2o8 FACTORS AFFECTING TRANSPIRATION 



may not pass across the walls with sufficient rapidity that an equilibrium 

 vapor pressure is maintained between the cell sap and the outer surface of 

 the cell walls. When the water content of a leaf is low the vapor pressure 

 of the cell wall surface in contact with the intercellular spaces may there- 

 fore be less than the vapor pressure of the water in the cell sap. Any such 

 reduction in the vapor pressure of the intercellular spaces results in decreasing 

 the steepness of the vapor pressure gradient between the intercellular spaces 

 and the outside atmosphere, and hence decreases the rate of transpiration. 

 The experimental results of Thut (1938) indicate that the vapor pressure 

 of the intercellular spaces is often at less than a saturation value. 



During the course of a day, however, the steepness of the vapor pressure 

 gradient through the stomates may be decreased in still another and perhaps 

 more important way. When the stomates open in the morning the vapor 

 pressure gradient between the saturated internal leaf atmosphere and the 

 outside atmosphere is short and hence steep, being approximately equal in 

 length to the depth of the guard cells. For some time after the stomates 

 open essentially such a condition is probably maintained. As the day advances 

 the rapid loss of water-vapor molecules out of the intercellular spaces, per- 

 haps coupled with a gradual reduction in the vapor pressure of the cell walls, 

 makes it less and less likely that a condition of saturation can be maintained 

 throughout the intercellular spaces. The zone in which a vapor pressure in 

 equilibrium with the water in the mesophyll cell walls is maintained almost 

 certainly retreats more and more deeply into the intercellular spaces. Eventu- 

 ally it may be restricted to a thin layer just above the evaporating surfaces 

 of the mesophyll cell walls. This gradual lengthening of the vapor pressure 

 gradient through the stomates is probably an important factor in bringing 

 about a reduction in the rate of transpiration during the afternoon hours. 



It has frequently been observed in studies of transpiration periodicity that 

 the transpiration rate often begins to decrease during the mid-day period 

 before any appreciable change occurs in the area of the stomatal apertures. 

 This indicates that the initial reduction in transpiration rate is induced by 

 some other factor than a change in the diffusive capacity of the stomates. As 

 the preceding discussion has shown this other factor is almost certainly a 

 reduction in the steepness of the vapor pressure gradient through the stomates. 



By late afternoon the air temperature and the intensity of the solar radia- 

 tion begin to decrease appreciably, thus inducing a decrease in the tempera- 

 ture of the leaf. This lowering of the leaf temperature may further decrease 

 the vapor pressure of the intercellular spaces, and hence further depress the 

 steepness of the vapor pressure gradient, since temperature changes have very 

 little influence on the vapor pressure of the outside atmosphere under "stand- 



