338 A TEXT-BOOK OF BOTANY 
202. Rock associations.—Certain plants are able to live 
upon rocks and boards exposed to direct sunlight. The 
conspicuous forms are lichens and mosses, which are 
found very commonly splotching rocks (Fig. 308) and old 
Fic. 308.—Rocks covered with lichens and mosses. 
fences. Associated with them are often crevice plants, 
which send their roots into crevices and so gain a foot- 
hold. 
203. Sand associations.—The plants grouped together 
on dry, sandy ground are quite different in appearance 
from others, and such areas may be found in almost every 
neighborhood, at least along streams. On certain borders 
of the Great Lakes and on seacoasts an interesting suc- 
cession of sand associations occurs. Nearest the water is 
the beach with such a poor display of plants that it looks 
bare. 
Beyond the beach are the dunes, which are billows of 
sand that have been formed by the prevailing winds; and in 
