98 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



A comparison of the transpiration-rates of the two groups of 

 plants brings out certain other interesting facts. At only one time 

 did the rate for the plants sealed in dry soil apparently exceed that 

 of the heavily watered plants. This probably happened between 5 

 and 6 a. m., although because the two sets were weighed at alternate 

 half hours it was not actually demonstrated. This was due to the 

 much greater stomatal opening at that time in the plants in dry 

 soil. Another point of importance is shown by the comparative rates 

 in the periods including 10 p. m. in both series. At this time the 

 stomata of both sets averaged 3 per cent opening, but the rate 

 for the heavily watered plants was practically twice that of the dry 

 plants. Throughout the experiment the rate was proportionally 

 lower for the plants in dry soil for the given degree of stomatal 

 opening. The most plausible explanation is to ascribe this to the 

 effect of a lower water-margin in the leaf. Moreover, it is very 

 evident that the much lower rate of water-loss in the dry containers 

 during the greater part of the day, when the rate for the wet ones was 

 very high, must be attributed to the control exerted by the nearly 

 closed stomata (fig. 52). 



7 8 9 10 II MT. 



Z 3 A 56 7 8 9 10 II NOON I Z 3 A- 567 



Fio. 52. Series 32, showing transpiration of wet (A) and dry (B) alfalfa 

 phytometers. 



The stomatal movement of plants of cow-beet growing in large 

 pots of dry soil was studied in a similar manner in series 33. This 

 experiment was started at 6 p. m. on August 8, 1919, and ended at 

 6 p. m. on August 9. The night was not cold, but lowest tempera- 

 tures were 59 F. at 2 a. m. and 59. 5 F. at 6 a. m., just before sunrise. 

 During the day it rose to a maximum of 85 F. at 2 and 3 p. m. The 

 relative humidity fluctuated a great deal, but within unusually 

 narrow limits, since it did not rise above 79 per cent or fall below 

 42 per cent. Passing clouds caused the sunlight to fluctuate con- 

 tinuously. Like the other factors, the wind was also variable, at 

 times rising to an average of 5 miles an hour and other times dying 

 away completely (fig. 32). 



