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 



