The Encyclopedia Americana (1920)/Lichens
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LICHENS, lī-kĕnz (Lat. lichen, lichen, Gr. λειχήν), a large but artificial group of the higher fungi (Carpophyta), characterized by parasitic growth upon the lower blue-green and yellow-green algæ (Protophyta, Chlorophyceæ). Lichens are of the wildest occurrence in nature, appearing as gray, yellow and brown crusts or masses almost everywhere upon trees, rocks and soil. The number of genera and species differs more or less with the authority cited: the valid genera number not far from 250, while the species are in the neighborhood of 4,000.
The vegetative body or thallus varies from a fraction of a millimeter to several decimeters in size, though it shows relatively little variation in thickness. In texture it is powdery, leathery, paper-like, or, in the case of many forms with blue-green algæ, gelatinous; the prevailing colors are gray, brown and yellow, while green and black sometimes occur. The shape of the thallus is typically orbicular or stellate; it is often irregular, especially in branched forms. In general appearance the thallus varies within wide limits; as a rule, however, three types, crustose, foliose and fruticose, may be clearly distinguished. The crustose type is the primitive one, showing in its granular, warty and areolate forms the various stages through which the thallus has passed in its development from the original mycelium. The crustose thallus is so closely in contact with its substratum that it cannot be separated from it without tearing. The foliose type is a higher development of the crustose. It is usually a definite, leaf-like structure, more or less lobed at the margin and attached to the stratum somewhat loosely or at but a single point. The fruticose type is a special modification of the foliose, in which the latter is more or less flattened or cylindrical and erect or pendulous. This form is probably an adjustment to conditions of diffuse light. It is especially characteristic of tree-lichens and of certain ground forms, such as Cladonia, where it is termed the podetium. In the latter there is also developed an accessory or secondary thallus, consisting of minute, leaf-like scales.
The simplest thallus consists merely of a few fungus threads enclosing the irregularly disposed cells of the host or alga. Ordinarily, however, the algal cells not only have a definite position, but the fungal portion of the thallus is likewise highly specialized. Naturally, this differentiation is least in the crustose forms and greatest in the fruticose ones. The structure of the foliose type may be taken as fairly representative, except of the gelatinous lichens, in which the algæ are scattered throughout the thallus. A definite epidermal layer is wanting except in a few of the higher lichens, where the outer filaments have been gelatinized, resulting in the formation of a structure closely resembling a cuticle. As a rule, however, the uppermost part of the thallus is the cortical layer. This consists of hyphæ (filaments) compacted in such a way as to produce a tissue which looks much like parenchyma and is called in consequence pseudoparenchyma. The function of the cortical layer is in part mechanical or supportive and in part protective. Its structure seems to depend primarily upon the latter function: it is least in those forms growing in forests and greatest in those found in the open. Below the cortical layer and continuous with it is found the host or algal layer, consisting of filaments more or less loosely intertwined with the algæ. This is the nutritive layer, in which the fungal hyphæ draw their nourishment from the host-cells. The connection between the two may be merely by contact or by penetration. In the latter case the fungal hyphæ either penetrate the protoplasm of the host and finally destroy it, or merely pierce the cell-membrane and lie in contact with the protoplasm. In either event, the hyphæ develop special branches for contact or penetration, which are called haustoria. The algal layer is a specialized portion of the medulla which lies just below it. The hyphæ of the two layers are continuous, but they do not develop haustoria in the medulla, where they tend also to run more or less parallel with the direction of growth. The medullary layer primarily serves the function of transport; it is likewise used for the storage of lichenin (lichen-starch) and fats. The lower surface of the thallus is covered with a cortical layer similar in structure to that of the upper surface. Generally, however, it is somewhat thinner and is designed rather for absorption than for protection. It is frequently produced into facicles of hyphæ termed rhizoids and cilia.
