[CJuip. XXIV TRANSPIRATION 229 



summer the temperatures of both the atmosphere and the leaf are gradu- 

 ally raised from 68° F. to 86° F. The concentration of water vapor in 

 the atmosphere changes very Httle, because the volume of air is so 

 enormous and the air is continually moved by currents. Inside the leaf 

 the same rise in temperature results in rapid increase in water-vapor 

 content, because the volume of the intercellular spaces is very small 

 compared with the evaporation surface. Calculations indicate that the 

 vapor diffusion gradient between the inside of the leaf and the atmos- 

 phere has increased 2.5 times. If now the internal temperature of the 

 leaf rises an additional 9° above that of the air, the gradient is increased 

 to 4 times what it was in the early morning. Assuming that there is an 

 abundant supply of water within the plant and the stomates are open, 

 the rate of transpiration would be about 4 times as great. Guard cells 

 may open and close stomates and thus accelerate or stop stomatal tran- 

 spiration. But when the stomates are even partially open the most impor- 

 tant factor is the water-vapor gradient from inside the leaf to the air 

 outside. 



Intensity of sunlight and transpiration. Under good growing conditions 

 light intensities naturally occurring in daytime are sufficient to result in 

 the opening of the stomates, except on extremely cloudy days and when 

 the plants are densely shaded. Sunlight thus indirectly accelerates tran- 

 spiration. Furthermore, the radiant energ)- of sunlight directly increases 

 transpiration in humid climates, because it raises the internal tempera- 

 tures of exposed leaves above that of the air, sometimes as much as 5° to 

 10° F. This in midsummer is enough to double or treble the vapor 

 diffusion gradient between the inside of the leaf and the atmosphere. 



In the shade, leaf temperatures are about the same as those of the 

 air, or lower. When the atmosphere is very dry, leaf temperatures in 

 the shade may be several degrees cooler than air temperature. 



Soil water and transpiration. Another factor that greatly influences 

 transpiration is the water content of the soil. Even though the stomates 

 are open and other conditions favor high transpiration, if the water 

 supply in the soil becomes low, transpiration may be markedly decreased. 

 If the soil water in immediate contact with the roots is soon exhausted, 

 water in the xylem vessels of the stem and roots fails to diffuse into the 

 leaf cells as rapidly as it evaporates from them. There is an increase of 

 tension in the water in the veins and vessels of the leaf, the stem, and 

 finally of the root; the mesophyll cell walls become less and less saturated 

 — hence fewer water molecules leave the cell surfaces. 



