AVAILABILITY OF SOIL WATER 



201 



A gentle breeze is relatively much more effective in increasing the trans- 

 piration rate than winds of greater velocity (Fig. 49). Winds of very high 

 velocity have been observed to have a retarding effect upon transpiration. 

 This is probably due to closure of stomates under such conditions. 



The effect of wind upon the transpiration of a leaf which is exposed to in- 

 tense insolation is further complicated by the fact that a wind may have a 

 cooling effect upon the leaves, due to their increased thermal emissivity under 

 such conditions. Reduction in the temperature of a leaf, as we have already 

 seen, exerts a decelerating effect upon transpiration. If conditions are such 



S 10 



WIND VELOCITY IN MILES PER HOUR 



Fig. 49. Relation between wind velocity and rate of transpiration of sunflower 

 plants expressed as ratio between plants in wind (Tj^) and plants in quiet air (T^^). 

 Data of Martin and Clements (1935). 



that this cooling effect of wind is predominant increased wind velocity may 

 result in a diminution in transpiration rate. 



Soil Conditions Influencing the Availability of Water. — Although 

 transpiration can continue for short periods at rates considerably in excess of 

 the rate of absorption of water (Chap. XVIII), in general, if soil conditions 

 are such that absorption of water is appreciably retarded, the rate of trans- 

 piration will soon show a corresponding retardation. The availability of 

 soil water to the plant is therefore an important and, in fact, often the limit- 

 ing factor in transpiration. The principal soil factors which affect the rate 

 of absorption of water by plants are: (i) available soil Avater, (2) soil tem- 

 perature, (3) aeration of the soil, and (4) concentration of solutes in the 

 soil solution (Chap. XVII). All of these factors indirectly influence the 

 rate of transpiration. 



