The Water Relations of Leaves 



59 



isr 



Adjustment to desert conditions by ability to withstand 

 drying. Another group of plants is adjusted to desert condi- 

 tions by being able to with- k nL t 

 stand complete drymg. The ^.-W- A-'-V. 

 resurrection plant (Fig. 39) 5«M^^^ 

 of Texas is an example of ^.^^^^^^i;^^ 

 group. During ' ^. - 



this group. During the 



rainy season it is green and 



has its many scale-leafed 



branches spread out for food 



manufacture and growth. 



When drought comes, the 



plant dries out completely 



and its branches curl upward 



until it is in the form of a 



ball. In this condition it 



may be blown about by the 



wind and remain dormant 



for weeks and months, all 



of its physiological processes 



having ceased. When the plant again becomes wet it imfolds, 



and its processes begin anew. In the eastern United States we 



find plants of this same type in the hchens, mosses, and small 



ferns that grow on the bark of trees and on bare, dry rocks. 



Plants classified according to their water relations. In the 

 preceding paragraphs the importance of the water require- 

 ments of plants has been made clear. We have seen that the 

 internal water balance of the plant is of great importance in 

 modifying its physiological processes and the size and struc- 

 ture of its organs. Three great classes of plants are dis- 

 tinguished on the basis of their water relations : 



Fig. 39. Resurrection plant (Selaginella), 

 in growing condition (above), and the same 

 plant in a dry and dormant condition 

 (below) . V. 



