TEMPERATURE EFFECTS ON TRANSPIRATION 199 



that the process of transpiration itself, occurrin^^ on a grand scale from a 

 vegetation-covered area of the earth's surface would be sufficient to increase 

 the vapor pressure of the lovi^er layers of the atmosphere during the daylight 

 hours. The atmosphere is so vast, however, in relation to the amount of 

 water-vapor lost by plants, that over short periods, transpiration has only a 

 slight effect on its vapor pressure except in deep ravines or other local habi- 

 tats in which movement of the air is restricted. 



Increase in the temperature of the atmosphere does result in an increase 

 in the speed of the water-vapor molecules present. If the volume of the at- 

 mosphere remained constant this would result in a small increase in its vapor 

 pressure. But even this effect is never fully realized in the atmosphere be- 

 cause an increase in temperature also results in an expansion of the atmosphere, 

 entirely or largely offsetting its influence in increasing vapor pressure. 



If we assume for the purpose of our specific example that the vapor 

 pressure of the atmosphere at 20° C. was half that of a saturated atmosphere 

 at that temperature — 8.77 mm. Hg — then the excess vapor pressure of the 

 leaf over that of the atmosphere at 20° C. was 8.77 mm. Hg (17.54.— 

 8.77). At 30° C, however, the vapor pressure of the intercellular spaces 

 would have increased to about 31.82 mm. Hg while the increase in the vapor 

 pressure of surrounding atmosphere would usually be so slight that it can be 

 disregarded. The excess vapor pressure of the intercellular spaces over the 

 atmosphere is now 23.05 mm. Hg (31.82 — 8.77) which will result in dif- 

 fusion of water-vapor out of the leaf at a rate nearly three times as fast as 

 at 20° C. The effect of a rise in temperature therefore is principally an 

 increase in the steepness of the diffusion gradient (vapor pressure gradient) 

 of water-vapor through the stomates, and hence an increase in the rate of 

 transpiration. 



So far we have considered only examples in which the temperature of a 

 leaf and the surrounding atmospheres are the same. We have already noted, 

 however, that the temperature of leaves exposed to direct sunlight is almost 

 always higher than that of the atmosphere. If the temperature of a leaf is 

 increased above that of the surrounding atmosphere by the absorption of solar 

 radiation, the usual effect is an increase in the magnitude of the excess vapor 

 pressure of the intercellular spaces over that of the outside atmosphere. Let 

 us continue the example which has been presented earlier. At 30° C. under 

 the conditions as stated the vapor pressure difference between the intercellular 

 spaces and the atmosphere was about 23.05 mm. Hg. Suppose, however, that 

 due to the absorption of radiant energ}', the temperature of the leaf is 35° C. 

 while that of the atmosphere remains at 30° C. Water would evaporate 

 from the walls of the mesophyll cells until the vapor pressure in the inter- 



