XEKOPHYTE SOCIETIES 189 
may occur sufficiently for the life-processes, but that it 
may not be wasteful. 
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 protect 
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 
