204 



PLANT STUDIES 



tions, but a few prominent illustrations will be given. Some 

 of the prominent associations are as follows : " rock associa- 

 tions," composed of plants living upon exposed rock surfaces, 

 etc., notably lichens and mosses (Fig. 184) ; "sand associa- 

 tions," including beaches, dunes (Fig. 185), etc.; "shrubby 

 heaths," characterized by heath plants ; " plains," the great 

 areas with dry air developed in the interiors of continents 

 (Fig. 186); " cactus deserts," still more arid areas of the Mex- 

 ican region, where the cactus, agave, etc., have learned to live 



Fig. 184. A rock covered with lichens. 



(Fig. 190); "tropical deserts," where xerophytic condi- 

 tions reach their extreme in the combination of maximum 

 heat and minimum water ; " xerophyte thickets," the most 

 impenetrable of all thicket-growths, represented by the 

 "chaparral" of the southwest (Fig. 187), and the "bush" 

 of Africa and Australia ; " xerophyte forests," also notably 

 coniferous. (See Figs. 192, 193.) 



