CONCLUSIONS. 133 



lating, and we may further suppose that the quality of the light may become 

 such in a few hours that the activity of the ferment, or the ferment itself, 

 is destroyed. Whatever the cause, accumulation of starch begins at a certain 

 time approximately each day, whether the whole of the starch content is 

 depleted or not. The action of the ferment is not, therefore, so closely related 

 to the presence or absence of starch as it is to some other factor, possibly 

 light. The stimulus of light in the early morning may, however, be directed 

 toward secretion, rather than the activity, of an enzyme already present; 

 and here again the early morning light may affect the protoplasm positively, 

 while the light of midday may be such as to inhibit the secretion of the fer- 

 ment. The continued presence of starch in the night seems to warrant the 

 view that the ferment as such is not present in the night, though the zymogen 

 may be. And the increase of starch in the latter part of the day, even though 

 the plant be in darkness, indicates that the ferment has disappeared by the 

 time the minimum starch content is reached, though it remains possible that 

 the cessation of its activity is due to the accumulation of the products of 

 amylolysis. The activity of the ferment in the stoma seems, then, to be confined 

 to the early part of the day, during which time the starch content of the plas- 

 tids is depleted, and its absence at night strengthens the view that the light 

 of the early morning stimulates the secretion of the ferment. We are, then, 

 led to the conclusion that the stomatal activity of the morning is due to the 

 secretion of an enzyme by the protoplasm in response to the light stimulation, 

 and not to the view that the light, through the chlorophyll, causes the accu- 

 mulation of osmotic substances by photosynthesis. The response differs, 

 however, from the usual phototropic responses in that the protoplasm of the 

 stoma does not differentiate between blue and red light (Francis Darwin, 

 1898, p. 615), and we are drifting toward the "nutritive theory" of stomatal 

 activity, though not wholly in the sense conceived by Kohl. I would hold 

 that the opening of the stoma is due to the stimulation of the protoplasm, 

 or of some part of the protoplasm, by the secretion of an enzyme which digests 

 the starch present. The cessation of this process is followed by a period of 

 stasis, in which there is little change in the dimensions of the stomata. The 

 closing of the stomata during the afternoon is due to the withdrawal of the 

 osmotic substances by reformation of starch (and by outward diffusion) and 

 is therefore passive in part. This reformation of starch may occur in the 

 dark, but if the plant is exposed to light the loss of material may be made up 

 in part by photosynthesis, which may be more active in the stomata of some 

 plants than of others. 



Further study may show that photosynthesis supplements the peculiar 

 processes of the stomata to a greater extent in those having a large amount of 

 chlorophyll, but the fact that they are not directly dependent upon light 

 must be reckoned with. 



