NORMAL DAILY PERIODICITY. 39 



ment 139 both commence simultaneously. The stomatal decrease is steady, 

 and is completed by 5 or 6 p. m., while the transpiration rate continues to 

 fall after this for several hours. The possible interpretation presents itself, 

 that were it not for the early decrease of the stomatal dimensions in the after- 

 noon, the rate of transpiration would continue high; and this suggestion 

 draws importance from the fact that the danger from wilting is reaching its 

 maximum at about mid-afternoon. It seems hardly likely, however, that 

 the stomata are mainly responsible for the decrease in transpiration, because, 

 in the cases before us, (1) the rate did not fall during the period of greatest 

 reduction of stomatal dimensions, and (2) the rate fell steadily for 5 hours 

 after the minimum openings had been reached. 



That the transpiration rates are not related within wide limits to the dimen- 

 sions of the stomata appears from the observed differences in the rate of 

 transpiration accompanied by constant condition of the stomata ; or by direct 

 variation of the stomata too small to explain the changes in transpiration 

 rate; or by inverse differences, which offer still more striking evidence. 

 Thus, with a narrow opening of 1 to 1.7 micra, the stomata allow observed 

 differences in the rate of transpiration from unity to 5. 



In experiment 139 the total increase in the rate of transpiration during 

 the morning of July 2 was from 54 to 336 units, while the transverse diameter 

 of the stomata increased from 3.5 to 10 micra. In experiment 140 the 

 change from 14 to 90 units in transpiration rate was accompanied by a change 

 in transverse stomatal dimension of from 2 to 9 micra. Assuming the data 

 in table 18 as approximately correct for Verbena ciliata, the ratios between 

 the maximum and minimum diffusive capacities for stomata with these 

 measurements are presented in table 19. 



Table 19. — Ratios between transpiration rates and diffusion capacities of stomata compared. 



An observed increase in the rate from 54 to 133 units (*". e., in the ratio 

 2.46) can not be explained by an increase in the average transverse diameter 

 of the stomata from 3.5 to 4.5 micra (i. e., in the ratio 1.28); or diffusive 

 capacity ratio i.2j±. The increased rate from 296 to 304 units is, of course, 

 not connected with the static condition. And, while the stomata decreased 

 from 6 to 2.5 micra, the transpiration rate increased somewhat (experiment 

 140, i2 h 38 m to 4 h 2o m p. m.). 



