June, 1919] Variations in Transpiration 501 



22nd hour. The curves for the size of the stomata rose rapidly 

 at the fifth hour, or sunrise, the hourly increment diminishing 

 slowly until a maximum was reached at the 10th hour. From 

 the maximum there was a gradual decrease until their night 

 values were reached between the 20th and 22nd hour. It is 

 apparent from the occurrence of the maxima of the different 

 factors and the rates of water loss that no single factor con- 

 trols the water. loss from the plants. 



A comparison of these curves does not give a true represen- 

 tation of their relations to each other because different scales 

 of ordinates are taken in plotting them. For comparing 

 curves of this kind the variation between the maximum and 

 minimum values should be reduced to the same scale in all 

 cases. The same curves are shown on pages 496 and 498 

 with the maximum and minimum values reduced to the 

 same scale, and with the transpiration curve (dotted line) 

 superimposed on each of the other curves. The maximum and 

 minimum values of each curve are indicated on the base lines. 

 A comparison of these maximum and minimum values shows 

 that a change of 23° F. and 35% in saturation deficit of the 

 air produced a change from 0.30 to 1.65 grams per hour in 

 evaporation, i. e. increased it about five times. Under the same 

 conditions of temperature and saturation deficit the transpira- 

 tion rates showed a change from 0.08 to 2.20 grams per hour 

 in mullein i. e. an increase of about 25 times and 0.06 to 2.60 

 grams per hour in tobacco i. e. an increase of about 40 times. It 

 is obvious that so great a variation in the rate of water loss 

 when compared with evaporation must be due to some physi- 

 ological change within the plant. 



An increase in leaf water deficit decreases transpiration by 

 increasing the resistance of the mesophyll cells to water loss and 

 as the maxim.um occurs a little later than the maximum water 

 loss it operates to decrease the difference between the maximum 

 and minimum rates of water loss. The stomatal openings are 

 never used to their full diffusion capacity under ordinary condi- 

 tions so the increase in their dimensions accounts for only the 

 general difference between the day and the night rates of water 

 loss. The rate of water loss from the leaf when the stomata are 

 open is determined by the diffusion gradient (Renner,^), i. e., 

 the difference between the saturation deficit of the intercellular 

 spaces of the leaf and the saturation deficit of the air. 



