164 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, etc.; 

 (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 Algae 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 Algae, 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. 



