesting that the diverse constituents are produced (whether spontaneously or as the result of stimulation) as secretions by the protoplasm. It is an obvious inference that if a small quantity of a substance can affect the development of an entire organ it probably acts after the manner of an enzyme. Beyerinck has, in fact, gone so far as to speak of “formative enzymes.”
It is not possible to go into all the facts that might be adduced in support of this view: one case, perhaps the most pregnant, must suffice. Beyerinck was led to take up the decided position just mentioned by his researches into the conditions determining the formation of plant-galls as the result of injury by insects. He found that the development of a gall is due to a temporary modification of the part affected, not, as is generally thought, in consequence of the deposition of an egg by the insect, but of the injection of a poisonous substance which has the effect of stimulating the protoplasm to develop a gall instead of normal structure. If this be so, it may justifiably be inferred that both normal and abnormal morphological features may be due to the presence of enzymatic substances secreted by the protoplasm that determine the course of development. At any rate this hypothesis suggests an explanation of many hitherto inexplicable facts. For instance, it has been pointed out in the article on the reproduction of plants that the effect of the fertilization of the female cell in the ovule of a phanerogam is not confined to the female cell, but extends more or less widely outside it, inducing growth and tissue-change. The ovule develops into the seed; and the gynaeceum and even more remote parts of the flower, develop into the fruit. The facts are familiar, but there is no means of explaining them. In the light of Sachs's theory the interpretation is this, that the act of fertilization causes the formation in the female cell of substances which are transmitted to adjacent structures and stimulate them to further development.
Literature.—As the scope of this article limits it to the general principles of the morphology of plants, comparatively few facts have been adduced. Full morphological and organographical details are given in the articles on the various groups of plants, such as those on the Algae, Bryophyta, Pteridophyta, Angiosperms, Gymnosperms, &c. The following works may also be consulted: Schimper, Plant-Geography (Clarendon Press, Oxford); Goebel, Organography (Clarendon Press, Oxford); Bower, The Origin of a Land Flora (Macmillan); Beyerinck, “Ueber Cecidien,” (Bot. Zeitung, 1888). (S. H. V*)
Distribution of Plants
Common experience shows that temperature is the most important condition which controls the distribution of plants. Those of warmer countries cannot be cultivated in British gardens without protection from the rigours of winter; still less are they able to hold their own unaided in an unfavourable climate. Temperature, then, is the fundamental limit which nature opposes to the indefinite extension of any one species. Buffon remarked “that the same temperature might have been expected, all other circumstances being equal, to produce the same beings in different parts of the globe, both in the animal and vegetable kingdoms.” Yet lawns in the United States are destitute of the common English daisy, the wild hyacinth of the woods of the United Kingdom is absent from Germany, and the foxglove from Switzerland. We owe to Buffon the recognition of the limitation of groups of species to regions separated from one another by “natural barriers.” When by the aid of man they surmount these, they often dominate with unexpected vigour the native vegetation amongst which they are colonists. The cardoon and milk thistle, both European plants, cover tracts of country in South America with impenetrable thickets in which both man and beast may be hopelessly lost. The watercress blocks the rivers of New Zealand into which it has been introduced from Europe. The problem, then, which plant distribution presents is twofold: it has first to map out the earth's surface into “regions” or “areas of vegetation,” and secondly to trace the causes which have brought them about and led to their restriction and to their mutual relations.
The earliest attempts to deal with the first branch of the inquiry may be called physiognomical. They endeavoured to define “aspects of vegetation” in which the “forms” exhibited an obvious adaptation to their climatic surroundings. This has been done with success and in great detail by Grisebach, whose Vegetation der Erde from this point of view is still unsurpassed. With it may be studied with advantage the unique collection at Kew of pictures of plant-life in its broadest aspects, brought together by the industry and munificence of Miss Marianne North. Grisebach declined to see anything in such “forms” but the production by nature of that which responds to external conditions and can only exist as long as they remain unchanged. We may agree with Schimper that such a point of view is obsolete without rejecting as valueless the admirable accumulation of data of which it admittedly fails to give any rational explanation. A single example will be sufficient to illustrate this. The genus Senecio, with some 1000 species, is practically cosmopolitan. In external habit these exhibit adaptations to every kind of climatic or physical condition: they may be mere weeds like groundsels or ragworts, or climbers masquerading like ivy, or succulent and almost leafless, or they may be shrubs and even trees. Yet throughout they agree in the essential structure of their floral organs. The cause of such agreement is, according to Grisebach, shrouded in the deepest obscurity, but it finds its obvious and complete explanation in the descent from a common ancestor which he would unhesitatingly reject.
From this point of view it is not sufficient, in attempting to map out the earth's surface into “regions of vegetation,” to have regard alone to adaptations to physical conditions. We are compelled to take into account the actual affinity of the plants inhabiting them. Anything short of this is merely descriptive and empirical, and affords no rational basis for inquiry into the mode in which the distribution of plant-life has been b1ought about. Our regions will not be “natural” unless they mark out real discontinuities both of origin and affinity, and these we can only seek to explain by reference to past changes in the earth's history. We arrive thus at “the essential aim of geographical botany,” which, as stated by Schimper, is “an inquiry into the causes of differences existing among the various floras.” To quote further: “Existing floras exhibit only one moment in the history of the earth's vegetation. A transformation which is sometimes rapid, sometimes slow, but always continuous, is wrought by the reciprocal action of the innate variability of plants and of the variability of the external factors. This change is due partly to the migrations of plants, but chiefly to a transformation of the plants covering the earth.” This transformation is due to new characters arising through variation. “If the new characters be useful, they are selected and perfected in the descendants, and constitute the so-called ‘adaptations’ in which the external factors acting on the plants are reflected.” The study of the nature of these adaptations, which are often extremely subtle and by no means merely superficial, is termed Ecology (see above).
The remark may conveniently find its place here that plants which have reached a high degree of adaptive specialization have come to the end of their tether: a too complicated adjustment has deprived them of the elasticity which would enable them to adapt themselves to any further change in their surroundings, and they would pass away with conditions with which they are too inextricably bound up. Vast floras have doubtless thus found their grave in geologic change. That wrought by man in destroying forests and cultivating the land will be no less effective, and already specimens in our herbaria alone represent species no longer to be found in a living state. Extinction may come about indirectly and even more surely. This is easy to happen with plants dependent on insects for their fertilization. Kronfeld has shown that aconites are dependent for this on the visits of a Bombus and cannot exist outside the area where it occurs.
The actual and past distribution of plants must obviously be controlled by the facts of physical geography. It is concerned with the land-surface, and this is more symmetrically disposed than would at first sight appear from a glance at a map of the world. Lyell points out that the eye of an observer placed above a point between Pembroke and Wexford, lat. 52° N. and long. 6° W., would behold at one view the greatest possible quantity of land, while the opposite hemisphere would contain the greatest quantity of water. The continental area is on one side of the sphere and the oceanic on the other. Love has shown (Nature, Aug. 1, 1907, p. 328) that this is the result of physical causes and that the existence of the Pacific Ocean “shows that the centre of gravity of the earth does not coincide with the centre of figure.” One half of the earth has therefore a greater density than the other. But “under the influence of the rotation the parts of greater density tend to recede further from the axis than the parts of less density . . . the effect must be to produce a sort of furrowed surface.” The furrows are the great ocean basins, and these would still persist even if the land surface were enlarged to the 1400 fathoms contour. These considerations preclude the possibility of solving difficulties in geographical distribution by the construction of hypothetical land-surfaces, an expedient which Darwin always stoutly opposed (Life and Letters, ii. 74-78) The furrowed surface of the earth gives the land-area a star-shaped figure, which may from time to time have varied in outline, but in the main has been permanent. It is excentric as regards the pole and sends tapering extensions towards the south. Sir George Darwin finds a possible explanation of these in the screwing motion which the earth would suffer in its plastic state. The polar regions travelled a little from west to east relatively to the equatorial, which lagged behind.
