The Water Relations of Leaves 



59 



Adjustment to desert conditions by ability to withstand 

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

 tions by being able to with- 

 stand complete drying. The 

 resurrection plant (Fig. 39) 

 of Texas is an example of 

 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 unfolds, 

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

 find plants of this same type in the lichens, 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 

 precedmg 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). 



