CHAPTER XXI. 



XEROPHYTES AND HALOPHYTES. 



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

 by imperceptible gradations with the mesophytes. They 

 reach their best development in desert and rocky regions. 

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

 that they must adapt themselves to become so dry at certain 

 periods that they may be powdered. Such, for example, are 

 a few algae, many lichens, mosses, and a few fernworts. 

 Adaptations in these cases must be looked for in the character 

 of the cell contents. 



Other plants must adapt themselves to endure dry periods, 

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

 and dry seasons, by retaining in their bodies sufficient water 

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

 by which plants adapt themselves to periodic 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 upon as an adaptation of this sort. Little evaporation 

 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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