96 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



11.4 mg. as before. During the period between 2 h 30 ra and 4 h 30 m 

 a. m. the average of stomatal opening rose to 6 per cent, but as the 

 humidity increased to the maximum and evaporation fell off to the 

 minimum, the rate of transpiration fell slightly, namely, to 10.2 

 mg. per minute. During the next period, 4 h 30 rn to 6 h 30 m a. m., the 

 average of stomatal opening rose to 25 per cent, but as evaporation 

 and relative humidity remained the same, this in part explains the 

 slight increase to 12.4 mg. During the period from 6 h 30 m to 8 h 30 m 

 a. m. the average rose to 85 per cent and the rate of water-loss to 

 48.3 mg. Evaporation increased somewhat during this period, but 

 was still very low. The average opening rose to 100 per cent during 

 the period from 8 h 30 m to 10 h 30 m a. m. and the rate of loss to 80 mg. 

 per minute. The stomata were wide open during the following 

 period as well, while the rate further increased to 108 mg. per minute 

 in response to increase of evaporation. From 12 h 30 m to 2 h 30 m p. m. 

 the evaporation was very high comparatively and the transpiration- 

 rate rose to the maximum of 132.7 mg. per minute, the stomata still 

 being wide open. During the interval from 2 h 30 m to 4 h 30 m p. m. 

 the rate fell to 106.7 mg., largely in response to a similar fall in the 

 intensity of the factors of evaporation, although at the end of the 

 period the stomata closed slightly. From 4 h 30 m to 6 h 30 m p. m. the 

 rate fell to 56.8 mg. per minute, in part due to the fall of evaporation 

 and in part to closure of the stomata, the average opening for the 

 period being 62 per cent. The fluctuations of the rate while the 

 stomata w r ere wide open are clearly caused by the changes in the 

 evaporating factors, and it is evident that these will control transpira- 

 tion while the stomata are at or near their maximum opening (fig. 50) . 



The plants in the dry containers were treated in the same manner 

 as the preceding ones. The plants stripped were small ones in large 

 earthenware pots, and differed from those heavily watered in having 

 had no water for 10 days. The water-content for this period was 

 10 per cent, and the echard was 9 per cent. The containers were 

 weighed, as before, but alternating with the watered plants. On 

 this account the intervals overlap those for the other set by 1 hour, 

 and hence the curves alone can be compared for the two. 



As before, the stomatal movement was averaged for the 2 hours 

 of the interval between weighings. Between 7 h 30 In and 8 h 30 p. m. 

 this average was 2 per cent, while the rate of transpiration was 7.91 

 mg. per minute. Between 9 h 30 m and Il h 30 m p. m. the rate fell to 

 5 h 40 m mg. per minute, in spite of the slight increase of opening to 

 3 per cent. This fall coincides with a great reduction in the rate 

 of evaporation as shown by the atmometers, the average for the 

 first period being 195 mg. per minute and for the second 46 mg. per 

 minute. In the interval from Il h 30 m to I h 30 m a. m. the rate was 

 practically unchanged, being 5.42 mg. per minute, although the 

 stomata showed again a slight opening. From I h 30 m to 3 h 30 m a. m. 



