264 The Physiology of Plants BOOK in 



allowing the cells to become flaccid. A further objection 

 to Von Mohl's hypothesis lies in Darwin's observation that 

 the stomata tend to open again in prolonged darkness, 

 and do not close in air containing no carbon dioxide. 



Kohl was the first observer to investigate the action of 

 the different rays of the spectrum on the opening and 

 closing of stomata. He showed in 1895 that the rays 

 absorbed by the chlorophyll, viz. the red and the blue, 

 cause the opening. These observations may be taken to 

 support Von Mohl, but only up to a certain point, for Kohl 

 also failed to identify the substances, the formation of which 

 the theory demands. The activity of the red rays in the 

 process weighs somewhat against the theory of a direct 

 stimulation of the protoplasm by the light, for the most 

 generally efficient rays in stimulation are those of high 

 refrangibility. 



F. Darwin suggested that the nutritive changes which are 

 excited by the light may form a stimulus to the proto- 

 plasm of the guard cells, and so bring about the varying 

 turgescence of opening and closing. Taking this view, 

 the stimulating action of the light is actually indirect. 

 Darwin admitted that he had no positive evidence in 

 favour of the hypothesis, but he held that the nutritive 

 changes are not of sufficient magnitude to act directly. 

 Darwin also found reason to suppose that the closure of 

 stomata accompanying or preceding flaccidity of the leaf 

 may be its response to the stimulation caused by the 

 commencing reduction of turgor. 



An important contribution to our knowledge of the 

 behaviour of the stomata of nyctitropic plants was made 

 by Stahl in 1897. In his paper he showed that the stomata of 

 such plants remain open at night so long as temperature 

 conditions remain favourable. F. Darwin agreed that they 

 are more widely open than those of other plants except 

 those of aquatics, which do not close at night at all. 