The great primary divisions of the earth's flora present themselves at a glance. The tropics of Cancer and Capricorn cut off with surprising precision (the latter somewhat less so) the tropical from the north and south temperate zones. The north temperate region is more sharply separated from the other two than the south temperate region from the tropical.
I. North Temperate Region (Holarctic).—This is the largest of all, circumpolar, and but for the break at Bering Straits, would be, as it has been in the past, continuous in both the old and new worlds. It is characterized by its needle-leaved Coniferae, its catkin-bearing (Amentaceae) and other trees, deciduous in winter, and its profusion of herbaceous species.
II. South Temperate Region.—This occupies widely separated areas in South Africa, Australia, New Zealand and South America. These are connected by the presence of peculiar types, Proteaceae, Restiaceae, Rutaceae, &c., mostly shrubby in habit and on the whole somewhat intolerant of a moist climate. Individual species are extremely numerous and often very restricted in area.
III. Tropical Region.—This is characterized by the presence of gigantic Monocotyledons, palms, Musaceae and bamboos, and of evergreen polypetalous trees and figs. Herbaceous plants are rare and mostly epiphytic.
A consideration of these regions makes it apparent that they are to a large extent adaptive. The boreal is cold, the austral warm, and the tropical affords conditions of heat and moisture to which the vegetation of the others would be intolerant. If we take with Drude the number of known families of flowering plants at 240, 92 are generally dispersed, 17 are more restricted, while the remainder are either dominant in or peculiar to separate regions. Of these 40 are boreal, 22 austral and 69 tropical. If we add to the latter figure the families which are widely dispersed, we find that the tropics possess 161 or almost exactly two-thirds of the large groups comprised in the world's vegetation. M. Casimir de Candolle has made an independent investigation, based on Hooker and Bentham's Genera plantarum. The result is unfortunately (1910) unpublished, but he informs the present writer that the result leads to the striking conclusion: “La végétation est un phénomène surtout intertropical, dont nous ne voyons plus que restes affaiblis dans nos régions tempérées.” In attempting to account for the distribution of existing vegetation we must take into account palaeontological evidence. The results arrived at may be read as a sequel to the article on Palaeobotany.
The vegetation of the Palaeozoic era, till towards its close, was apparently remarkably homogeneous all over the world. It was characterized by arborescent vascular Cryptogams and Gymnosperms of a type (Cordaiteae) which have left no descendants beyond it. In the southern hemisphere the Palaeozoic flora appears ultimately to have been profoundly modified by a lowering of temperature and the existence of glacial conditions over a wide area. It was replaced by the Glossopteris flora which is assumed to have originated in a vast continental area (Gondwana land), of which remnants remain in South America, South Africa and Australia.
The Glossopteris flora gradually spread to the northern hemisphere and intermingled with the later Palaeozoic flora which still persisted. Both were in turn replaced by the Lower Mesozoic flora, which again is thought to have had its birth in the hypothetical Gondwana land, and in which Gymnosperms played the leading part formerly taken by vascular Cryptogams. The abundance of Cycadean plants is one of its most striking features. They attained the highest degree of structural complexity in the Bennettiteae, which have been thought even to foreshadow a floral organization. Though now on the way to extinction, Cycadeae are still widely represented in the southern hemisphere by genera wh1ch, however, have no counterpart in the Mesozoic era. Amongst Conifers the archaic genera, Ginkgo and Araucaria still persist. Once widely distributed in the Jurassic period throughout the world, they are now dying out: the former is represented by the solitary maiden-hair tree of China and Japan; the latter by some ten species confined to the southern hemisphere, once perhaps their original home. With them may be associated the anomalous Sciadopitys of Japan.
So far the evolution of the vegetable kingdom has proceeded without any conspicuous break. Successive types have arisen in ascending sequence, taken the lead, and in turn given way to others. But the later period of the Mesozoic era saw the almost sudden advent of a fully developed angiospermous vegetation which rapidly occupied the earth's surface, and which it is not easy to link on with any that preceded it. The closed ovary implies a mode of fertilization which is profoundly different, and which was probably correlated with a simultaneous development of insect life. This was accompanied by a vegetative organization of which there is no obvious foreshadowing. As Clement Reid remarked: “World-wide floras, such as seem to characterize some of the older periods, have ceased to be, and plants are distributed more markedly according to geographical provinces and in climatic zones.” The field of evolution has now been transferred to the northern hemisphere. Though Angiosperms become dominant in all known plant-bearing Upper Cretaceous deposits, their origin dates even earlier. In Europe Heer's Populus primaeva from the Lower Cretaceous in Greenland was long accepted as the oldest dicotyledonous plant. Other undoubted Dicotyledons, though of uncertain affinity, of similar age have now been detected in North America. The Cenomanian rocks of Bohemia have yielded remains of a sub-tropical flora which has been compared with that existing at present in Australia. Upper Cretaceous formations in America have yielded a copious flora of a warm-temperate climate from which it is evident that at least the generic types of numerous not closely related existing dicotyledonous trees had already come into existence. It may be admitted that the identification of fragmentary leaf-remains is at most precarious. Even, however, with this reservation, it is difficult to resist the mass of evidence as a whole. And it is a plausible conjecture that the vegetation of the globe had already in its main features been constituted at this period much as it exists at the present moment. There were oaks, beeches (scarcely distinguishable from existing species), birches, planes and willows (one closely related to the living Salix candida), laurels, represented by Sassafras and Cinnamomum, magnolias and tulip trees (Liriodendron), myrtles, Liquidambar, Diospyros and ivy. This is a flora which, thinned out by losses, practically exists to this day in the southern United States. And one essentially similar but adapted to slightly cooler conditions existed as far north as the latitude of Greenland.
The tertiary era opens with a climate in which during the Eocene period something like existing tropical conditions must have obtained in the northern hemisphere. The remains of palms (Sabal and Nipa) as well as of other large-leaved Monocotyledons are preserved. Sequoia (which had already appeared in the American Upper Cretaceous) and the deciduous cypress (Taxodium distichum) are found in Europe. Starkie Gardner has argued with much plausibility that the Tertiary floras which have been found in the far north must have been of Eocene age. That of Grinnell Land in lat. 81° consisted of Conifers (including the living spruce), poplars and willows, such as would be found now 25° to the south. The flora of Disco Island in lat. 70° contained Sequoia, planes, maples and magnolias and closely agrees with the Miocene flora of central Europe. Of this copious remains have been found in Switzerland and have been investigated with great ability by O. Heer. They point to cooler conditions in the northern hemisphere: palms and tropical types diminish; deciduous trees increase. Sequoia and the tulip-tree still remain; figs are abundant; laurels are represented by Sassafras and camphor; herbaceous plants (Ranunculaceae, Cruciferae, Umbelliferae) are present, though, as might be expected, only fragmentarily preserved.
