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 
