74 THE PHYSIOLOGY OF STOMATA. 



INDUCED RHYTHM IN STOMATAL MOVEMENT. 



Darwin favors the conclusion that there is exhibited by stomata an induced 

 rhythm of movement. The evidence cited by him is found in the more 

 rapid closure of stomata when darkened in the morning than in the afternoon, 

 and conversely, the more rapid opening when, after being darkened, they are 

 illuminated in the morning than when the illumination takes place in the 

 afternoon. 



Curtis (1902) supports Darwin (1898), offering in evidence the data obtained 

 in the study of transpiration. The exposure of plants when kept in the dark 

 and subjected to a given period of illumination (this being a constant in the 

 two cases in which electric light was used) in the morning and in the afternoon 

 resulted in a greater response in the morning. This is interpreted by Curtis 

 as an expression of stomatal response, but as the stomata were not examined 

 directly the conclusion seems scarcely justified. The same objection may 

 be urged, too, against Darwin's conclusions, for we may hold that the effect 

 upon the hygroscope was due to a greater response in transpiration, without 

 any change in the condition of the stomata. The evidence may, then, be 

 regarded as evidencing induced rhythm in transpiration and not in stomatal 

 action. Nevertheless, I am inclined, on a priori grounds, to support Darwin's 

 view. The occurrence of rhythm in other processes (I believe that I have 

 shown it to occur in transpiration) would lead us to expect it in the behavior 

 of stomata, and there appears to be no reason why it should not. The 

 evidence here presented bearing on this question is, however, rather more 

 confusing than convincing. 



Table 42. — Verbena ciliata — Transverse diameter measurement 0} stomatal pores. 



r 



I. I 



II. 



III. { 



fjuly 4, 8 a. m., 3 to 8.5 micra; mean 5.7. 

 1 1 a. m., 2 to 3 micra; mean 2 . 5. 

 1 p. m., 2 to 3 micra; mean 2.5. 

 I July 5, 8 a. m., 2 to 5.5 micra; mean 3.7. 



July 10, 8 h 55 m a. m., 5.5 to 10 micra; mean 7.7. 

 io h 30 m a. m., 3 to 5.5 micra; mean 4.2. 

 i h i5 m p. m., 2 to 5 micra; mean 3.5. 

 I July 11, 8 a. m., o to 5.5 micra; mean 2.7. 

 [_ 11 a. m., 2 to 4 micra; mean 3. 



fjuly 11, 8 h 35 m a. m., 2 . 8 to 9 micra; mean 5.9. 

 July 12, 8 h 5o m a. m., o to 2.8 (7) micra, 8 per cent closed; mean 1.4 

 o to 4 micra, 70 per cent closed; mean 2. 

 [.July 13, 8 a. m., o to 2 micra, 90 per cent closed; mean 1 — . 



I was led to look for the occurrence of periodicity in stomatal movement 

 upon finding a maximum in the transpiration curve at an early hour in the 

 morning. The regularity of its occurrence in total darkness and with other 

 conditions constant can only with difficulty be explained in any other way 