The thallus of many lichens exhibits several peculiar structures, which are the direct result of the symbiosis of fungus and alga. The most frequent and most important of these is the soredium. This is a minute irregular mass of fungal hyphæ and algal cells, readily carried by wind or water, and able to grow directly into a lichen thallus under the proper conditions of moisture and warmth. Soredia occur upon the upper face of the thallus of many lichens as elevated powdery masses or tubercles. They arise in the algal layer of the thallus by the repeated branching of a fungal filament in such fashion as to completely enclose one or more cells of the alga, which also increase in number. The hyphæ become more or less gelatinized and compacted into a surface very resistant to desiccation. The soredia are pushed upward through the thallus by the growth of the filaments below and are finally extruded through a rift in the cortical layer, constituting a sorus. Normally, the soredia are carried away from the sorus and develop independently, but in some cases they grow while still in contact with the mother-thallus, producing minute, leaf-like scales upon the latter. These are the so-called isidioid growths or phylloclades, found in Usnea and related genera. Soralia are structures which arise from the medulla or even from the lower-most layer by the upgrowth of a mass of parallel filaments which penetrate the algal layer and there develop into normal soredia. Cephalodia bear the general appearance of soredia, but in origin and function they are quite different. They are distinguished as external and internal. The cause of their development is unknown: they are said to arise from the soredia of other lichens, which have lodged upon the thallus. They have never been produced experimentally, however, and it seems much more probable that they are modifications of the thallus due to a change in the life form of the algal element. The cyphellæ are flat or concave gaps in the lower cortical layer of the thallus of Sticta and Stictina. They are filled with variously branched hyphæ of the medullary layer and probably function as organs of absorption and respiration. The spermagonia are minute black dots occurring on the upper face of many lichens, especially near the margin of the thallus. Structurally they are identical with those propagative organs of black fungi that are termed pycnidia. They are spherical bodies with a membranous or carbonaceous envelope, containing a layer of rod-like filaments which bear at their tips tiny spore-like bodies called spermatia. As the names indicate, the spermagonia were supposed to be male reproductive organs and the spermatia the fertilizing cells. There now seems to be little doubt, however, that they are propagative organs or pycnidia inherited from fungus ancestors. In a few cases they may be pycnidial parasites, such as Phyllosticta.
The fruit (sporocarp) of those lichens which bear spore-sacs (ascolichens) is called a perithecium when the fungus is one of the black fungi (Pyrenomycetales), a hysterothecium, when it belongs to the cleft fungi (Hysteriales), and an apothecium when the lichen is a cup fungus (Discomycetales). All these agree essentially in structure, though they differ in form: the perithecium is globoid, opening by a minute pore at the top, the hysterothecium, linear or irregular, opening by a cleft, and the apothecium usually open and disc-shaped. The essential parts in each are the same, namely, spore-sacs (asci), the spores, and the sterile threads (paraphyses). The apothecium is the highest type of spore-fruit and the most widely distributed. It consists usually of the following parts: the thecium, the central portion consisting of the asci, sometimes called thekes, and the paraphyses; the epithecium, which lies above the asci; and the hypothecium which is found below them. The latter often extends around the sides of the thecium also, and is there termed the exciple (proper exciple, parathecium). All of these consist of densely compacted elongate hyphæ (prosenchyma) which are without algæ and are often more or less dark colored. In most of the higher lichens the exciple is surrounded by the tissue of the algal layer, producing a thalline margin (thalline exciple) about the apothecium. The apothecium usually sits directly upon the thallus; occasionally it is stalked, and less frequently it is immersed or innate. Lichens show the same differences with respect to paraphyses, asci and spores that are to be found among the other cup-fungi and black fungi. The paraphyses are simple or repeatedly branched, continuous or septate, gelatinized or non-gelatinized, persistent or evanescent. They are often compacted and dark-colored at the tip, appearing to be continuous with the epithecium. The spore-sacs of lichens are cylindrical or calvate in form, more rarely obovoid. Their walls are thin, though sometimes gelatinized; they do not react to iodine as a rule, except when very young. The asci usually open by a terminal slit; in a few cases the entire wall breaks away. The number of spores in an ascus varies from one (Pertusaria) to many (Acarospora); the normal number is eight, six or four. The arrangement of the spores is usually irregular, though they are sometimes in one row (monostichous), or in two rows (distichous). Lichen spores are ordinarily colorless and simple, less frequently dark brown and many-celled. They may be two-celled (bilocular), several-celled (plurilocular), or muriform, when the partitions run in both directions. The wall of the spore is smooth and without appendages; the contents may be hyaline, granular or guttulate.
