X*- 



CHAPTER XXIII 



XEROPHYTES 



201. Adaptations. The adaptations of plants to meet 

 drought are numerous and striking. The meager supply 

 of water available for the plant must not escape from it 

 too freely, and hence most of the special adaptations are to 

 check the loss of water. In addition to this, there is often 

 developed water-storage tissue, which acts as a reservoir, 

 receiving water at a time of supply and doling it out accord- 

 ing to the needs of the plant. 



Drought conditions vary in different regions, and may 

 be grouped under three heads: (1) possible drought, which 

 occurs at irregular intervals, or which in some seasons may 

 not occur at all; (2) periodic drought, which occurs at regular 

 intervals; and (3) perennial drought, which is a constant 

 condition, as in arid or desert regions. For the first con- 

 dition plants are poorly prepared, but by various temporary 

 expedients may resist until the drought ceases. For the 

 second condition plants are well prepared, enduring the 

 regularly recurring drought as definitely as a regularly re- 

 curring winter. In the third condition plants not only must 

 endure drought, but also must be able to work in such con- 

 ditions. 



Some of the conspicuous methods of protection against 

 drought have been described ( 17). These should be kept 

 in mind when the following illustrations of xerophytic asso- 

 ciations are considered. 



337 



