160 A TEXT-BOOK OF BOTANY 
89. Lichens.—Lichens are abundant everywhere, form- 
ing splotches of various colors on tree trunks, rocks, old 
Fic. 154.—A common lichen growing on bark; the numerous dark disks are lined 
by a layer of asci. 
boards, etc., and growing also upon the ground (Figs. 154 
and 155). They have a general greenish-gray color, but 
brighter colors also may be observed. 
The great interest connected with lichens is that they 
Fic. 155.—A common foliose lichen growing upon a board. 
