The Water Relations of Leaves 



77 



W. S. Cooper 



Fig. 43. A xerophytic morning-glory and a succulent-leafed Mesembryanthetnum (above at 

 right) growing on the dunes near Coronado, California. The thick cuticle of the morning- 

 glory leaf reduces the transpiration rate. The other plant has a relatively small leaf area 

 and holds its water tenaciously because of substances within its cells. 



high transpiration rate if the stomata are open. In leaves with 

 a heavy cuticle the stomata are usually small and do not open so 

 freely as in leaves with a very thin cuticle ; consequently trans- 

 piration is generally less from hard, thick, and heavily cutinized 

 leaves, even though the cuticle prevents evaporation only from 

 the outer leaf surfaces. 



(2) Compact leaves. A plant may become adjusted to an inade- 

 quate water supply by the development of leaves with compact 

 tissues, as a result of exposure to drought or bright sunshine. In 

 such leaves the intercellular spaces are much reduced, and evapo- 

 ration from the mesophyll cells is greatly lessened. In extreme 

 cases the mesophyll cehs are all of the compact pahsade type, 

 which leaves the minimum of air -space within the leaf. Compact 

 tissues reduce the rate of transpiration through the stomata. 

 The tissues are compact under these circumstances simply because 

 drought prevented the further expansion of the leaf, leaving the 

 cells close together (Fig. 42). 



(3) Small leaf area. A third way in which plants become ad- 



