204 



PLANT STUDIES 



inent illustrations will be given. Some of the prominent 

 societies are as follows : " rock-societies," composed of 

 2)lants 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 



Fic 1S4. A rock covi'vril willi lirli. 



live ; " troj^ical deserts," where xerophytic conditions reach 

 their extreme in the combination of maximum heat and 

 minimirm 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-.) 