We may draw with some certainty the conclusion that a general movement southward of vegetation had been brought about. While Europe and probably North America were occupied by a warm temperate flora, tropical types had been driven southward, while the adaptation of others to arctic conditions had become accentuated. A gradual refrigeration proceeded through the Pliocene period. This was accompanied in Europe by a drastic weeding out of Miocene types, ultimately leaving the flora pretty much as it now exists. This, as will be explained, did not take place to anything like the same extent in North America, the vegetation of which still preserves a more Miocene facies. Torreya, now confined to North America and Japan, still lingered, as did Ocotea, now profusely developed in the tropics, but in north temperate regions only existing in the Canaries; the evergreen oaks, so characteristic of the Miocene, were reduced to the existing Quercus ilex. At the close of the Pliocene the European flora was apparently little different from that now existing, though some warmer types such as the water chestnut (Trapa natans) had a more northern extension. The glacial period effected in Europe a wholesale extermination of temperate types accompanied by a southern extension of the arctic flora. But its operation was in great measure local. The Pliocene flora found refuges in favoured localities from which at its close the lowlands were restocked while the arctic plants were left behind on the mountains. During the milder interglacial period some southern types, such as Rhododendron ponticum, still held their own, but ultimately succumbed.
The evidence which has thus been briefly summarized, points unmistakably to the conclusion that existing vegetation originated in the northern hemisphere and under climatic conditions corresponding to what would now be termed sub-tropical. It occupied a continuous circumpolar area which allowed free communication between the old and new worlds. The gradual differentiation of their floras has been brought about rather by extermination than specialization, and their distinctive facies by the development and multiplication of the surviving types.
The distribution of mountain barriers in the Old and New Worlds is in striking contrast. In the former they run from east to west; in the latter from north to south. In the Old World the boreal zone is almost sharply cut off and afforded no means of escape for the Miocene vegetation when the climate became more severe. Thus in the Mediterranean region the large groups of palms, figs, myrtles and laurels are each only represented by single surviving species. The great tropical family of the Gesneraceae has left behind a few outliers: Ramondia in the Pyrenees, Haberlea in the Balkans, and Jankaea in Thessaly; the Pyrenees also possess a minute Dioscorea, sole European survivor of the yams of the tropics.
In North America there is no such barrier, the Miocene flora has been able to escape by migration the fluctuations of climate and to return when they ameliorated. It has preserved its characteristic types, such as Magnolia, Liriodendron, Liquidambar, Torreya, Taxodium and Sequoia. While it has been customary to describe the Miocene flora of Europe as of a North American type, it would be more accurate to describe the latter as having in great measure preserved its Miocene character.
If mountains serve as barriers which arrest the migration of the vegetation at their base, their upper levels and summits afford lines of communication by which the floras of colder regions in the northern hemisphere can obtain a southern extension even across the tropics. They doubtless equally supply a path by which southern temperate types may have extended northwards. Thus the characteristic assemblage of plants to which Sir Joseph Hooker has given the name Scandinavian “is present in every latitude of the globe, and is the only one that is so” (Trans. Linn. Soc. xxiii. 253). In the mountains of Peru we find such characteristic northern genera as Draba, Alchemilla, Saxifraga, Valeriana, Gentiana and Bartsia. High elevations reproduce the physical conditions of high latitudes. The aqueous vapour in the atmosphere is transparent to luminous but opaque to obscure heat-rays. The latter are retained to warm the air at lower levels, while it remains cold at higher. It results that besides a horizontal distribution of plants, there is also an altitudinal: a fact of cardinal importance, the first observation of which has been attributed to Tournefort.
Speaking generally, all plants tend to exhaust particular constituents of the soil on which they grow. Nature therefore has provided various contrivances by which their seeds are disseminated beyond the actual position they occupy. In a large number of cases these only provide for migration within sufficient but narrow limits; such plants would be content to remain local. But other physical agencies come into play which may be briefly noticed. The first of these is wind: it cannot be doubted that small seeds can be swept up like dust and transported to considerable distances. This is certainly the case with fern-spores. The vegetation of Krakatoa was completely exterminated in 1883 by a thick coat of red-hot pumice. Yet in 1886 Treub found that it was beginning to cover itself again with plants, including eleven species of ferns; but the nearest source of supply was 10 m. distant. Seeds are carried with more facility when provided with plumes or wings. Treub found on Krakatoa four species of composites and two grasses. Water is another obvious means of transport. The littoral vegetation of coral islands is derived from sea-borne fruits. The seeds of West Indian plants are thrown on the western shores of the British Isles, and as they are capable of germination, the species are only prevented from establishing themselves by an uncongenial climate. Travers picked up a seed of Edwardsia in the Chatham Islands, evidently washed ashore from New Zealand (Linn. Soc. Journ. ix. 1865). Rivers bring down the plants of the upper levels of their basins to the lower: thus species characteristic of the chalk are found on the banks of the Thames near London. Birds are even more effective than wind in transporting seeds to long distances. Seeds are carried in soil adhering to their feet and plumage, and aquatic plants have in consequence for the most part an exceptionally wide range. Fruit-pigeons are an effective means of transport in the tropics by the undigested seeds which they void in their excrement. Quadrupeds also play their part by carrying seeds or fruits entangled in their coats. Xanthium spinosum has spread from the Russian steppes to every stock-raising country in the world, and in some cases has made the industry impossible. Even insects, as in the case of South African locusts, have been found to be a means of distributing seeds.
The primary regions of vegetation, already indicated, and their subordinate provinces may now be considered more in detail.
I. North Temperate Region.—Many writers on the distribution of animals prefer to separate this into two regions of “primary rank”: the Palaearctic and the Nearctic. But to justify such a division it is necessary to establish either an exclusive possession or a marked predominance of types in the one which are correspondingly deficient in the other. This cannot apparently be done for insects or for birds; Newton accordingly unites the two into the Holarctic region. It equally fails for plants. To take, for example, one of the most characteristic features of the Palaearctic region, its catkin-bearing deciduous trees: in North America we find precisely the same genera as in the Old World—oaks, chestnuts, beeches, hazels, horn beams, birches, alders, willows and poplars. Or to take the small but well defined group of five-leaved pines, all the species of which may be seen growing side by side at Kew under identical conditions: we have the Weymouth pine (Pinus Strobus) in eastern North America, P. monticola and the sugar pine (P. Lambertiana) in California, P. Ayacahuste in Mexico, the Arolla pine (P. Cembra) in Switzerland and Siberia, P. Peuce in Greece, the Bhotan pine (P. excelsa) in the Himalayas, and two other species in Japan. Amongst broad-leaved trees Juglans has a similar Holarctic range, descending to the West Indies; so has Aesculus, were it not lacking in Europe; it becomes tropical in South America and Malaya. If we turn to herbaceous plants, Hemsley has pointed out that of the thirteen genera of Ranunculaceae in California, eleven are British.
While the tropics preserve for us what remains of the pre-Tertiary or, at the latest, Eocene vegetation of the earth, which formerly had a much wider extension, the flora of the North Temperate region is often described as the survival of the Miocene. Engler therefore calls it Arcto-Tertiary. We must, however, agree with Starkie Gardner that it is only Miocene as regards its present position, which was originally farther north, and that its actual origin was much earlier. There has been in effect a successive shifting of zones of vegetation southwards from the pole. Their distinctive and adaptive characteristics doubtless began to be established as soon as the phanerogamic flora was constituted. There is no reason to suppose that the peculiarities of the arctic flora are more modern than those of any other, though there is no fossil evidence to prove that it was not so.
