38 



THE PHYSIOLOGY OF STOMATA. 



- 300 



repeatedly verified by daily observation for several weeks. During the 

 cooler weather of early spring, the opening appears to take place more 

 slowly (see table 49, p. 100). From 8 a. m. till 12 day or 1 p. m. the maximum 

 opening is maintained, but at about 1 p. m. the closure of the stomata com- 

 mences and proceeds till the minimum average opening of about 1.5 micra 

 is reached at about 5 p. m. The data obtained on two different days are 

 consistent. 



The rate of transpiration commences to increase at about 2 a. m., a phenom- 

 enon of very general occurrence. For the first few hours the increase is slow. 

 After 5 a. m. the rate increases much more rapidly and continues to do so 

 till the maximum is reached, in one case at 10 a. m. and in the other some- 

 what later. After the hour of maximum transpiration is passed the rates 

 fall till midnight or later, but the decrease is interrupted between 1 and 4 

 p. m., during which period the decrease is less rapid, or there may be a slight 



increase. 



Figure 10 shows the curves of 

 transpiration and stomatal move- 

 ment for the period between 4 a. m. 

 and 3 p. m., constructed upon the 

 data of experiment 139, Verbena 

 ciliata. Numerals opposite abscissas 

 on the left are to be applied to the 

 curve of stomatal movement ; on the 

 right to the transpiration curve. 



If, now, we compare the curves 

 constructed to represent the two 

 sets of data, it will be seen that the 

 maximum stomatal dimensions are 

 reached 2 to 3 hours earlier than 

 the maximum transpiration rate. 

 That the rise in transpiration dur- 

 ing the morning is not due in the 

 main to the opening of the sto- 

 mata is indicated by the continued 

 rise in the curve of transpiration 

 after 8 a.m., the hour of maximum 

 stomatal opening. The slight 



200 



— 100 



3a.m 



3 p.m. 



Fig. 10.— Experiment 139. Verbena ciliata. Curves change in the transpiration CUrve of 

 of transpiration (/) and of stomatal movement (s). experiment ^q at this hour might 



be regarded as due to the static condition of the stomata, but there are 

 other facts which deny this. 



Again, a decrease in stomatal dimensions commences, in experiment 140, 

 about 2 hours later than the decrease in transpiration rate, though in experi- 



