8o 



THE PHYSIOLOGY OF STOMATA. 



The very obvious difficulty which intervenes to prevent the drawing of any 

 safe conclusion is the variation in the size of the stomata, and the consequent 

 large error in measurement as compared with the amount of movement which 

 we may expect. Another objection which obtains in my own as well as in 

 Darwin's experiments is the unequal stimulus applied. To be certain of the 

 data, therefore, a large number of stomata must be accurately measured, and 

 the changes in the condition of these stomata must be induced by a known 

 constant stimulus. This is, then, a subject which awaits further inquiry, 



300 



200 



100 



a.m. 



p.m. 



FIG. 30. Diagram illustrating stomatal responses when placed 

 in total darkness for given periods in the morning and in 

 the afternoon. A particular species is represented by one 

 kind of line. 



which, however, may be expected, by means of a refinement of the method 

 here used, to afford an answer to the problem. The method of the hygro- 

 scope, on the other hand, certainly can not be relied upon to give us safe 

 evidence, since the possible variation in rate of transpiration, with a given 

 stomatal opening somewhere between the extreme limits is so great that 

 the rhythmic movements of the stomata, if present, might not be detected. 

 Darwin's results, interpreted to mean an induced rhythm in transpiration, 

 receive the support of Curtis's work; but neither of these observers may 

 interpret their data as indicating stomatal behavior. 