The North Temperate region admits of subdivision into several well-marked sub-regions. The general method by which this is effected in this and other cases is statistical. As A. de Candolle, however, points out, exclusive reliance on this may be misleading unless we also take into account the character and affinities of the plants dealt with (Geogr. Bot. i. 1166). The numerical predominance of certain families or their absence affords criteria for marking out boundary lines and tracing relationships. The analysis of the flora of the British Isles will afford an illustration. This was first attempted in 1835 by H. C. Watson, and his conclusions were enforced ten years later by Edward Forbes, who dealt also with the fauna. Watson showed that Scotland primarily, and to a less extent the north of England, possessed species which do not reach the south. Such are the crow berry (Empetrum nigrum), Trientalis europaea, Rubus saxatilis and the globe-flower (Trollius europaeus). He further found that there was an element which he termed “boreal . . . in a more intense degree,” which amounted to about “a fifteenth of the whole flora.” This was not confined to the north but may occur on the mountains of England and Wales: Salix herbacea, Silene acaulis and Dryas octopetala will serve as examples. Even so small an area as that of Britain illustrates what has already been pointed out, that the species of a flora change both with latitude and altitude. Watson further brought out the striking fact that the west and east of Britain each had species peculiar to it; the former he characterized as Atlantic, the latter as Germanic. The Cornish heath (Erica vagans) and the maiden-hair fern (Adiantum Capillus-Veneris) may serve as instances of the one, the man-orchis (Aceras anthropophora) and Reseda lutea of the other. Ireland illustrates the same fact. It possesses about 1000 species, or about two-thirds the number of Britain. On its western shores there are some twenty, such as Saxifraga umbrosa, Erica mediterranea and Arbutus unedo, which are not found in Britain at all. The British Phanerogamic flora, it may be remarked, does not contain a single endemic species, and 38% of the total number are common to the three northern continents.
The analysis of larger areas yields results of the same kind. Within the same region we may expect to find considerable differences as we pass from one meridian to another. Assuming that in its circumpolar origin the North Temperate flora was fairly homogeneous, it would meet in its centrifugal extension with a wide range of local conditions; these would favour the preservation of numerous species in some genera, their greater or less elimination in others. Thus comparing the Nearctic and Palaearctic floras we find striking differences overlying the points of agreement already indicated. The former is poor in Cruciferae, Caryophyllaceae, Umbelliferae, Primulaceae and Labiatae; but for the occurrence of Calluna, in Newfoundland it would have no heaths. On the other hand, it is rich in Compositae, especially Solidago and Aster, Polemoniaceae, Asclepiadaceae, Hydrophyllaceae and Cyperaceae, and it has the endemic Sarracenia, type of a family structurally allied to poppies, of which of the remaining genera Darlingtonia is Californian, and Heliamphora Venezuelan. These distinctions led Sir Joseph Hooker to claim for the two divisions the rank of primary regions. Darwin doubted, however, whether they ought to be separated (Life, iii. 230). Lyell, discussing the facts of zoological distribution, admits that “the farther we go north . . . the more the discordance in genera and species diminishes” (Principles, ii. 340); and Hemsley finds that not less than 75% of the genera in the flora of eastern North America “are represented in the old world,” and, according to Asa Gray, 50% in Europe.
Latitudinally the region subdivides naturally into several well marked sub-regions which must be briefly discussed.
1. The Arctic-Alpine sub-region consists of races of plants belonging originally to the general flora, and recruited by subsequent additions, which have been specialized in low stature and great capacity of endurance to survive long dormant periods, sometimes even unbroken in successive years by the transitory activity of the brief summer. It is continuous round the pole and roughly is bounded by the arctic circle. Mature seeds are highly tolerant of cold and have been shown to be capable of withstanding the temperature even of liquid hydrogen. Arctic plants make their brief growth and flower at a temperature little above freezing-point, and are dependent for their heat on the direct rays of the sun. Characteristic representatives are Papaver nudicaule, Saxifraga oppositifolia, which forms a profuse carpet, and Dryas octopetala. Such plants perhaps extend to the most northern lands at present known. On May 30th, in Ward Hunt's Island, lat. 83° 5′, Sir George Nares found that “vegetation was fairly represented as regards quantity in the poppy, saxifrage and small tufts of grass.” We may compare this with extreme alpine conditions: on a spot above the Aletsch glacier at a height of 10,700 ft. Ball found the temperature one inch below the surface to be 83°, and he collected “over forty species in flower.” Taking the whole arctic flora at 762 species, Hooker found that 616 occurred in arctic Europe, and of these 586 are Scandinavian. Beyond the arctic circle some 200, or more than a quarter, are confined to the mountains of the northern hemisphere and of “still more southern regions.” This led Hooker to the striking observation already quoted. The arctic flora contains no genus that is peculiar to it, and only some fifty species that are so. Christ has objected to terming the arctic flora Scandinavian, but the name implies nothing more than that Scandinavia has been its chief centre of preservation.
A detailed examination of mountain floras shows that a large local element is present in each besides the arctic. The one is in fact the result of similar physical conditions to that which has produced the other. Thus Saxifraga cernua is regarded as an alpine form of the lowland S. granulata. Comparing the Alps with the Pyrenees, according to Ball, each has about half its flora common to the other: “the Alps have 172 endemic species and at least 15 genera that are not found in the Pyrenees, while the latter range counts about 100 endemic species with several (six or seven) genera not found in the Alps.” Drude has accordingly suggested the substitution of the term “High-mountain floras” for Alpine, which he regards as misleading. Its meaning has, however, become synonymous and is consecrated by usage.
The repetition of the same species in the arctic regions and in the high mountains of the North Temperate region is generally attributed to the exchange which took place during the glacial period. This was first suggested by Edward Forbes in 1846, though the idea had earlier suggested itself to Darwin (Life, i. 88). It took place southwards, for the arctic flora is remarkably uniform, and, as Chodat points out, it shows no evidence of having been recruited from the several mountain floras. That the arctic flora was driven south into Central Europe cannot be contested in the face of the evidence collected by Nathorst from deposits connected with the boulder-clay. And Reid has shown that during the glacial period the existing flora was replaced by an arctic one represent by such plants as Salix polaris, S. herbacea, S. reticulata and Betula nana. At the same time the then existing alpine floras descended to lower levels, though we may agree with all that they did not necessarily become extinct at higher ones as long as an land-surface remained uncovered by ice. At the close of the glacial period the alpine floras retreated to the mountains accompanied by an arctic contingent, though doubtless many species of the latter, such as Salix polaris, failed to establish themselves. Christ, while admitting an ancient endemic element, such as Campanula excisa in the arctic-alpine flora of Europe, objects that a Scandinavian colonization could not furnish such characteristic plants as the larch and edelweiss. He traces the oriinal home of the bulk of existing alpine plants to northern Asia, the mountains of which appear to have escaped glaciation. At the close of the glacial epoch the north Asiatic flora spread westwards into Europe and intermingled with the surviving vegetation. Some species, such as Anemone alpina, which are wanting in the Arctic flora of the Old World, he thinks must have reached Europe by way of Greenland from north-east America.
2 The Intermediate sub-region comprises the vegetation of the large area occupied by the steppes of the Old World, the prairies of the new and the forest region of both. The former support a copious herbaceous flora, the characteristics of which in the Old and New Worlds have been already briefly summarized. In the former that of Europe and of Central Asia are continuous. Of species common to the two, Maximowicz finds that Manchuria possesses 40% and scarcely 9% that are endemic. Of a collection of about 500 species made in that country by Sir Henry James nearly a third are British. This confirms the theory of Christ that Europe was restocked mainly from Asia after the close of the glacial epoch, the south being closed to it. In the new world no southern barriers existed and it is more difficult to draw the line between contiguous sub-regions.
