

CHAPTER XIII. 



XEEOPHYTE ASSOCIATIONS. 



144. General character. — Strongly contrasted with the 

 hydrophytes are the xerophytes, which are adapted to dry 

 air and soil. The xerophytic conditions may be regarded 

 in general as drouth conditions. It is not necessary for 

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

 xerophytic conditions. These conditions may be put under 

 three heads : (1) possible drouth, 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 drouth period 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 may occur, the problem 

 of the plant is always one of water supply, and many strik- 

 ing structures have been developed to answer it. Plants 

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

 (1) collection and retention of water, and (2) prevention of 

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

 loss of water through transpiration (see §26) tends to be 

 much increased. This tendency in the presence of a very 

 meager water supply is a menace to the life of the plant, 

 for it is impossible to stop transpiration entirely, as it 

 must take place so long as the plant is alive. The adapta- 

 tions on the part of the plant, therefore, are directed 

 towards the regulation of transpiration, that it may occur 



