THE RELATION OF PLANTS TO WATER 143 



Leaves in dry climates, in a location where soil water is not 

 readily available, are wont to be smaller than in regions and 

 localities where water is abundant. This contraction of the 

 leaf results in a diminution in size of the intercellular spaces, 

 which thus reduces the danger of excessive evaporation into the 

 intercellular spaces from the mesophyll cells. The outer walls 

 of the epidermal cells may also become greatly thickened and 

 coated over with a waxy secretion called the cuticle. This pre- 

 vents all loss of water except through the stomata. The stomata 

 in most plants are also able to limit the amount of transpiration 

 by effecting a closure when the loss of water from the mesophyll 

 cells is not balanced by that received from the veins. This 

 opening and closing of the stomata is partly explainable on phys- 

 ical grounds, but is not as yet fully understood. Then again 

 many plants, like the mullein, have leaves in which the epidermal 

 cells grow out into a thick coating of hairs which prevent loss 

 of water (Fig. 72). These are only a few of the innumerable 

 structural devices for controlling the excessive loss of water from 

 leaves by transpiration. 



Control. The external factors which control transpiration are 

 light, heat, humidity, air currents, and the available water in 

 the soil. Light affects transpiration largely through its control 

 of the stomata, which, as was stated above, are usually open 

 during sunlight and closed in darkness. Temperature plays an 

 important role in water loss on account of its effect on the leaf 

 tissues and on the water content of the air. If the leaf tissues 

 are heated by the sun's rays, the result is an increased evapora- 

 tion from the mesophyll cells and an acceleration of diffusion 

 from the internal atmosphere of the leaf through the stomata 

 into the external air. This external air will also take up more 

 moisture when heated than when cool. These facts are con- 

 firmed by experience with plants grown in warm, dry living 

 rooms in the home, where the greatest care must be exercised 

 to prevent them from wilting on account of the excessive loss 

 of water. It is a well-known fact also that plants in humid 

 regions lose very little water by transpiration, on account of the 

 high relative humidity of the air. For the same reason there is 



