XEROPHYTE ASSOCIATIONS 189 



sufficiently for the life-processes, but that it may not be 

 wasteful to the point of danger. 



The regulation of transpiration may be accomplished 

 in two general ways. It will be remembered that the 

 amount of transpiration holds some relation to the 

 amount of leaf exposure or exposure of green tissue. 

 Therefore, if the amount of leaf exposure be diminished, 

 the total amount of transpiration will be reduced. Another 

 general way for regulating transpiration is to jDrotect 

 the exposed surface in some way so that the water does 

 not escape so easily. In a word, therefore, the general 

 method is to reduce the extent of exposed surface or to 

 protect it. It must be understood that plants do not differ 

 from each other in adopting one or the other of these 

 methods, for both are very commonly used by the same 

 plant. 



Adaptations 



134. Complete desiccation. — Some plants have a very re- 

 markable power of completely drying up during the drouth 

 period, and then reviving upon the return of moisture. 

 This power is strikingly illustrated among the lichens and 

 mosses, some of which can become so dry that they may be 

 crumbled into powder, but revive when moisture reaches 

 them. A group of club mosses, popularly known as " res- 

 urrection plants,'' illustrates this same power. The dried 

 up nest-like bodies of these plants are common in the 

 markets, and when they are placed in a bowl of water they 

 expand and may renew their activity. In such cases it can 

 hardly be said that there is any special effort on the part of 

 the plant to resist drouth, for it seems to yield completely 

 to the dry conditions and loses its moisture. The power 

 of reviving, after being completely dried out, is an offset, 

 however, for protective structures. 



135. Periodic reduction of surface. — In regions of periodic 



