Lichens
These are thalloid plants, which live attached to stones, or the bark of trees, or on the ground. Their growth is extremely slow, and their period of existence considerable. They consist of two structural elements: (1) Filaments, or hyphae; (2) green cells, or gonidia (which may be solitary, in groups, or in chains). These elements may be arranged in layers, or irregularly intermixed. In form, Lichens present the following modifications, viz.: (1) Crustaceous, in which the whole under surface is firmly adherent to the body upon which it grows; (2) Foliaceous are attached by many small processes from the under side, called rhizines; (3) Fruticose generally consist of slender branches, and are attached by one point only; (4) Gelatinous, so called from the nature of their substance when moist. Lichens are reproduced by spores, borne, generally, eight together, in club-shaped filaments (asci), which are found in large numbers, together with hairs (paraphyses), in external receptacles. In addition to these, other receptacles are formed, called spermagonia, which contain filaments (sterigmata), bearing short rod-like bodies (spermatia). Similar cavities bear larger spore-like bodies, called pyenidia, whose precise nature, as well as that of the spermatia, is not known. A second method of reproduction is by soredia, which consists of one or more gonidia, surrounded by hyphae. The whole, or only a part, of the Lichen thallus can thus be converted into a powdery mass. The exact relation of the hyphae to the gonidia is still disputed. Some writers assert that the gonidia are produced by the hyphae; others (of whom Schwendener was the leader) say that the gonidia are Algae, upon which fungi (the hyphae) are growing parasitically. This differs from parasitism in general, inasmuch as the host is not destroyed, but seems to profit by the union. Many Lichens furnish excellent dyes, e.g., Roccella tinctoria, from which litmus is obtained. Cladonia rangiferina supplies the reindeer with food. Lichens are, in some cases, useful as food or medicine--e.g., Iceland Moss (Catraria islandica),--but their principal value consists in their properties as dyes. Lecanora esculenta is frequently met with in immense quantities in the most arid, desert regions of Asia and North Africa; it occurs in rounded masses about the size of a filbert, and is largely used as food. It possesses, too, a peculiar interest, on account of its being supposed, by some commentators, to be the "manna" which fed the children of Israel during their wanderings in the wilderness. "Lichens are not parasitic; but when they clothe trees they impede the circulation of air, and hasten decay. They further intercept light when enveloping young shoots, and interfere with the development of cambium and the evolution of the foliage" (Hooker).