200 FACTORS AFFECTING TRANSPIRATION 



cellular spaces approximates that of a saturated atmosphere at 35° C. (42.18 

 mm. Hg). The gradient between the intercellular spaces and the atmosphere 

 is therefore increased to about 33.41 mm. Hg (42.18 — 8.77) and the rate 

 of transpiration is correspondingly increased. The sudden increases in trans- 

 piration rate which are often observed when plants are shifted from shade 

 to direct sunlight, as for example in potometer experiments, are undoubtedly 

 due largely if not entirely to effects of this type. Similarly it has been ob- 

 served that a sudden decrease may occur in the transpiration rate when a pass- 

 ing cloud obscures the sun, which is followed by just as sudden an increase 

 when the sun again emerges from behind the cloud. For further analysis of 

 the effects of leaf temperature upon transpiration rates see Curtis (1936b). 



Wind. — The effect of wind upon the rate of transpiration is far from 

 simple, and depends in part upon the other prevailing environmental condi- 

 tions. Usually, however, increase in wind velocity, within limits, results in an 

 increase in the rate of transpiration (Wrenger, 1935)- This is usually 

 explained by assuming that water-vapor often accumulates in the vicinity of 

 transpiring leaves in a quiet atmosphere, especially if they are not exposed 

 to direct sunlight. The result of such an accumulation of water-vapor is a 

 decrease in the steepness of the vapor pressure gradient through the stomates 

 and hence a decrease in the rate of transpiration. If, however, the leaves are 

 exposed to a wind, any accumulation of water-vapor molecules in the imme- 

 diate vicinity of the leaf surfaces will be dispersed. The effective result will 

 be an increase m the steepness of the vapor pressure gradient through the 

 stomates, and a consequent increase in the rate of loss of water-vapor. 



Whenever a temperature differential exists between a leaf and the sur- 

 rounding atmosphere, convection currents are set up in the gases in the vicinity 

 of a leaf which may largely or entirely prevent any accumulation of water- 

 vapor in the immediate vicinity of the leaf. The influence of wind in raising 

 the transpiration rate of leaves is probably more effective, therefore, when they 

 are subjected to such conditions that their temperature does not depart appre- 

 ciably from that of the surrounding atmosphere. 



The effect of wind in dispersing accumulated water-vapor in the vicinity 

 of the leaf surfaces is probably of less importance in its influence upon trans- 

 piration rates than its effect in causing the swaying of branches and shoots, 

 and the bending, twisting, and fluttering of leaf blades. It has been shown 

 experimentally that immobile leaves usually transpire less than similar leaves 

 allowed to bend and move freely when both are exposed to wind of equal 

 velocity. Such bending and contortion of leaves may increase the rate of 

 water-vapor loss in part by compressing the intercellular spaces, thus forcmg 

 water-vapor and other gases out through the stomates. 



