324 PLANT STRUCTURES 



parts, deciduous trees and shrubs dropping their leaves, 

 etc. ; temporary reduction of surface, the leaves rolling up 

 or folding together in various ways ; profile position, the 

 leaves standing edgewise and not exposing their flat sur- 

 faces to the most intense light ; motile leaves which can 

 shift their position to suit their needs ; small leaves, a very 

 characteristic feature of xerophytic plants ; coverings of 

 hair ; dwarf growth ; anatomical adaptations, such as 

 cuticle, palisade tissue, etc. Probably the most conspicu- 

 ous adaptation, however, is the organization of "water- 

 reservoirs," which collect and retain the scanty water sup- 

 ply, doling it out as the plant needs it. 



Some of the prominent societies are as follows : "rock- 

 societies " composed of plants living upon exposed rock sur- 

 faces, walls, fences, etc., notably lichens and mosses ; 

 "sand societies," including beaches, dunes, and sandy 

 fields ; " shrubby heaths," characterized by heath plants ; 

 "plains," the great areas of dry air and wind developed in 

 the interiors of continents; "cactus deserts," still more 

 arid areas of the Mexican region, where the cactus, agave, 

 yucca, etc., have learned to live by means of the most ex- 

 treme xerophytic modifications ; " tropical deserts," where 

 xerophytic conditions reach their extreme in the combina- 

 tion of maximum heat and minimum water ; " xerophyte 

 thickets," the most impenetrable of all thicket-growths, 

 represented by the " chaparral " of the Southwest, and the 

 "bush" and "scrub" of Africa and Australia; "xero- 

 phyte forests," also notably coniferous. (See Figs. 285, 

 286, 287.) 



181. Mesophyte societies. Mesophytes make up the com- 

 mon vegetation, the conditions of moisture being medium, 

 and the soil fertile. This is the normal plant condition, 

 and is the arable condition that is, best adapted for the 

 plants which man seeks to cultivate. If a hydrophytic 

 area is to be cultivated, it is drained and made mesophytic ; 

 if a xerophytic area is to be cultivated, it is irrigated and 



