EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 97 



the average stomatal opening increased to 25 per cent, while the 

 rate rose to 8.73 mg. per minute. A slight increase in opening to 

 28 per cent from 3 h 30 m to 5 b 30 m a. m. corresponded to slight increase 

 in the rate to 8.75 mg., although the rate of evaporation decreased. 

 The maximum of opening occurred from 5 h 30 m to 7 h 30 m a. m., the 

 average being 58 per cent, and the greatest transpiration also oc- 

 curred at this time, the water-loss being 24.21 mg. per minute. 

 Closure to 8 per cent from 7 h 30 m to 9 h 30 m a. m. caused in part the 

 drop in the rate to 4.75 mg., but the lowest rate of evaporation 

 observed was partly also responsible. From 9 h 30 m to Il h 30 m a. m. 

 the average opening decreased to 6 per cent, but the rise in evapora- 

 tion to three times its former rate caused a slight increase in the 

 rate of transpiration to 5.11 mg. per minute. From Il h 30 m a. m. 

 to I h 30 m p. m. the average opening and evaporation were unchanged, 

 and the rate of water-loss also remained the same. From I h 30 m to 

 3 h 30 p. m. the stomata were again unchanged, but the great increase 

 of evaporation from 85 mg. to 268 mg. per minute caused a rise in the 

 rate of transpiration to 10.67 mg. per minute. During the period 

 from 3 h 30 m to 5 h 30 m p. m. the stomata opened to 8 per cent, and 

 although the evaporation-rate dropped considerably, the rate of 

 transpiration rose to 10.75 mg. per minute. From 5 h 30 m to 7 h 20 m 



7 a 9 IO It MX. I Z 3 * 5 6 7 8 9 10 II NOON I E 3 <V 5 



FIG. 51. Series 32, showing stomatal movement averaged per 2-hour period 

 (A) and transpiration in milligrams per minute (B) of dry alfalfa 

 phytometers; evaporation 1=2 milligrams per minute (C). 



the stomata closed to 4 per cent and the rate of evaporation fell 

 below 100 mg. per minute, and, in consequence, the average water- 

 loss fell to 5.06 mg. per minute. While this shows that the factors 

 of evaporation play an important part in changing the rate of trans- 

 piration, the effect of these factors is as clearly shown to be regulated 

 by the stomata (fig. 51). 



