EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 83 



6.7 mg. per minute, while the stomata remained at 5 per cent opening. 

 From midnight to 1 a. m. the average loss was 8.7 mg. per minute, 

 corresponding to an increase to 10 per cent opening of the stomata. 

 Between 1 and 2 a. m. the rate of loss decreased to 7.5 mg. per 

 minute, while the stomata closed to 5 per cent. A fall in the rate to 

 4.5 mg. during the next hour occurred as the stomata closed. As the 

 stomata opened to 10 per cent at 4 a. m. the average rate of trans- 

 piration was 4.2 mg., but as a result of this opening it rose to 6.7 

 mg. between 4 and 5 a. m. At 5 a. m. the stomata closed again and 

 the rate of loss dropped to 1.5 mg. This very low rate was also due 

 to the rise of relative humidity to 80 and 90 per cent. Between 6 and 



7 a. m. the stomata opened from 5 to 35 per cent and the rate of 

 transpiration rose to 13.5 mg. The following hour opening increased 

 from 35 to 80 per cent, relative humidity fell below 70 per cent, and, 

 in consequence, the rate increased to 47.5 mg. per minute. Between 



8 and 9 a. m. the stomata opened from 80 per cent to maximum, and 

 the rate of loss to 100 mg. per minute. The next hour the stomata 

 closed to 25 per cent and transpiration fell to 37.0 mg. This was in 

 spite of the fact that relative humidity had fallen to 46 per cent and 

 sunlight had reached the average maximum for the day. Further 

 closure to 10 per cent at 11 a. m. was accompanied by a fall to 26.7 

 mg. per minute. From this time slow closure of the stomata, com- 

 pleted at 5 p. m., was accompanied by a similar fall in the transpira- 

 tion-rate. The stomata were closed from 5 to 10 p. m., at which 

 time the series ended. The rate of water-loss remained rather con- 

 stant from 5 to 7 p. m., since the relative humidity was also as con- 

 stant, but after 7 p. m. the humidity rose rapidly and the trans- 

 piration-rate again fell. Between 9 and 10 p. m., when the humidity 

 again became fairly stable, changing only from 74 to 76 per cent, 

 transpiration decreased only from 4.7 to 4.5 mg. per minute. 



The correlation in this experiment between the stomatal movement 

 and the rate of transpiration of cut stems is remarkably clear. 

 Where there is deviation in the rate of water-loss from the changes 

 of stomatal opening this is clearly due to fluctuations of relative 

 humidity. When the fluctuations are rapid and irregular they 

 introduce complications which are hard to measure. But in this 

 experiment the humidity changed slowly and rather uniformly, and 

 was nearly constant for long periods. Because this did not occur 

 in the other experiments, and because of other reasons, the later 

 series failed to show as clear a relation between stomatal movement 

 and transpiration. The stomatal movement of the field plants, 

 watered and unwatered, did not in any manner resemble that of the 

 cut stems, and the transpiration from the potometers in turn showed 

 no relationship to the changes in stomatal opening of these plants. 

 For both these reasons, in the case of alfalfa at least, potometers 

 can not be used to represent the transpiration of rooted plants. 



