THE PHYSIOLOGY OF PLANTS. 75 



when the air is saturated with moisture. The farther below 

 the point of saturation the amount of moisture falls, the greater 

 the transpiration. From delicate and thin-walled cells the 

 evaporation would be excessive, but considerable hindrance is 

 offered by the epidermis. This in most plants is a layer one 

 cell thick, but in some cases it consists of several layers of 

 cells. The outer wall of each epidermal cell is thickened and 

 besides sometimes covered with a waxy coating, or a thin im- 

 pervious layer called the cuticle. In such cases there could be 

 no exhalation except through the stomates. In fact the sto- 

 mates may be considered as the special organs of exhalation. 

 The guard-cells become flaccid or turgid under the varying 

 influences of light and atmospheric conditions, thus closing 

 or dilating the stomates. Moreover the water in plants holds 

 in solution various substances, and the rate of evaporation is 

 thereby decreased. 



3. The rate of transpiration differs much in different plants 

 and under varying conditions, but it is always slow. A few 

 from the many published examples are as follows : In twelve 

 hours of daylight the amount evaporated from a Vine was 

 equal to a film only 0.005 inch thick, that from a Cabbage in 

 the same time equalled a film 0.012 inch and from an Apple 

 0.01 inch thick. The amount of water exhaled from a close- 

 topped Oak tree 20 feet high during the growing season of five 

 and a half months was a little over 30,000 gallons. This is 

 equal to a layer about 1.3 inches thick over the whole evapor- 

 ating surface. The transpiration from leaves is usually given 

 as one-sixth to one-third as much as the evaporation from an 

 equal surface area of water. This would be a much larger loss 

 of water from the plant than the amount of annual rainfall 

 on the ground surface the plant covers. 



Fasten a leafy shoot or single leaf with roundish petiole in a closely fitting 

 stopper in one end of a U-tube filled with water and furnished with a small 

 horizontal tube as shown in Fig. 114. The water soon recedes in the hori- 

 zontal tube, indicating loss by transpiration. 



4. In midsummer the transpiration may be so great that 

 the roots cannot furnish the requisite amount of water to sup- 



