MECHANISM OF STOMATA 



107 



assimilation is further shown by the fact that opening takes 

 place under bhie glass screens which greatly lower the rate 

 of assimilation, and also in light in an atmosphere free from 

 carbon dioxide in which no assimilation takes place. Lloyd 

 and Loftfield suppose that light activates an enzyme of the 

 nature of diastase, which then converts the starch into sugar. 

 Recently Sayre (1923) claims to have shown that light acts 

 by changing the hydrogen ion concentration, and thus 

 favouring enzymatic conversion of starch into sugar ; in the 

 dark a reverse change in the acidity reverses the reaction. 



100 



Fig. II. — Starch content and stomatal opening; the heavy line 

 shows the change in per cent, opening of the stomata. of the Lombardy 

 poplar through the day, the broken Hne shows the changes in starch 

 content. (After Loftfield, modified.) 



Experimental alterations in hydrogen ion concentration 

 lead to the same result. Sayre also found that decrease 

 in water content on wilting led to a change in hydrogen ion 

 concentration, and to an increase in starch. How far 

 closure on wilting is due to such changes rather than 

 directly to loss of water remains to be investigated. 



Water Content o£ Lea£. — The behaviour of the stoma as 

 the leaf wilts is not simple ; there are many conflicting 

 statements on this point. Knight (1917) has, however, 

 shown conclusively that in the initial stages of wilting, 

 when the leaf is losing more water than is supplied to it, the 