The dominant characteristics of the arboreous vegetation are, besides deciduous and amentiferous trees, Coniferae, especially the more recent tribe of Abietineae—pines, silver-firs, hemlocks, spruces and larches, of which, unlike the older types, no representative crosses the tropic. The prominent deciduous trees of Europe appear to be of eastern origin, and the progress of western migration has continued to historic times. The evidence of the peat bogs shows that the Scots fir, which is now extinct, was abundant in Denmark in the Roman period. It was succeeded by the sessile-fruited oak, which was in turn supplanted by the pedunculate form of the same tree. Quercus Robur has left no trace in the Tertiary deposits of Europe and it is most nearly allied to east Asiatic species. The oak in turn has been almost superseded in Denmark by the beech, which, if we may trust Julius Caesar, had not reached Britain in his time, though it existed there in the pre-glacial period, but is not native in either Scotland or Ireland. Its eastern limit in Europe is a line from Königsberg to the Caucasus; thence through China it is continued by varietal forms to Japan. It has a solitary representative in North America.
Broadly speaking, the American portion of the sub-region consists of an Atlantic and Pacific forest area and an intervening non-forest one, partly occupied by the Rocky Mountains, partly by intervening plains. Its arboreal vegetation is richer both in genera and species than that corresponding to it in the Old World. Glacial elimination has been less severe, or rather there has been, at any rate on the Atlantic side, an unimpeded return of Miocene types. The Atlantic area has five magnolias, a tulip tree, an Anonacea (Asimina), two Ternstroemiaceae (Stuartia and Gordonia), Liquidambar, Vitis (the fox-grape, V. Labrusca, reappears in Japan), and others; an assemblage, as long ago pointed out by Asa Gray, which can only be paralleled in the Chino-Japanese region, another centre of preservation of Miocene types. All these are wanting in the Pacific area, though there are indications in its gold-bearing gravels that it once possessed them. On the other hand, the latter “is rich in coniferous types beyond any country except Japan” (A. Gray), but till we reach California these are boreal in type. The Atlantic flora has also numerous oaks and maples, signalized by their autumnal coloration. These were abundant in Tertiary Europe, as they are now in Japan, and represent perhaps a cooler element in the flora than that indicated above. The highlands of Central America and the West Indies have preserved a number of Chino-Japanese types—Bocconia, Deutzia, Abelia, &c.—not met with elsewhere in the New World.
3. The Medrterraneo-Oriental sub-region contrasts no less vividly with the Intermediate than the Arctic-Alpine. It includes the Azores and Canaries, the Mediterranean basin, northern Africa as far as the Atlas and Sahara, Asia Minor, Persia and the countries eastward as far as Sind, being bounded to the north by the mountains which run from the Caucasus to the Hindu-Kush. Its extreme richness in number of species (it comprises six-sevenths of the European flora) and the extremely restricted areas of many of them point to a great antiquity. The Mediterranean basin has been a centre of preservation of Miocene vegetation: the oleander is said to have been found in local deposits of even earlier age, and the holm oak (Quercus Ilex) is the living representative of a Miocene ancestor. Extensions of the flora occur southwards of the high mountains of tropical Africa; Adenocarpus, a characteristic Mediterranean genus, has been found on Kilimanjaro and 2000 m. distant on the Cameroons. Two British plants may be added which both reach North Africa: Sanicula europaea, extends from Abyssinia to the Cameroons and southwards to Cape Colony and Madagascar; Sambucus Ebulus reaches Uganda. The Mediterranean, however, has apparently been a barrier to the southward passage of the arcto-alpine flora which is totally wanting on the Atlas. The vegetation of the sub-region is rich in shrubs: myrtle, bay, Cistus, Pistacia, Arbutus, heaths in its western portion, and the ground-palm (Chamaerops). It is even richer in more herbaceous plants tolerant of a hot summer; giant Umbelliferae (such as Ferula) are especially characteristic and yield gum-resins which have long been reckoned valuable. Of the 1000 known species of Astragalus it possesses 800. Evergreen oaks and Conifers form the forests. Asia Minor has a Liquidambar. The Argan tree (Argania Sideroxylon), which forms forests in Morocco, is a remarkable survivor of a tropical family (Sapotaceae). Amongst Conifers Cedrus is especially noteworthy; it is represented by geographical races in the north-west Himalaya, in Syria, Cyprus and North Africa.
This well-marked sub-region has a deeper interest than the botanical. It has been the cradle of civilization, and to it is due the majority of cultivated plants. Such are the date in Mesopotamia (a second species of Phoenix occurs in the Canaries); most European fruits, e.g. the vine, fig, mulberry, cherry, apricot, walnut; pulses, e.g. the bean, pea and lentil; pot-herbs, e.g. lettuce, endive, beet, radish, cress; cereals; and fodder plants such as lucerne and carob.
4. The Chino-Japanese sub-region.—Of the vegetation of China till recently very little has been known. In 1873 Elwes pointed out that the Himalayan avifauna extended into north-west China and established the Himalo-Chinese sub-region. Shortly afterwards the collections of Prejewalsky confirmed it for the flora. And we now know that, excluding the southern tropical area, it has the same character throughout the whole of China proper. We may therefore regard the Himalayan flora as a westward extension of the Chinese rather than the latter as a development of the former. Of four genera which Hooker singles out as the largest in Sikkim, in China Corydalis has 76 species, Saxifraga 58, Pedicularis 129, and Primula 77. Of Rhododendron there are 134 species. Upwards of 8000 species are known out of a probable total of not less than 12,000, and of these more than half are endemic. The number of species to a genus, 3, is only half that found in other large areas. This aggregation of genera and of endemic species is characteristic of the circumferential portions of the earth's land surface, the expansion of the one and the further advance of the other is arrested. The sub-region is probably sharply cut off from the Intermediate. Maximowicz finds that 40% of the plants of Manchuria are common to Europe and Asia, but the proportion falls sharply to 16% in the case of Japan. Its connexion with the Mediterraneo-Oriental sub-region is still more remote. China has no Cistus or heath, only a single Ferula, while Astragalus is reduced to 35 species. There are to species of Pistacia and four of Liquidambar. The affinity to Atlantic North America is strongly marked as it has long been known to be in Japan. China has 66 species of Quercus, 35 of Vitis, 2 of Aesculus, 42 of Acer, 33 Magnoliaceae (including two species of Liriodendron), 12 Anonaceae, 71 Ternstroemiaceae (including the tea-plant), and 4 of Clethra, which has a solitary western representative in Madeira. Trachycarpus and Rhapis are characteristic palms, and Cycadeae are represented by Cycas.