Physiology and Reproduction. — The functions of lichens are essentially those involved in the relation between parasite and host, modified to an important degree in those forms with well-developed thallus. Absorption of water takes place readily upon both surfaces of the thallus, but especially on the under side, where hyphæ, cilia and rhizoids all act more or less efficiently as absorptive agents. According to Zukal, the hyphal hairs are capable also of absorbing moisture directly from the air, in Physcia, Peltigera, Sticta, etc. Water-storage takes place in the algal and medullary layers. It is effected primarily by the algal membranes, especially of the blue-green slimes, and to a degree also by the lichenin of the hyphæ. Zukal has suggested that the cephalodia, because of their blue-green algæ, are probably to be regarded as structures for the storage of water. The ability of the lichen thallus to retain water arises from its complexity, and from the presence of the gelatinized cortex. Lichens exhibit a number of somewhat primitive devices for the exchange of gases. These are often mere rifts in the thallus, or degenerate pycnidia; sometimes definite openings are present, or absorption takes place through loose protuberances. A specialized organ for this purpose is found in the cyphellæ of Sticta and Stictina, which are primitive breathing pores, making direct connection between the air and the medullary layer. The latter serves as a pathway for the transport of water and gases to the various parts of the thallus.
The relation of the lichen thallus to the environment is obscure. Of all macroscopic plants lichens resist drying-out the most successfully. With respect to the temperature extremes which they can endure, they are surpassed only by the bacteria. Many lichens withstand temperatures greater than 65° C., and nearly all are able to resist the intense cold of Arctic and Alpine winters, with minima of —40° to —60° C. Lichens exhibit very different sensibility to light: the majority of them grow in the fullest sunlight, while some, Evernia, Usnea, Peltigera, Graphis, etc., are adapted to more or less intense shade. In Alpine regions especially, orange and yellow thalli occur almost exclusively on the under, or shaded, side of rocks. This fact is explained by Zukal's researches, in which he found that the algal layer was most highly developed under a cortex orange or yellow in color, these colors being most penetrable by the rays active in carbon assimilation. The color of the cortex is also thought to be a protection against excessive illumination, though this explanation can scarcely hold for those lichens in which the lower cortex is highly colored. According to Schwendener, the growth of the thallus is largely intercalary, marginal or apical growth being relatively insignificant. In lichens with yellow-green algæ, the growth of the thallus is determined by the fungus, and the development of the algal layer takes place subsequently. In this process, the algæ and hyphæ show a tendency to aggregate into tubercles, which modify the surface of the thallus. Somewhat similar sculpturings are produced by tensions in the growing thallus, especially by the alternation of wet and dry periods. In nearly all the gelatinous lichens, and particularly those parasitic on filamentous algæ, growth is controlled by the alga, and the fungus has little or no influence upon it.
