LIMITATION OF TRANSPIRATION 175 



by Fitting (191 1), a certain protective effect is obtained ; 

 it is unlikely that it is very pronounced or general. 



E. B. Shreve (1920) found that the xerophytic summer 

 leaves of the desert composite Encelia farinosa contained 

 in their cell sap a brown substance, which appreciably 

 diminished evaporation rate, and which was absent from 

 the more mesophytic type of leaf produced in the rainy 

 season. Such cases are probably rare. 



Conclusions on Regulation. — We must conclude, then, 

 that a slight fall in the water content of the leaf acts as an 

 automatic check on transpiration, but that, despite this 

 internal regulation, wilting may set in. On wilting, stomatal 

 closure, after a preliminary temporary opening, follows, and 

 ultimately cuts down water loss to a low value, conserving 

 the supply or even allowing absorption to make up the loss. 

 The mesophyte grows in conditions where such regulation 

 is normally sufficient to obviate serious damage, but the 

 plant which exists in more extreme conditions can do so 

 only by virtue of special safeguards. 



§ 23 . Limitation of Transpiration by Form and Position 

 OF Stoma and by Cuticle 



We have assumed so far that the stoma is of the 

 " normal " type in structure and position. Frequently, 

 however, the structure is such that diffusion is slowed down, 

 and the stoma may be so placed that direct impact of extreme 

 atmospheric conditions is avoided. Most simply this 

 happens when the stomata are confined to the less exposed 

 lower surface of the leaf, as is the rule in our broad-leaved 

 trees. 



A common mode of protection is seen where the stomata 

 are sunk in pits or grooves below the surface of the epiderm. 

 Grooved leaves are very common among the grasses and 

 sedges ; they are prominent in our two dune grasses, 

 Psamma arenaria and Elymiis arenarius. In such cases the 

 stomata are almost or entirely confined to the bottom and 

 sides of the grooves. Grooves are also a common feature 