5. The Mexico-American sub-region has as its northern boundary the parallel of lat. 36° as far as New Mexico and then northwards to the Pacific coast at lat. 40°. The eastern and western halves are contrasted in climate—the former being moist and the latter dry—and have been distinguished by some zoologists as distinct sub-regions. They are 1n fact in some degree comparable to sub-regions 3 and 4 in the Old World. The absence of marked natural boundaries makes any precise north and south limitation difficult. But it has been a centre of preservation of the Taxodieae, a tribe of Coniferae of great antiquity. Taxodium (with single species in China and Mexico) is represented by the deciduous cypress (T. distichum), which extends from Flor1da to Texas. The two species of Sequoia, the “Wellingtonia” (S. gigantea) and the redwood (S. sempervirens), are confined to California. In the eastern forests the prevalence of Magnoliaceae and of Clethra and Rhododendron continues the alliance with eastern Asia. Florida derives a tropical element from the Antilles. Amongst palms the Corypheae are represented by Sabal and Thrinax, and there is a solitary Zamia amongst Cycads. The western dry areas have the old-world leguminous Astragalus and Prosopis (Mesquit), but are especially characterized by the northward extension of the new-world tropical Cactaceae, Mammillaria, Cereus and Opuntia, by succulent Amaryllideae such as Agave (of which the so-called “American aloe” is a type), and by arborescent Liliaceae (Yucca). Amongst palms Washingtonia, Brahea and Erythea (all Corypheae) replace the eastern genera. On the west coast Cupressus Lawsoniana replaces the northern Thuya gigantea, and a laurel (Umbellularia of isolated affinity) forms forests. California and Arizona have each a species of Platanus, a dying-out genus. Elsewhere it is only represented by P. occidentalis, the largest tree of the Atlantic forests from Maine to Oregon, and by P. orientalis in the eastern Mediterranean. Otherwise the Californian flora is entirely deficient in the characteristic features of that of eastern North America. Nor, with perhaps the interesting exception of Castanopsis chrysophylla, the solitary representative in the New World of an east Asiatic genus, which ranges from Oregon to California, has it any affinity with the Chino-Japanese sub-region. Its closest connexion is with the South American Andine.
II. The Tropical Region.—The permanence of continents and great oceans as first insisted upon by J. D. Dana, but, as already stated, has later received support on purely physical grounds. It precludes the explanation of any common features in the dissevered portions of the tropical area of vegetation by lateral communications, and throws back their origin to the remotest geological antiquity. In point of fact, resemblance is in the main confined to the higher groups, such as families and large genera; the smaller genera and species are entirely different. No genus or species of palm, for example, is common to the Old and New Worlds. The ancient broad-leaved Gymnosperm Gnetum has a few surviving species scattered through the tropics of both worlds, one reaching Polynesia.
1. African sub-region.—Western Arabia must be added to the African continent, which, with this exception and possibly a former European connexion in the far west, has had apparently from a very early period an almost insular character. Bentham remarks (Journ. Linn. Soc. xiii. 492): “Here, more perhaps than in any other part of the globe, in Compositae as in so many other orders, we may fancy we see the scattered remains of ancient races dwindling down to their last representatives.” It is remarkable that the characteristic features of the Miocene flora, which in other parts of the world have spread and developed southwards, are conspicuously absent from the African tropical flora. It has no Magnoliaceae, no maples, Pomaceae, or Vacciniaceae, no Rhododendron and no Abietineae. Perhaps even more striking is the absence of Cupuliferae; Quercus, in particular, which from Tertiary times has been a conspicuous northern type and in Malayan tropical conditions has developed others which are widely divergent. Palms are strikingly deficient: there are only three out of 79 genera of Areceae, and the Corypheae are entirely absent. But including the Mascarene Islands and Seychelles the Borasseae are exclusively African. Aroideae are poorly represented compared with either South America or Malays. A peculiar feature in which tropical Africa stands alone is that at least one-fifth and probably more of the species are common to both sides of the continent and presumably stretch right across it. An Indian element derived from the north-east is most marked on the eastern side: the Himalayan Gloriosa will suffice as an example, and of more tropical types Phoenix and Calamus amongst palms. The forest flora of Madagascar, though including an endemic family Chlaenaceae, is essentially tropical African, and the upland flora south temperate.
2. The Indo-Malayan sub-region includes the Indian and Malayan peninsulas, Cochin-China and southern China, the Malayan archipelago, and Philippines, with New Guinea and Polynesia, excluding the Sandwich Islands. Probably in point of number of species the preponderant family is Orchideae, though, as Hemsley remarks, they do not “give character to the scenery, or constitute the bulk of the vegetation.” In Malaya and eastward the forests are rich in arborescent figs, laurels, myrtles, nutmegs, oaks and bamboos. Dipterocarpeae and Nepenthaceae only extend with a few outliers into the African sub-region. Screw pines have a closer connexion. Compositae are deficient. Amongst palms Areceae and Calameae are preponderant. Cycads are represented by Cycas itself, which in several species ranges from southern India to Polynesia. In India proper, with a dryer climate, grasses and Leguminosae take the lead in the number of species. But it has few distinctive botanical features. In the north-west it meets the Mediterraneo-Oriental and in the north-east the Chino-Japanese sub-regions, while south India and Ceylon have received a Malayan contribution. Bengal has no Cycas, oaks or nutmegs. Apart from the occurrence of Cycas, the Asiatic character of the Polynesian flora is illustrated by the distribution of Meliaceae. C. de Candolle finds that with one exception the species belong to genera represented in one or other of the Indian peninsulas.
3. The South American sub-region is perhaps richer in peculiar and distinctive types than either of the preceding. As in the Indo-Malayan sub-region, epiphytic orchids are probably most numerous in point of species, but the genera and even sub-tribes are far more restricted in their range than in the Old World; 4 sub-tribes with 74 genera of Vandeae are confined to South America, though varying in range of climate and altitude. Amongst arboreous families Leguminosae and Euphorbiaceae are prominent; Hevea belonging to the latter is widely distributed in various species in the Amazon basin, and yields Para and other kinds of rubber. Amongst Rubiaceae, Cinchoneae with some outliers in the Old World have their headquarters at cooler levels. In Brazil the myrtles are represented by “monkey-pots” (Lecythideae). Nearly related to myrtles are Melastomaceae which, poorly represented in the Old World, have attained here so prodigious a development in genera and species, that Ball looks upon it as the seat of origin of the family. Amongst Ternstroemiaceae, the singular Marcgravieae are endemic. So also are the Vochysiaceae allied to the “milkworts.” Cactaceae are widely spread and both northwards and southwards extend into temperate regions. Screw pines are replaced by the nearly allied Cyclanthaceae. The Amazon basin is the richest area in the world in palms, of which the Cocoineae are confined to South America, except the coco-nut, which has perhaps spread thence into Polynesia and eastward. The singular shrubby Amaryllids, Vellozieae, are common to tropical an South Africa, Madagascar and Brazil. Aroids, of which the tribes are not restricted in their distribution, have two large endemic genera, Philodendron and Anthurium. Amongst Cycads, Zamia is confined to the New World, and amongst Conifers, Araucaria, limited to the southern hemisphere, has scarcely less antiquity; Pinus reaches as far south as Cuba and Nicaragua.
The flora of the Hawaiian Islands has complicated relations. Out of the 860 indigenous plants, 80% are endemic, but Hillebrand finds that a large number are of American affinity.
III. The South Temperate Region contrasts remarkably with the northern. Instead of large continuous areas, in which local characteristics sometimes blend, it occupies widely dissevered territories in which specialization, intensified by long separation, has mostly effaced the possibility of comparing species and even genera, and compels us to seek for points of contact in groups of a higher order. The resemblances consist, in fact, not so much in the existence of one general facies running through the regions, as is the case with the northern flora, but in the presence of peculiar types, such as those belonging to the families Restiaceae, Proteaceae, Ericaceae, Mutisiaceae and Rutaceae.
