130 THE PHYSIOLOGY OF STOMATA. 



the opening in blue light ; but in the event that the amount of chlorophyll is 

 small we may justifiably doubt that the substances which effect the opening 

 are produced in great enough quantities by photosynthesis alone. The ques- 

 tion opens a distinct and well-defined field for research which should enable 

 us to get exact information leading to the solution of the problem at large. 



The dissolution of the starch contained within the plastids of the guard- 

 cells, in regard to which there can be no doubt, introduces another phase of 

 the problem that, namely, as to the cause of this dissolution and of the con- 

 ditions under which it may proceed. The question concerns the intimate 

 physiology of the stoma, and is one upon which we have at present far too 

 little light to enable us to do more than to formulate a working theory. The 

 only contribution in this direction is that of F. G. Kohl (1895), whose argu- 

 ment is as follows: Recognizing that stomatal movements are caused by 

 changes in turgor, he points out that the direct cause of these changes remained 

 till that moment undiscovered. The fact that the guard-cells always con- 

 tain an abundance of starch indicates that this plays an important role. The 

 ever-present chlorophyll bodies produce in the course of their development a 

 continually increasing amount of starch,* which is quickly changed under 

 appropriate stimuli into powerfully osmotic substances. That these are 

 sugars, or organic acids, failed of proof, because the former can not be demon- 

 strated to be present in the guard-cells, and the latter are not more abundant 

 in them than in the epidermal cells. That sugar takes a part may perhaps 

 be inferred from the opening of stomata on being treated with a solution 

 of diastase. 



The change of starch into the problematical osmotic substances takes place 

 for the most part in consequence of the effect of light. Kohl now asks what 

 conditions favor stomatal opening. He shows that a rise in temperature 

 will do so while relative humidity does not. Also the dark heat rays, and 

 light deprived of these, cause opening, f Of the visible spectrum, he found 

 that the effective light is that which lies between B and C and between F and 

 the beginning of the violet. In the yellow, green, violet, and ultra-violet, 

 no movement whatever occurred. This behavior brings the stomatal move- 

 ment and the maxima of photosynthesis into coincidence. 



The failure of the stomata to open in the region of the violet is referred 

 to the destructive effect of this light upon diastase, as held by J. R. Green 

 (1897) to be the case, while the red light, especially, has the contrary effect. 



* Not all of this is, however, the product of photosynthesis within the guard-cell, as above 

 shown. 



fPfeffer, however, had previously shown that the dark heat rays are not "assimilatory" 

 and, therefore, can not effect the opening of stomata by their being used in this way by the 

 chlorophyll (Kohl, 1885, p. 77). The effectiveness of the dark rays may, however, be 

 related to their effect upon the enzyme. 



