40 THE VEGETATIVE FUNCTIONS OF PLANTS 



Warm air can contain more water- vapor in a given space 

 than colder air. For this reason, other things being 

 equal, plants on which the sun is shining will transpire 

 more rapidly than those in the shade, or than on a cool, 

 cloudy day. Florists take advantage of this fact by 

 keeping cut flowers in a place artificially cooled by ice. 



Certain structural features of the plant operate to 

 reduce transpiration. The epidermal hairs, as for ex- 

 ample on the mullein leaf, tend to retain the more humid 

 air near the surface of the leaf, even when the wind 

 blows. In some plants {e.g., the tropical gasterias, Fig. 

 30) the cuticle is greatly thickened, so that water can 

 pass off only very slowly. The very curling of leaves, 

 when they begin to wilt, also tends to reduce transpira- 

 tion by reducing the amount of surface exposed (Fig. 31). 

 The arrangement of leaves in a compact rosette accom- 

 plishes the same result (Fig. 32). 



Evidence obtained by recent studies of transpiration 

 in several different species of flowering plants indicates 

 that there is no necessary nor uniform relation between 

 the amount of transpiration and the number of stomata 

 per unit of leaf-surface, nor between the amount of 

 transpiration and the total area of the stomata. These 

 studies indicate that, contrary to our earlier conceptions, 

 the amount of transpiration is probably regulated by a 

 complex of several factors, among which the stomata are 

 less important than was formerly supposed. 



40. Advantages of Transpiration. — It might seem, at 

 first thought, that the loss of water by transpiration is a 

 disadvantage to plants. Of course this would be the case 

 were it not possible for the roots to take in water as fast 

 as it is lost. When this is not possible, transpiration 



