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
