132 PLANT LIFE 
that a closer examination of the matter will 
reveal much that is of intense interest, and 
of great importance in its theoretical bearings 
on the problems already adumbrated. 
The urgent need of water, common to all 
vegetation, is especially great on the part of 
the green plants, although the larger portion of 
that which is absorbed is not used directly in 
the synthetic functions, but is exhaled through 
the stomata with which most leaves are so 
plentifully provided. Its value to the plant 
stands even before that of light, for photo- 
synthesis, like other characteristically vital 
functions, is practically arrested as soon as 
the supply of water falls below a critical 
amount. 
Some of the higher and many of the lower 
green plants are able to tolerate long periods 
of drought; but they do so by passing into a 
condition of suspended animation, during 
which many of their chemical processes 
are slowed down and others are completely 
arrested. Thus a large number of lichens, 
certain mosses, and various other plants, may 
all become so far desiccated during dry periods 
that they can be easily reduced to powder. 
A shower of rain, however, serves to restore 
them in a few minutes to a condition of 
renewed and active vitality. 
Nearly all land plants are liable to encounter 
periods during which the supply of available 
moisture runs short. The shortage may be 
due to seasonal or climatic causes, or it 
