36 



THE DAILY MAECH OF STOMATAL MOVEMENT. 



may be attributed to a less ready supply of water to their guard-cells. 

 Reversing the leaves of alfalfa did not cause their stomata to show 

 the results found in sugar-beet, but the stomata of each surface still 

 behaved differently from those of the other surface of normal leaves. 

 Earlier opening and later closing of the lower stomata were often 

 induced by reversing the leaves, and at tunes greater midday closure 

 was brought about. 



It is thus evident that the difference in stomatal behavior of the 

 upper and lower epiderms of alfalfa is due to differences in the struc- 

 ture of the leaf, and, in particular, of the stomata and surrounding 

 epidermal cells. The position of the veins and the number of cells 

 through which the leaf-water must pass before reaching the guard- 

 cells of the stomata of each surface must have considerable influence 

 upon the functioning of the stomata, at least under conditions of 

 high evaporation. The air-spaces in the spongy chlorenchyma and 

 fewer paths or bridges of cells through these air-spaces from the 

 veins to the epidermal cells must play a part in the rate at which 

 water passes to the lower epiderm and in the loss by evaporation 



IO II NOON I 3456769 10 II MT. I 



34S6789IOII 



Fio. 19. Series 16, showing effect on movement of reversing leaves of sugar-beet; 

 normal leaves, lower stomata (A), upper stomata (B); reversed 

 leaves, lower stomata (C), upper stomata (A), as in lower stomata 

 of normal leaves. 



during its passage. As leaf sections show, the water on its way to 

 lower and upper epiderm guard-cells passes through the same num- 

 ber of cells in sugar-beet, and the air-spaces in the sponge tissue are 

 not nearly so large or numerous as in alfalfa (plates 5 and 6). 



No series was made of sugar-beet under conditions where normal 

 stomatal behavior could be expected, nor was this found. However, 

 it can not be doubted that since the general behavior of the stomata 

 of this species resembles that found in alfalfa, sugar-beet stomata 

 would show essentially the same behavior under conditions of low 

 evaporation, medium temperature, bright sunshine, and high water- 

 content, as found in alfalfa, series 26 (fig. 6). The type of curve 

 shown in each series is like the type found in alfalfa of the same 

 series, in that the stomata respond to changes of physical factors in 



