MECHANISM OF STOMATA 109 



the extreme wilted condition, and Darwin confirmed this in 

 some cases, though not in all. Linsbauer (19 17) found the 

 stomata to close in all cases. 



Temperature. — Loftfield has shown that temperature 

 has an important effect on the rate of movement of the 

 stomata. At temperatures of 1° C, 10° C, 20° C, and 

 30° C, the times of opening of the stomata of alfalfa in light 

 were 6 hours, 4 hours, 2 hours, and i hour respectively ; 

 a rise of 10° C. doubles the rate of the movement ; this is 

 what one would expect with the enzymatic conversion of 

 starch into sugar as the fundamental reaction. 



We have noted the effect of mechanical shock in causing 

 closure. 



Rhythm. — The result of the relation to light is that the 

 stoma is normally open through the day. It is possible, 

 however, that the daily opening at the normal hour is 

 partly due to an inherent rhythm. This possibility is 

 suggested by Darwin (1898). Lloyd, using more exact 

 methods, did not obtain conclusive results, though some of 

 his experiments may be taken to support Darwin's view. 



Metabolism o£ Guard Cell. — If the chlorophyll of the 

 guard cell is not directly concerned with the opening of the 

 stomata in light, we may still suppose that, through it, the 

 great amount of starch on which the mechanism depends 

 is built up and maintained ; but this conclusion has not been 

 established. The guard cells of white-margined Pelargo- 

 nium leaves contain plastids, but no chlorophyll. Yet 

 Kiimmler (1922) has shown that they contain abundant 

 starch — rather more than normal guard cells — and that they 

 open in light ; wide opening only occurs if the conditions 

 of water supply are very favourable. From this it would 

 appear that the presence of chlorophyll is not an essential 

 part of the equipment of the guard cell, though it is difficult 

 to believe, in absence of decisive evidence to the contrary, 

 that it does not function, and so assist in the formation of 

 the carbohydrate reserve in ordinary cases. 



The guard cell must be looked on as speciaHsed, not 

 only in its structure, but in its metabolism. In this it is 