1. The South African sub-region has a fora richer perhaps in number of species than any other; and these are often extremely local and restricted in area. It exhibits in a marked degree the density of species which, as already pointed out, is explicable by the arrest of further southern expansion. Hemsley remarks that “the northern genus Erica, which covers thousands of square miles in Europe with very few species, is represented by hundreds of species in a comparatively small area in South Africa.” There is an interesting connexion with Europe through the so-called Iberian flora. Bentham (Pres. addr. Linn. Soc., 1869, p. 25) points out that “the west-European species of Erica, Genisteae, Lobelia, Gladiolus, &c., are some of them more nearly allied to corresponding Cape species than they are to each other; and many of the somewhat higher races, groups of species and genera, have evidently diverged from stocks now unrepresented anywhere but in South Africa.” This flora extends from Ireland to the Canaries and reappears on the highlands of Angola. On the eastern side the southern flora finds representatives in Abyssinia, including Protea, and on the mountains of equatorial Africa, Calodendron capense occurring on Kilimanjaro. This is the most northern representative of the Rutaceous Diosmeae, which are replaced in Australia by the Boronieae. The Proteaceous genus, Faurea, occurs in Angola and Madagascar. The characteristic genus Pelargonium has a few Mediterranean representatives, and one even occurs in Asia Minor. In all these cases it is a nice question whether we are tracing an ascending or descending stream. Darwin thought that the migration southwards would always be preponderant (Origin of Species, 5th ed., 458). Other characteristic features of the flora are the abundance of Compositae, Asclepiadeae, and petaloid Monocotyledons generally, but especially Orchideae (terrestrial species predominating) and Iridaceae. There is a marked tendency towards a succulent habit. The nearly related Ficoideae replace the new-world Cactaceae, but the habit of the latter is simulated by fleshy Euphorbias and Asclepiads, just as that of Agave is by the liliaceous Aloe. South Africa has only two palms (Phoenix and Hyphaene). In the Gnetaceous Welwitschia it possesses a vegetable type whose extraordinary peculiarities make it seem amongst contemporary vegetation much as some strange and extinct animal form would if suddenly endowed with life. Conifers are scantily represented by Callitris and Podocarpus, which is common to all three sub-regions; and Cycads by the endemic Encephalartos and Stangeria.
2. The Australian sub-region consists of Australia, Tasmania, New Caledonia and New Zealand, and, though partly lying within the tropic is most naturally treated as a whole. They are linked together by the presence of Proteaceae and of Epacrideae, which take the place of the nearly allied heaths in South Africa. The most dominant order in Australia is Leguminosae, including the acacias with leaf-like phyllodes (absent in New Zealand). Myrtaceae comes next with Eucalyptus, which forms three-fourths of the forests, and Melaleuea; both are absent from New Caledonia and New Zealand; a few species of the former extend to New Guinea and one of the latter to Malaya. Cupuliferae are absent except Fagus in Australia and New Zealand. The so-called “oaks” of Australia are Casuarina, which also occurs in New Caledonia, but is wanting in New Zealand. The giant rushes Xanthorrhoea and Kingia are peculiar to Australia. Palms are poorly represented in the sub-region and are of an Indo-Malayan type. Amongst Conifers, Podocarpus is found throughout, Agathis is common to Australia, New Zealand and New Caledonia; Araucaria to the first and last. Of Cycads, Australia and New Caledonia have Cycas, and the former the endemic Macrozamia and Bowenia. The Australian land-surface must be of great antiquity, possibly Jurassic, and its isolation scarcely less ancient. In Lower Eocene times its flora appears to have been distinctly related to the existing one. Little confidence can, however, be placed in the identification of Proteaceous or, indeed, of any distinctively Australian plants in Tertiary deposits in the northern hemisphere. The Australian flora has extensions at high levels in the tropics; such exists on Kinabalu in Borneo under the equator. The Proteaceous genus Helicia reaches as far north as China, but whether it is starting or returning must as in other cases be left an open question.
While the flora of New Caledonia is rich in species (3000), that of New Zealand is poor (1400). While so many conspicuous Australian elements are wanting in New Zealand, one-eighth of its flora belongs to South American genera. Especially noteworthy are the Andine Acaena, Gunnera, Fuchsia and Calceolaria. By the same path it has received a remarkable contribution from the North Temperate region; such familiar genera as Ranunculus, Epilobium and Veronica form more than 9% of the flowering plants. And it is interesting to note that while the tropical forms of Quercus failed to reach Australia from Malaya, the temperate Fagus crept in by a back door. Three-quarters of the native species are endemic; they seem, however, to be quite unable to resist the invasion of new-comers, and already 600 plants of foreign origin have succeeded in establishing themselves.
3. The Andine sub-region extends from Peru to the Argentine and follows roughly the watershed of the Amazon. In the New World, as already explained, the path of communication between the northern and southern hemispheres has always been more or less open, and the temperate flora of southern America does not exhibit the isolation characteristic of the southern region of the Old World. Taking, however, the Andean flora as typical, it contains a very marked endemic element; Ball finds that half the genera and four-fifths of the species are limited to it; on the other hand, that half the species of Gamopetalae belong to cosmopolitan genera such as Valeriana, Gentiana, Bartsia and Gnaphalium. The relation to the other sub-regions is slight. Ericeae are wholly absent, and it has but a single Epacrid in Fuegia. Proteaceae are more marked in Guevina and Embothrium. Of Restiaceae, a single Leptocarpus has been found in Chile. On the other hand, it is the headquarters of Mutisiaceae, represented in South Africa by such genera as Oldenburgia and Gerbera and by Trichocline in Australia. Tropaeolum takes the place of the nearly allied South African Pelargonium. There has been an interchange between it and the Mexico-American sub-region, but as usual the northern has been preponderant. Prosopis extends to the Argentine; other characteristic genera are Oenothera, Godetia Collomia, Heliotropium and Eritrichium. In the ascending stream may be mentioned—Larrea, a small genus of Zygophylleae with headquarters in Paraguay and Chile, of which one species, L. mexicana, is the creosote plant of the Colorado desert, where it forms dense scrub; Acaena; the Loasaceae, of which Mentzelia reaches North America, Petunia and Lippia. Compositae compose a quarter of the Andean flora, which is a greater proportion than in any in the world. Baccharis, with some 250 species, ranges over the whole continent from the Straits of Magellan and, with seven species, to California. Melastomaceae, copiously represented in tropical America, are more feebly so in Peru and wholly wanting in Chile. A few Cactaceae extend to Chile. Of Cupuliferae, Quercus in three species only reaches Colombia, but Fagus, with only a single one in North America, is represented by several from Chile southwards and thence extends to New Zealand and Tasmania. The Magnoliaceous genus Drimys, with a single species in the new world, follows the same track. Bromeliaceae are represented by Rhodostachys and the temperate Puya. Palms as usual are few and not nearly related. Wettinia occurs in Peru, Trithrinax in Chile with the monotypic Jubaea, Juania, also monotypic, is confined to Juan Fernandez. Amongst Coniferae Podocarpus is common to this and preceding sub-regions; Libocedrus extends from California to New Zealand and New Caledonia; Fitzroya is found in Chile and Tasmania; and Araucaria in its most familiar species occurs in Chile.
4. The Antarctic-Alpine region is the complement of the Arctic-Alpine, but unlike the latter, its scattered distribution over numerous isolated points of land, remote from great continental areas, from which, during migrations like those attending the glacial period in the northern hemisphere, it could have been recruited, at once accounts for its limited number of species and their contracted range in the world. On the whole, it consists of local species of some widely distributed northern genera, such as Carex, Poa, Ranunculus, &c., with alpine types of strictly south temperate genera, characteristic of the separate localities. The monotypic Pringlea antiscorbutica, the “Kerguelen Island cabbage,” has no near ally in the southern hemisphere, but is closely related to the northern Cochlearia.
Such a summary of the salient facts in the geographical distribution of plants sufficiently indicates the tangled fabric of the earth's existing floral covering. Its complexity reflects the corresponding intricacy of geographical and geological evolution.