The propagation of lichens occurs ordinarily by means of soredia. It may take place also by fragments of the thallus, whether abstricted naturally or cut off by accident. The propagative value of the pycnidium (spermogonium) in nature is unknown. The conidia have been germinated in cultures with difficulty, and at present there is no evidence that they grow more readily under normal conditions. Reproduction is a characteristic feature of the lichens: a few species produce apothecia rarely, while in certain sterile forms they are never developed. It is still an open question whether the apothecium is the result of fertilization. Some investigators have thought to demonstrate the presence of a carpogone and trichogyne, and to follow the development of a spore fruit, which results from the contact of spermatium and trichogyne. The germination of many spermatia points strongly to the conclusion that these are mere conidia and not male sexual cells. No fusion of sexual cells has yet been seen in lichens, and, until this is seen, it is impossible to settle the question of their sexual nature. The apothecium arises usually from certain more or less distinctly twisted hyphæ lying between the medulla and the algal layer. Sometimes the point of origin is just beneath the cortex, especially near the margin, and in crustose forms it is in the hyphal layer just above the substratum. The development is essentially the same as in the other cup-fungi; the hyphal fundament increases in size, and becomes differentiated above into two sorts of threads. The first to grow up are the paraphyses, in the centre of which push up the club-shaped branches, which become the asci. Spore formation in the ascus follows the method typical of all sac-fungi. The escape of the spores takes place through a terminal rift in the ascus or by the breaking up of the latter. The mature spores germinate readily under the proper conditions, usually sending out a single filament from each cell; large spores, however, such as those found in Megalospora and Pertusaria, produce many germinating filaments. The young mycelium is capable of slight development only, unless it comes in contact with the proper alga, when it grows at once into the thallus. The uncertainty that the spores will germinate in a place where the proper host occurs makes multiplication by spores much less sure than by soredia. In a few cases (Endocarpon, Staurothele, etc.), this disadvantage has been overcome by the development of algæ in the thecium between the paraphyses and spore-sacs (hymenial gonidia). These are ejected with the spores, and, clinging to the latter, furnish a certain substratum for the germination of the spores.
Origin and Classification. — Lichens are sac-fungi and rod-fungi which show more or less similarity in their vegetative body because of their parasitism upon certain algæ. The clew to their origin and relationship is to be found in the inherited reproductive organ, the sporocarp, and not in the thallus. From this standpoint, the group is highly artificial, containing representatives of two distinct classes of fungi, the Ascomycetes and the Basidiomycetes. The basidiolichens are a small group, containing but a few genera; the ascolichens consist of the representatives of several unrelated families. It is evident that the lichens have not arisen from a single point, as members or offshoots of one line of development, but that they have originated at several widely separated points. They are of multiple origin; that is, they are polyphyletic. In ascolichens, the form of the sporocarp indicates the main places of origin: the Verrucariaceæ, with perithecia, are Pyrenomycetales; the Graphidaceæ, which show the hysterothecium, belong to the Hysteriales; the remaining families, Caliciaceæ, Collemaceæ, Parmeliaceæ, etc., belong to the Pezizales.
The following synopsis will indicate the relationship and limits of the various families of lichens:
Class Ascomyceteæ: fruit a sporocarp, spores borne in sacs (asci).
Order Pyrenomycetales: sporocarp a perithecium.
Family Spharioceæ: mycelium filamentous, saprophytic or parasitic on tissues.
Family Verrucariaceæ: mycelium thalloid, parasitic on yellow-green algæ.
Order Hysteriales: sporocarp a hysterothecium.
Family Hysteriaceæ: mycelium filamentous, hysterothecium carbonaceous.
Family Hybodermataceæ: mycelium filamentous, hysterothecium membranaceous.
Family Graphidaceæ: mycelium thalloid, on algæ, hysterothecium membranaceous or carbonaceous.
Order Pezizales: sporocarp an apothecium.
Apothecia and thallus leathery, waxy or carbonaceous, never gelatinous.
Family Patellariaceæ: mycelium filamentous, mostly saprophytic.
Family Lecidiaceæ: mycelium thalloid, apothecium sessile, exciple without algæ (proper).
Family Cladoniaceæ: mycelium thalloid of two sorts, primary and secondary; apothecia borne on stalks (podetia), proper exciple.
