CHAPTER XIV 

 FACTORS AFFECTING TRANSPIRATION 



The rate of transpiration of a plant or any leaf on a plant varies from 

 day to day, from hour to hour, and, frequently, from minute to minute. 

 Variations in the rapidity with which water is lost by plants are due to the 

 effects of environmental factors upon physiological conditions within the plant. 

 The important environmental factors influencing the rate of transpiration are: 

 (i) solar radiation, (2) humidity, (3) temperature, (4) wind, (5) soil con- 

 ditions influencing the availability of water, and (6) atmospheric pressure. 

 This last factor is relatively much less important than the other five listed. 

 While the general effect of variations in the intensity or magnitude of each 

 of these factors upon transpiration is well known, and has frequently been 

 demonstrated by experimentation, the precise mechanism of the effect of each 

 is not so easily amenable to experimental treatment. The following interpreta- 

 tion of the mechanism of the effects of these environmental factors upon the 

 rate of transpiration is therefore a somewhat theoretical one, but is in accord 

 with the experimental data available at the present time. 



Solar Radiation. — This term refers to the visible light and other forms 

 of radiant energy (infrared and ultraviolet radiations) reaching the earth 

 from the sun (Chap. XIX). The principal effects of solar radiation upon 

 transpiration result from the influence of light upon the opening and closing 

 of the stomates. In most of the species of plants which have been studied 

 the stomates are usually closed in the absence of light, thus causing a virtually 

 complete cessation of stomatal transpiration during the hours of darkness. 

 Since none of the other environmental factors can have any influence upon 

 stomatal transpiration except when the stomates are open, light occupies a 

 position of prime importance among the environmental conditions influencing 

 transpiration. 



A second important effect of solar radiation upon transpiration operates 

 through its influence on leaf temperatures. In direct sunlight the temperature 

 of leaves is almost invariably higher than the air temperature. This effect 

 will be analyzed later in this chapter. 



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