MECHANISM OF STOMATA 



105 



cells, and to maintain its relatively greater turgor. It has 

 been asserted by F. Darwin (1898) that in plants brought 

 into a dry atmosphere the stomata close. Knight (19 17) has, 

 however, shown that mechanical shocks, due to shaking of 

 the leaves, may lead to rapid closure, and this is more likely 

 to have been the cause of the closure observed by Darwin 

 than the change in atmospheric humidity. 



Action of Light. — The external factor which most 

 markedly influences stomatal condition is light. In plants 

 with a sufficient water supply the stomata normally open 

 through the day and close at night, though, as we shall see, 

 other conditions may occur. The action of Hght might be 



A 



B 



Fig. 10. 



-Starch content of guard cells oi Fouquieria splendens ; A, starch 

 abundant ; B, starch almost absent. (After Lloyd.) 



supposed to be due to its effect on photosynthesis. The 

 guard cells contain chlorophyll ; in light they should 

 accumulate carbohydrates with a consequent increase in 

 osmotic pressure, giving the condition favourable to open- 

 ing. This, however, is not the case. Lloyd (1908), 

 Loftfield (1921), and Iljin (191 5) have found that the open- 

 ing in light is accompanied by a great decrease in the amount 

 of starch in the guard cells ; as the stomata close in the dark 

 the starch is reformed (cp. Fig. 10). This is, of course, not 

 the behaviour of the ordinary assimilating cell. Cases have 

 been observed, e.g. by Ursprung (1917), where intense in- 

 solation has led to disappearance of starch ; but, normally, 



