LICHENS, a natural order of acotyiedonous plants, allied to fungi and to alga. They are thallogenous, consisting mainly of a thallus (q.v.), and without stem and leaves; wholly cellular, and nourished through their whole surface by the medium in which they live, which is a;r, and not water, although a certain amount of moisture in the air is always necessary to their active growth; and when the air becomes very dry. they become dormant, ready to resume their growth on the return of more favorable weather. The thallus of some is pulverulent; that of others is crustaceous; of others, leaf-like; of others, fibrous. Reproduction takes place by spores, usually contained in sacs ((mei, theca), embodied in repositories of various form, often shield-like or disk-like, called apothecia (or shields), which arise from the outer layer of the thellus, and are generally very different in color i'rom the thallus., But there is also another mode of propagation by gonidia, separated cells of the inner or medullary layer of the thallus, usually spherical or nearly so, and always of a green color. This seems to be a urovis ion for the propagation of lichens, even in circumstances—as of the absence of light • unfavorable to the formation of them and spores. Lichens are plants of long life, dif fering in this very widely from fungi. They are most widely diffused, growing equally in the warmest and the coldest regions. On the utmost limits of vegetation, in very
high latitudes, or on the very highest mountains, they cover the soil in great masses. Some grow on earth, others on stones, others on the bark of trees, and some of the trap.
' ical species ou evergreen leaves. In the great economy of nature they serve for the first commencement of vegetation, especially to prepare the soil for plants of higher organiza tion. The gray, yellow, and brown stains on old walls are produced by lichens. which have begun to vegetate where nothing else could. The curiously scattered apothecia of some present the appearance of written characters often seen on the bark of trees. Some hang as tufts or shaggy beards from old trees; some grow amidst heaths and mosses to cover the soil of the most frigid regions. Lichens contain a peculiar gelatinous substance resembling starch, and called lichenin or lichen starch ; generally also a bitter substance called cetrarine; resin; a red, bright yellow, or brown coloring matter; oxalate and phosphate of lime. etc.; and are therefore adapted to purposes of domestic economy, medicine, and the arts. Some are used for food, as Iceland moss (q.v.) and tripe de cache (q.v.); some afford food for cattle, as reindeer moss (q.v.); some are medicinal, as Iceland moss; some afford dye-stuffs, as archil (q.v.), cudbca• (q.‘.) etc.