16d A TEXT-BOOK OF BOTANY 
is chiefly concerned in forming lichens. Some Basidio- 
mycetes also have learned to form lichens. 
Various forms of lichens can be distinguished as fol- 
lows: (1) crustaceous lichens, in which the body resembles 
an incrustation upon its substratum of rock, soil, ete.; 
(2) foliose lichens, with flattened, leaf-like, lobed bodies, 
attached only at the middle or irregularly to the substra- 
tum (Fig. 155); (3) fruticose lichens, with slender bodies 
branching like shrubs, either erect, hanging, or prostrate 
(Fig. 159). 
Lichens are often very important in starting a humus 
formation on bare rocks and sterile soil. In such exposed 
situations Algze could not endure alone, and of course 
Fungi could not exist alone in any situation. The lichen 
combination can exist, however, since the fungus obtains 
its food from the Alge, while the latter are protected against 
drying out by the enveloping meshwork of the fungus. As 
the lichens grow and decay, enough humus is collected for 
higher forms of plant life to start; and these in turn con- 
tribute to a more rapid accumulation of humus, until 
presently a respectable soil may be the result. 
