CHAPTER XIII 



XEEOPHYTE SOCIETIES 



133. General character. — Strongly contrasted with the 

 hydrophytes are the xeroiAytes, which are adapted to dry 

 air and soil. The xerophytic conditions may he regarded 

 in general as drouth conditions. It is not necessary for 

 the air and soil to he dry throughout the year to develop 

 xerophytic conditions. These conditions may l)e put under 

 three heads : (1) j^ossihle droutJi. in which a season of 

 drouth may occur at irregular intervals, or in some seasons 

 may not occur at all; (2) periodic drouth, in which there 

 is a (li'outh jjeriod as definite as the winter period in cer- 

 tain regions ; (3) perennial drouth, in which the dry con- 

 ditions are constant, and the region is distinctly an arid 

 or desert region. 



However xerophytic conditions nuiy occur, the problem 

 of the plant is always one of water supply, and man)^ strik- 

 ing sti'uctures have been developed to answer it. Plants 

 in such conditions must provide, therefore, for two things: 

 (1) (Millection and I'etenlion of watei'. and (2) prevention of 

 its loss. It is evident that in these drouth conditions the 

 loss of water through transpiration (see §20) tends to be 

 much increased. This tendeni\y in the iiresence of a very 

 meager water su])ply is a menace to the life of the plant. 

 It is impracticable to stop ti'ans])iration cntirelv. for it 

 must take place in connection witli a, necessary life-process. 

 The ada])tations on the part of the plant, tlu'refore, are 

 directed towards tlie regulation of transpiration, that it 

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