ORDER HYMENOMYCETALES : hymenophore ex posed on a pileus.
Family Thelephoracea: hymenophore smooth, mycelium filamentous or thalloid (in Cora, Rhipidonema, Dictyonema, and Laudatea).
Dins Gasreawitvcrraus: hymenophore en closed in peridium.
Family Scierodermatacea: peridium broad subsessile, gleba excavate, mycelium filamen tous, or thalloid in Emericella.
Distribution and Lichens are dis tributed over the entire earth; they are least numerous in the tropics and reach their maxi mum development in Alpine and polar lands, where they often form the principal vegetation over immense stretches. Many species are widespread, especially in the northern hem isphere: some of these, such as Cladonia rangi ferina, 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 ma terial by their decay. In all rocky habitats they are the pioneers which prepare the way for the appearance of more highly organized 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 mechan ically. 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 consti tutes an invaluable supply of food for the rein deer and caribou. In Japan, Gyrophora escu
lents, 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 quan tities 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-roches 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 valve 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 Roc cella tinctoria and held in high esteem for their brilliant purples have been entirely replaced by the aniline dyes. Litmus, which is a similar 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) ; Sar gent, F. L., 'Lichenology for Beginners) (Cambridge 1906).