204 PLANT STUDIES 
inent illustrations will be given. Some of the prominent 
societies are as follows : " rock-societies," composed of 
plants living upon exposed rock surfaces, walls, fences, etc., 
notably lichens and mosses ; " sand societies," including 
beaches, dunes, and sandy fields ; " shrubby heaths," char- 
acterized by heath plants ; " plains," the great areas with 
dry air and wind developed in the interiors of continents ; 
" cactus deserts," still more arid areas of the Mexican re- 
gion, where the cactus, agave, yucca, etc., have learned to 
FIG. 184. A rock covered with lichens. 
live ; " tropical deserts," where xerophytic conditions reach 
their extreme in the combination of maximum heat and 
minimum water ; " xerophyte thickets," the most impene- 
trable of all thicket-growths, represented by the " chapar- 
ral " of the Southwest, and the " bush " and " scrub " of 
Africa and Australia; "xerophyte forests," also notably 
coniferous. (See Figs. 193, 194.) 
