2o6 FACTORS AFFECTING TRANSPIRATION 



transpiration in the morning is due to the opening of the stomates, resulting 

 in a gradual increase in their aggregate diffusive capacity. As each stomate 

 opens a vapor pressure gradient is estabhshed through it between the atmos- 

 phere of the intercellular spaces and the outside atmosphere. During the 

 hours of darkness the leaf cells regain most or all of the water which they 

 have lost during the preceding day, and thus attain approximately their normal 

 turgidity. This facilitates the evaporation of water into the intercellular 

 spaces, so that when the stomates open in the morning the internal atmosphere 

 of the leaf is usually saturated with water-vapor, or practically so. The vapor 

 pressure of the intercellular spaces is therefore at the maximum possible for 

 that leaf temperature. This is, on clear days, almost always in excess of the 

 vapor pressure of the atmosphere when the stomates open, and often consid- 

 erably so. Hence outward diffusion of water-vapor through the stomates 

 usually begins as soon as they are open. 



Once a stomate is well open, minor variations in the size of the stomatal 

 aperture have little or no effect on the rate of water-vapor loss through it. 

 The rate of transpiration continues to increase, however, for some time after 

 the stomates, in the aggregate, have attained their maximum diffusive capacity. 

 This is due to a gradual increase in the steepness of the vapor pressure gra- 

 dient through the stomates. As the day progresses the temperature of both 

 the atmosphere and the leaf increases; if the latter is in direct sunlight its 

 temperature is invariably somewhat in excess of that of the atmosphere. On 

 clear days, as previously described, the vapor pressure of the intercellular 

 spaces usually increases relative to that of the atmosphere with a rise in tem- 

 perature. The steepness of the vapor pressure gradient between the internal 

 atmosphere of the leaf and the external atmosphere therefore usually increases 

 progressively during the earlier part of the day, and this is the important 

 factor accounting for a rise in the transpiration rate once the stomates have 

 attained approximately their maximum diffusive capacity. 



Almost from the moment when the stomates begin to open in the morning, 

 a train of events is set in operation in the plant which ultimately causes a 

 reduction in the rate of transpiration. During approximately the first half 

 of the day, however, these factors are more than offset by the factors result- 

 ing in an increase in the rate of transpiration. In most plants, while trans- 

 piration is occurring rapidly, the rate of absorption of water does not keep 

 pace with the rate at which water-vapor is lost from the leaves. This results 

 in a reduction in the water content of the entire plant, and especially that of 

 the leaves. In more extreme cases wilting results, but under standard day 

 conditions the leaves seldom pass beyond the stage of incipient wilting (Chap. 

 XVIII), which corresponds only to a partial loss of turgor by the leaf cells. 



