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 nection with which the spores are borne. These aërial parts are the only ones we ordinarily see, and which constitute the "mushroom" part (Fig. 131). Only asexual spores (basidiospores) are produced, and on short stalks (basidia) (Fig. 286). In the puff-*balls the spores are inclosed and constitute a large part of the "smoke." In the mushrooms and toadstools they are borne on gills, and in the shelf fungi (Fig. 134) on the walls of minute pores of the underside. The mycelium of these shelf fungi frequently lives and grows for a long time concealed in the substratum before the visible fruit bodies are sent out. Practically all timber decay is caused by such growth, and the damage is largely done before the fruiting bodies appear. For other accounts of mushrooms, see Chap. XIV.

tr, trama tissue; sh, hymenium; b, basidium; st, sterigma; sp, spore. (Atkinson.)

(A species of Physcia.)

Lichens are so common everywhere that the attention of the student is sure to be drawn to them. They grow on rocks, trunks of trees (Fig. 287), old fences, and on the earth. They are thin, usually gray ragged objects, apparently lifeless. Their study is too difficult for beginners, but a few words of explanation may be useful.

Lichens were formerly supposed to be a distinct or separate division of plants. They are now known to be organisms, each species of which is a constant association of a fungus and an alga. The thallus is ordinarily made up of fungous mycelium or tissue within which the imprisoned alga is definitely distributed. The result is a growth unlike either component. This association of