OPENING AND CLOSING OF STOIVIATES 185 



The work of Sayre (1926), Scarth (1932) and others indicates that the 

 mechanism whereby light brings about stomatal opening and the mechanism 

 whereby its absence causes stomatal closure are primarily osmotic, but are 

 conditioned by changes in the H-ion concentration of the guard cells. Illumi- 

 nation of the guard cells has been found to result in a decrease in their H-ion 

 concentration ; failure of illumination in an increase. Thus Scarth found the 

 pH of the guard cells of the Wandering Jew {Zebrina pendula) to range 

 from 5.0 or less in the dark to between 6.0 and 7.4 in the light. The other 

 cells of the leaf do not change appreciably in H-ion concentration upon the 

 advent or failure of illumination, suggesting that the guard cells are less 

 effectively buffered than the other leaf cells. Although several explanations 

 have been offered to account for the effects of light and darkness on the pH 

 of the guard cells none of them are strongly supported by experimental evi- 

 dence and they will not be discussed. 



The guard cells apparently always contain starch, but the quantity 

 present is not constant from one hour of the day to the next. Sayre (1926) 

 has shown that the starch content of the guard cells is at its maximum dur- 

 ing the night, decreases rapidly during the daylight hours, and increases again 

 towards evening. The sugar content of the guard cells bears a reciprocal 

 relationship to the starch content; when the latter is high, the sugar content 

 is low, and vice versa. These reciprocal changes are apparently the result of 

 reversible reactions in which the total amount of carbohydrate involved does 

 not vary greatly. Conversion of starch to sugar and of sugar to starch results 

 from the action of the complex of enzymes known as "diastase" (Chap. 

 XXVII). Increase in the pH of the guard cells such as occurs upon the 

 incidence of light, appears to favor the hydrolytic (starch to sugar) action of 

 diastase. On the contrary a decrease in their pH, such as occurs in the eve- 

 ning, favors the synthetic action of this enzyme whereby sugar is converted into 

 starch. 



Increase in the sugar concentration of the guard cells results in an increase 

 in their osmotic pressure while a decrease in their sugar concentration has 

 the opposite effect. That such changes in the osmotic pressure of the guard 

 cells actually occur has been shown by many investigations. The diurnal 

 changes in the osmotic pressures at incipient plasmolysis of the guard cells 

 and epidermal cells of the English Ivy (Hedera helix) are shown in Fig. 47. 

 In general the osmotic pressure of the guard cells is usually relatively high 

 during the daylight hours, and relatively low at night. The osmotic pressure 

 of the epidermal cells does not change appreciably during the course of the 

 day and approximates that of the guard cells at night. 



Increase in the osmotic pressure of the guard cells in the morning results 



