io6 THE BIOLOGY OF FLOWERING PLANTS 



starch is formed by day and disappears at night. In the case 

 of the guard cell it is quite easy to see that, if the large 

 store of starch is converted into sugar, an increase in osmotic 

 pressure will occur, giving the conditions for stomatal open- 

 ing. Iljin found, in fact, that the osmotic pressure of the 

 guard cells falls to that of the epidermal cells as the starch 

 increases and the stomata close. Wiggans (1921) determined 

 the osmotic pressure of guard and epidermal cells of several 

 mesophytes, at various times throughout the day. The 

 results for Cyclamen and the beet are given in Table XXV. 



TABLE XXV 

 Osmotic Pressure of Guard and Epidermal Cells 



The difference in behaviour of the two kinds of cell is 

 striking ; the guard cells alone show a regular increase in 

 osmotic pressure, and this is accompanied by stomatal 

 opening. In neither of the plants cited were the stomata 

 found completely closed. From other experiments it seems 

 probable that, with complete closure, the osmotic pressure 

 would fall to that of the epidermal cells, as Iljin found it to 

 do. Fig. II shows the relation between starch content and 

 stomatal opening in the Lombardy poplar. 



There has been difficulty in demonstrating the formation 

 of sugar in the expanded guard cells, though there is little 

 doubt that it is formed. Lloyd saw oil globules appear as 

 the starch disappeared (in Verbena ciliata) ; the function 

 of this oil is unknown. 



That the action of light is not due to increased 



