CHAPTER XXI. 



XEROPHYTES AND HALOPHYTES. 



433. II. Xerophytes. — The jilants of dry regions blend 

 b\' imperceptible gradations witli tlie mesophytes. They 

 reach their best de\elopment in desert and rocky regions. 

 Some, especially of the lower forms, grow in such situations 

 that the\- must adapt themselves to become so dry at certain 

 periods that thev may be ]iowdered. Such, for example, are 

 a lew alg;e, many lichens, mosses, and a few fernworts. 

 Adaptations in these cases must be looked lor in the character 

 of the cell contents. 



Other plants nuist adapt themselves to endure dry periotis, 

 such as those occurring from day to day, or between the wet 

 and drv seasons, by retaining in their l)odies sufficient water 

 to sustain life. The following are some of the chief methods 

 by which plants adapt themselves to pieriodic or continuous 

 drought. 



A. Adaptations for reducing transpiration. 



434. I. Periodic reduction of surface exposed. — The 



dying away of an annual plant after forming its seed may be 

 looked u])on as an adaptation of this sort. Little ewaporation 

 occurs from the surface of the seed, which is thus adapted to 

 withstand prolonged dryness. Perennial plants accomplish 

 the same results when their annual shoots die off and leave 

 only the rhizomes, tubers, and similar parts buried in the soil. 

 Perennial plants with perennial shoots may drop their leaves 



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