Family Parmeliaceæ: mycelium thalloid, algæ yellow-green, exciple with algæ (thalline).
Family Pannariacaæ: mycelium thalloid, algæ blue-green, proper exciple.
Apothecia and mycelium gelatinous.
Family Bulgariaceæ: mycelium filamentous.
Family Collemaceæ: mycelium thalloid, on blue-green algæ.
Class Basidiomyceteæ: fruit a hymenophore spores borne on stalks (basidia).
Order Hymenomycetales: hymenophore exposed on a pileus.
Family Thelephoraceæ: hymenophore smooth, mycelium filamentous or thalloid (in Cora, Rhipidonema, Dictyonema, and Laudatea).
Order Gasteromycetales: hymenophore enclosed in peridium.
Family Sclerodermataceæ: peridium broad subsessile, gleba excavate, mycelium filamentous, or thalloid in Emericella.
Distribution and Rôle. — Lichens are distributed over the entire earth; they are least numerous in the tropics and reach their maximum development in Alpine and polar lands, where they often form the principal vegetation over immense stretches. Many species are widespread, especially in the northern hemisphere: some of these, such as Cladonia rangiferina, Urceolaria scruposa, Usnea barbata, etc., are truly cosmopolitan. In the tropical and temperate zones, the greatest wealth of lichens is found upon bark and wood. In Alpine and polar regions the stone and earth forms are predominant. In these places lichens play their most important part in the economy of nature. They take the initiative in the disintegration of the hardest rock by virtue of the acids secreted by the thallus; they are likewise very effective in binding together the new soils which result in this way and in contributing organic material by their decay. In all rocky habitats they are the pioneers which prepare the way for the appearance of more highly organized plants, mosses, grasses, etc. In the case of tree-lichens, the tree is not affected by the lichen, except in so far as the bark may be ruptured by it mechanically. It is a question whether lichens exert any really injurious effect upon timber, though they probably hasten the decay of boards, posts, etc., by increasing the amount of moisture present.
A few lichens are of value as food. The most important among these is the so-called “reindeer moss,” Cladonia rangiferina, which covers vast stretches in the north and constitutes an invaluable supply of food for the reindeer and caribou. In Japan, Gyrophora esculenta, which is collected in abundance in the mountains, is of sufficient importance to be an article of export. The arid regions in northern Africa and western Asia produce large quantities of the manna-lichen, which is used to make bread, especially by the Tartars. This lichen is readily torn away from the substratum by the wind and is carried often to considerable distance before falling as “manna rain.” This phenomenon has been observed repeatedly in modern times, and probably accounts for the manna of the Israelites. “Trip-de-roche” is an edible Umbilicaria of Arctic America, but the presence of the bitter principle so common in lichens restricts its use as a food. Lichens owe their food value almost wholly to their high content of lichenin, or lichen-starch. Lichens, though once of extensive application in dyeing and in medicine, have fallen almost completely into disuse in both. The various kinds of orseille, which were made from Roccella tinctoria and held in high esteem for their brilliant purples have been entirely replaced by the aniline dyes. Litmus, which is a similar dye made from a species of Lecanora, is still extensively used in chemistry because of its red coloration in the presence of an acid. “Iceland moss,” Cetraria islandica, is still used officially: it contains cetrarin, a bitter principle which is tonic and astringent, and a large amount of lichenin.
Consult Hale, E. H., ‘Flowerless Plants’ (New York 1909); Massee, G. E., ‘British Fungi’ (ib. 1912); Marshall, ‘Mosses and Lichens’ (ib. 1907); Tuckerman, E., ‘Synopsis of the North American Lichens’ (Amherst 1882); Schneider, A., ‘A Textbook of General Lichenology’ (1897); Schneider, A., ‘Guide to the Study of Lichens’ (Boston 1898); Sargent, F. L., ‘Lichenology for Beginners’ (Cambridge 1906).