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 



FIG. 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 



