CHAPTER XIII 
XEBOPHYTE SOCIETIES 
133. 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. 
It is impracticable to stop transpiration entirely, for it 
must take place in connection with a necessary life-process. 
The adaptations on the part of the plant, therefore, are 
directed towards the regulation of transpiration, that it 
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