82 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



they opened 10 per cent and closed again at 5 a. m. The sun appeared 

 through the clouds over the mountains at 5 h 45 m a. m., but it began 

 to be light shortly after 5 a. m. In consequence, the stomata of the 

 field plants opened 10 per cent at 6 a. m., 40 per cent at 7, 80 per 

 cent at 8, and were wide open by 9 a. m. The stomata of the cut 

 stems acted in practically the same manner, except that opening 

 was somewhat slower at the start. Maximum opening lasted until 

 2 p. m. in the field plants. The stomata of the cut stems, however, 

 closed immediately to 25 per cent at 10 a. m., 10 per cent at 11 a. m. 

 and noon, and then slowly to 5 per cent at 2 p. m. They remained in 

 this condition until 3 p. m., and then closed very slowly and com- 

 pletely by 5 p. m., remaining closed to the end of the series. The 

 watered field plants started to close after 2 p. m. This continued 

 uniformly throughout the afternoon, and was completed at 6 p. m. 

 (fig. 38). 



The stomatal movement of the unwatered field plants showed even 

 greater divergence from that of the cut stems. The considerable 

 night opening found in these plants did not occur in the cut stems, 

 and the maximum opening at 7 a. m. was 2 hours earlier than that 

 found in either the heavily watered field plants or the cut stems. 

 Complete closure occurred at 9 a. m., when the stomata of the cut 

 stems and watered plants had reached maximum opening (fig. 38). 

 It must be concluded, therefore, that the water-loss from alfalfa 

 potometers can not be used as a measure of the water-loss from 

 naturally growing plants, nor can stomatal regulation be judged 

 ineffective by reason of such comparison. 



The transpiration of the cut stems as measured in the potometers 

 is distinctly controlled by stomatal movement. For several reasons, 

 the correlation between the rate of water-loss and the degree of 

 stomatal opening is among the best found in these experiments. The 

 complications entering into the problem by sudden changes of wind 

 velocity, wide fluctuations of relative humidity within brief inter- 

 vals, and sudden changes of sunlight caused by passing clouds did 

 not occur. In addition, the difference between the maximum and 

 minimum temperatures was not nearly as great as usual, changes were 

 slow and gradual, and the temperature remarkably constant during 

 the greater part of the day. The stomatal opening described in 

 connection with transpiration in this section is the average for both 

 surfaces of the leaf, calculated for the number of stomata per unit 

 area on each surface. 



Between 10 and 11 p. m. the average water-loss from the alfalfa 

 potometers or, more accurately, the water absorbed, was 7.0 mg. 

 per minute per square decimeter of leaf-surface, which is the upper 

 and lower surface of 0.5 square decimeter of leaf area. During this 

 time the stomata opened to 4 per cent, most of the opening occurring 

 in the upper surface. During the next hour the water-loss was 



