198 FACTORS AFFECTING TRANSPIRATION 



of cacti, etc. under the influence of direct insolation may often greatly exceed 

 those of the surrounding atmosphere. The side of an apple fruit exposed to 

 direct sunlight, for example, may have a temperature of from 12 to 25° C. 

 higher than the air temperature (Brooks and Fisher, 1926). 



2. The Influence of Temperature upon Transpiration Rates. — The effects 

 of temperature upon the rate of stomatal transpiration can be most clearly 

 analyzed in terms of its effect upon the difference in vapor pressures between 

 the intercellular spaces and the outside atmosphere (Renner, 1915). In this 

 part of the discussion it is again assumed that the air movement is sufficient 

 to prevent any appreciable accumulation of vi^ater-vapor in the vicinity of the 

 leaf surfaces. Suppose that the temperature of both the leaf and the sur- 

 rounding atmosphere increases from 20° C. to 30° C. Unless the leaf is 

 markedly deficient in water this will result in an increase in the vapor pres- 

 sure of the intercellular spaces from approximately 17.54 mm- Hg to ap- 

 proximately 31.82 mm. Hg, these being the values for a saturated atmos- 

 phere at 20° C. and 30° C, respectively. The atmosphere of the leaf inter- 

 cellular spaces is in direct contact with the relatively extensive evaporating 

 surface of the mesophyll cell walls, hence the vapor pressure in the intercel- 

 lular spaces tends to remain in equilibrium with the water in the mesophyll 

 cells. In order to simplify this part of the discussion, it will be assumed that 

 the atmosphere of the leaf intercellular spaces maintains essentially a satura- 

 tion vapor pressure for the prevailing leaf temperature. Under certain condi- 

 tions, as shown later in this chapter, maintenance of even an approximately 

 saturated atmosphere throughout the intercellular spaces probably does not 

 occur, but for the time being this possibility will be disregarded. 



In the surrounding atmosphere, however, vapor pressure conditions are 

 very different. On clear days, that is, on the very type of day upon which 

 the highest rates of transpiration occur, there is frequently little change in the 

 vapor pressure of the atmosphere over land surfaces during the course of a 

 single day.^ Evaporation into the atmosphere is insuflficient to permit a rapid 

 building up of the vapor pressure towards the value for a saturated atmosphere 

 as the temperature of the air increases during the day. It might be thought 



2 This statement should not be misinterpreted to read that the vapor pressure 

 of the atmosphere is invariable. The magnitude of the vapor pressure of the 

 atmosphere varies greatly from day to day and from season to season, depending 

 upon the prevailing climatic conditions. On cloudy or rainy days, the atmospheric 

 vapor pressure is generally greater than on clear days during the same season; 

 in the summer months it is generally greater than in the winter months, etc. 

 Nevertheless the statement made above that on clear, bright days there is often 

 little change in the vapor pressure of the atmosphere is essentially correct 

 (Day, 1917). 