If the surface of the globe had been symmetrically divided into sea and land, and these had been distributed in bands bounded by parallels of latitude, the character of vegetation would depend on temperature alone; and as regards its aggregate mass, we should find it attaining its maximum at the equator and sinking to its minimum at the poles. Under such circumstances the earth's vegetation would be very different from what it is, and the study of plant distribution would be a simple affair.
It is true that the earth's physical geography presents certain broad features to which plants are adapted. But within these there is the greatest local diversity of moisture, elevation and isolation. Plants can only exist, as Darwin has said, where they must, not where they can. New Zealand was poorly stocked with a weak flora; the more robust and aggressive one of the north temperate region was ready at any moment to invade it, but was held back by physical barriers which human aid has alone enabled it to surpass.
Palaeontological evidence conclusively proves that the surface of the earth has been successively occupied by vegetative forms of increasing complexity, rising from the simplest algae to the most highly organized flowering plant. We find the ultimate explanation of this in the facts that all organisms vary, and that their variations are inherited and, if useful, perpetuated. Structural complexity is brought about by the superposition of new variations on preceding ones. Continued existence implies perpetual adaptation to new conditions, and, as the adjustment becomes more refined, the corresponding structural organization becomes more elaborate. Inheritance preserves what exists, and this can only be modified and added to. Thus Asclepiadeae and Orchideae owe their extraordinary floral complexity to adaptation to insect fertilization.
All organisms, then, are closely adapted to their surroundings. If these change, as we know they have changed, the organisms must change too, or perish. In some cases they survive by migration, but this is often prohibited by physical barriers. These, however, have often protected them from the competition of more vigorous invading races. Fagus, starting from the northern hemisphere, has more than held its own in Europe and Asia, but has all but died out in North America, finding conditions favourable for a fresh start in Australasia. The older types of Gymnosperms are inelastic and dying out. Even Pinus has found the task of crossing the tropics insuperable.
The whole story points to a general distribution of flowering plants from the northern hemisphere southwards. It confirms the general belief on geological grounds that this was the seat of their development at the close of the Mesozoic era. It is certain that they originally existed under warmer conditions of climate than now obtain, and that progressive refrigeration has supplied a powerful impulse to migration. The tropics eventually became, what they are now, great areas of preservation. The Northern Temperate region was denuded of its floral wealth, of which it only retains a comparatively scanty wreck. High mountain levels supplied paths of communication for stocking the South Temperate region, the floras of which were enriched by adapted forms of tropical types. Such profound changes must necessarily have been accompanied by enormous elimination, the migrating hosts were perpetually thinned by falling out on the way. Further development was, however, not stopped, but in many cases stimulated by migration and settlement in new homes. The northern Quercus, arrested at the tropic in the new world, expanded in that of the old into new and striking races. And it cannot be doubted that the profusion of Melastomaceae in South America was not derived from elsewhere, but the result of local evolution. There is some evidence of a returning stream from the south, but as Hooker and A. de Candolle have pointed out, It is insignificant as compared with the outgoing one Darwin attributes this to the fact that “the northern forms were the more powerful” (Origin of Species, 5th ed, p. 458).
The result of migration is that races of widely different origin and habit have had to adapt themselves to similar conditions. This has brought about superficial resemblance in the floras of different countries. At first sight a South African Euphorbia might be mistaken for a South American Cactus, an Aloe for an Agave, a Senecio for ivy, or a New Zealand Veronica for a European Salicornia. A geographical botany based on such resemblances is only in reality a study of adaptations. The investigation of these may raise and solve interesting physiological problems, but throw no light on the facts and genetic relationship which a rational explanation of distribution requires. If we study a population and sort it into soldiers, sailors, ecclesiastics, lawyers and artisans, we may obtain facts of sociological value but learn nothing as to its racial origin and composition.
In the attempt that has been made to map out the land surface of the earth, probable community of origin has been relied upon more than the possession of obvious characters. That sub-regions framed on this principle should show interrelations and some degree of overlapping is only what might have been expected, and, in fact, confirms the validity of the principle adopted. It is interesting to observe that though deduced exclusively from the study of flowering plants, they are in substantial agreement with those now generally adopted by zoologists, and may therefore be presumed to be on the whole “natural.”
Authorities—A. de Candolle, La Géographie botanique raisonnée, (Paris and Geneva, 1855); A Grisebach, La Végetation du globe, transl. by P. de Tchihatchef (Paris, 1875); Engler, Versuch einer Entwicklungsgeschichte der Pflanzenwelt (Leipzig, 1879-1882); Oscar Drude, Manuel de géographre botanique, transl. by G. Poirault (Paris, 1897), A. F. W. Schimper, Plant Geography, transl. by W. R. Fisher, (Oxford, 1903).
- Flahault and Schröter, Phytogeographical Nomenclature: reports and propositions (Zurich, 1910).
- Warming, Oecology of Plants (Oxford, 1909).
- Humboldt and Bonpland, Essai sur la géographie des plantes (Paris, 1807).
- Flahault and Schröter (op. cit.).
- Warming, Plantesamfund, Kjöbenhavn, 1895. (See German trans. by Knoblauch, “Lehrbuch der ökologischen Pflanzengeographie” (Berlin, 1896), new German ed. by Graebner (Berlin, 1902).
- Schouw, Grundtraek til en almindelig Plantegeografie (Kjöbenhavn, 1822); German trans., “Grundzüge einer allegemeinen Pflanzengeographie” (Berlin, 1823).
- Schimper, Pflanzengeographie auf physiolagischer Grundlage (Berlin, 1898); Eng. trans. by Fisher, “Plant Geography upon a Physiological Basis” (Oxford, 1903-1904).
- Warming (1909, op. cit.).
- Ibid. (1894, op. cit.).
- See Moss, “The Fundamental Units of Vegetation: historical development of the concepts of the plant association and the plant formation.” Botany School (Cambridge, 1910).
- F. E. Clements, Research Methods in Ecology (1905), Lincoln, Neb., U.S.A.
- Warming (1909, op. cit.).
- Schimper (1898, op. cit.).
- The nomenclature of the terms (floristic as well as ecological) used in geographical botany is in a very confused state. In the present article, the term “district” is used in a general sense to indicate any definite portion of the earth's surface. For a discussion of such phytogeographical terms, see Flahault “Premier essai de nomenclature phytogéographique,” in Bull. Soc. languedocienne de Géogr. (1901); and also in Bull. Torr. Bot. Club (1901).
- Humboldt, Eng. trans. by Sabine, Aspects of Nature (London, 1849.
- Eng. trans. by Seward, The New Flora of the Volcanic Island of Krakalau (Cambridge, 1908).
- See Moss, Rankin, and Tansley, “British Woodlands.” Botany School (Cambridge, 1910).
- As very little experimental work has been done with regard to physiological dryness in physically wet habitats, any classification such as the above must be of a tentative nature.
- Volkens, Die Flora der ägyptisch-arabischen Wüste (Berlin 1887).
- Lesage, “Recherches expérimentales sur les modifications des feuilles chez les plantes maritimes,” in Rev. gén. de bot. (1890), vol. ii.
- T. G. Hill, “Observations on the Osmotic Properties of the Root-Hairs of certain Salt Marsh Plants,” in The New Phytologist (1908), vol. vii.
- Ewart, On the Physics and Physiology of Protoplasmic Streaming in Plants. (Oxford, 1903), gives an excellent account of the phenomena of protoplasmic streaming with a full discussion of the probable causes to which it is due.
- See Halliburton. Science Progress in the 20th Century (1909), vol. iv.
- Strasburger (1909) states very definitely that he has observed the entrance of the male nucleus into the egg without a trace of cytoplasm.