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 associations are as follows : 

 "rock-associations," composed of plants living upon exposed 

 rock surfaces, walls, fences, etc., notably lichens and mosses ; 

 " sand associations," 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 associations. — Mesophytes make up the 

 common vegetation, the conditions of moisture being me- 

 dium, and the soil fertile. This is the normal plant condi- 

 tion, 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 



