STOMATAL REGULATION 169 



very nearly closed. This particular experiment is typical 

 of the normal march of transpiration and stomatal movement 

 through the 24 hours. Its analysis shows that while the 

 two agree in their general course, there can be no question 

 of a delicate regulation of transpiration by stomatal move- 

 ment ; or, to put it in another way, stomatal aperture is not 

 in general the factor limiting transpiration. In fact, it seems 

 more likely that the loss of water about noon is responsible 

 first for restriction of transpiration, and then for stomatal 

 closure. Lloyd measured transpiration by the potometer. 



Artificial changes in the conditions made the dis- 

 crepancies clearer. To quote only one experiment : a 

 shoot of Verbena kept overnight in a dark room was placed 

 in the sun at 7.15 a.m. ; at this time the stomata were 

 scarcely 50 per cent, open, at 7.45 a.m. they had opened to 

 75 per cent., and at 8 a.m. had closed to 60 per cent. Yet 

 during this period the transpiration rate had steadily 

 increased to four times its original value. 



Trelease and Livingston (19 16) followed the daily 

 march of transpiration by the cobalt paper method (it is 

 possible to time the rate of colour change in a standardised 

 paper, and so to obtain accurate relative values), by which 

 they determine an " index of transpiring power," which is 

 proportional to relative transpiration in still air. They 

 compare this with the changes in stomatal aperture deter- 

 mined by the porometer. The graphs they give for five 

 series of experiments show that, in two, variation in stomatal 

 aperture agrees closely with change in transpiration rate, 

 while in three it does not. F. Darwin (19 16) concludes, from 

 the results of a series of experiments with the porometer, 

 that regulation by the stomata is important. 



Knight (1917), in the most exact work so far done on 

 the subject, finds that sometimes the graph of the T : E 

 ratio runs parallel with the graph of stomatal aperture, but 

 that usually this is not so. Graphs for one of his experiments 

 are given in Fig. 17. 



Loftfield (1921) has carried out a long series of investiga- 

 tions with American crop plants, which have yielded results 



