VAPOR PRESSURE AND TRANSPIRATION 195 



of the water-vapor. Let us suppose that the vapor pressure of the intercellular 

 spaces is 31.82 mm. Hg which is the value for a saturated atmosphere at 

 30° C. Such a vapor pressure is frequently attained in the internal air spaces 

 of leaves. Let us further assume that at the same time the vapor pressure of 

 the atmosphere is only half as great (15.91 mm. Hg). Such a value would 

 be a representative one for a warm summer's day in the eastern United States. 

 In very quiet air the vapor pressure in the neighborhood of transpiring leaf 

 surfaces may be greater than that in the atmosphere in general but in this 

 discussion it is assumed that there is sufHcient air movement to prevent any 

 appreciable accumulation of water-vapor in the vicinity of the leaves. 



Under the conditions as stated diffusion of water-vapor would occur 

 through the stomates at a relatively rapid rate. If the vapor pressure of the 

 atmosphere were lowered below this value the rate of diffusion of water-vapor 

 out of the leaf would be increased ; conversely increase in the vapor pressure 

 of the atmosphere would result in a decrease in the diffusion rate of water- 

 vapor out of the leaf. Similarly, an increase in the vapor pressure of the 

 intercellular spaces would result in an increase in the rate of transpiration 

 while a decrease in the vapor pressure of the intercellular spaces relative to 

 that of the atmosphere would have the converse effect. On the rare occa- 

 sions when the vapor pressures of the atmosphere and of the intercellular 

 spaces are equal, no transpiration will occur, even if the stomates are open. 



The final statement in the preceding paragraph, should not, however, 

 be interpreted to mean that transpiration can never occur into a saturated 

 atmosphere. If the temperature of the leaf is higher than that of the sur- 

 rounding atmosphere this will, if the intercellular spaces be saturated with 

 water-vapor, result in a higlier vapor pressure in them than in the atmos- 

 phere, even when the latter is also saturated with water-vapor. Under such 

 conditions outward diffusion of water-vapor occurs, resulting in a localized 

 region of supersaturated atmosphere around the leaf in which condensation 

 of water-vapor may take place. 



Temperature Effects on Transpiration. — i. Thermal Relations of 

 Leaves. — While leaf temperatures often do not deviate greatly from surround- 

 ing atmospheric temperatures the discrepancy between the two is frequently 

 sufficiently great to make it necessary to take it into account in careful experi- 

 mental work. Leaves exposed to direct solar or artificial radiation usually 

 have temperatures of from 2 to 10° C. (sometimes even more) in excess of 

 that of the atmosphere. Under other conditions, to be described later, leaf 

 temperatures may be less than that of the atmosphere.'- 



^ Leaf temperatures are generally measured by means of thermocouples. A 

 thermocouple is made by twisting together the ends of two fine wires of dissimilar 



