366 A TEXTBOOK OF GENERAL BOTANY 
CLASS LICHENS 
Lichens are thallus plants (Figs. 392, 393) the bodies of which _ 
are composed of interlacing fungus hyphe, among which are found 
algal cells on which the fungus is parasitic (Fig. 394). Most lichens 
are greenish gray, the color being the result of a combination of 
the colors of the two components. In one genus of the lichens 
the fungus is a basidiomycete, while in the other genera the fungi 
are ascomycetes (Figs. 395, 396). The alga may be either Chloro- 
phyceae or Cyanophyceae. The combination of fungus and alga 
is able to live in places, such as bare rocks, where neither of the 
constituents could survive alone. The habitat of lichens varies 
greatly. They may grow on rocks, or as epiphytes on branches 
or leaves, or on the ground. One of the lichens, the so-called 
reindeer moss, is abundant in northern latitudes and serves as 
food for reindeer and caribou. 
