24 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



gradually to 60 per cent at 10 a. m. and for a time again showed 

 the same movement as did the upper stomata. At 3 p. m., however, 

 they continued to open, reaching 50 per cent at 4 p. m., while the 

 upper stomata remained at 30 per cent. From this time they closed 

 slowly and uniformly, coinciding at 8 p. m. with the 10 per cent 

 opening found in the upper stomata (fig. 8). From this time to the 

 end of the series there was no appreciable difference between the 

 upper and the stem stomata. 



\\ 



5 6 7 8 9 IO II NOON 



34 567 89 10 II MT I Z 3 * 



Fio. 8. Series 10, showing movement of lower (A), upper (B), and stem 

 stomata (C) of alfalfa. 



An examination of the previous series showed that the movements 

 of the stomata in the upper and lower epiderms of the same leaf did 

 not necessarily have any relation to each other. In series 10 the 

 general differences in behavior are quite clearly shown. The stomata 

 in the lower surface neither opened as widely nor stayed open as 

 long as the stomata of the upper epiderm. One reason for this is that 

 the lower surface of the leaf receives much less light than the upper, 

 but it seems largely to be a matter of water-supply to the guard-cells. 

 The slower opening in the morning and earlier closing in the evening 

 typical of the lower surface are due to the difference in light, but this 

 is not true of the prolonged mid-day closure and slight afternoon 

 opening in the lower stomata. Experiments made by turning the 

 leaf over and keeping it upside down during the day seem to bear 

 this out, but have not been conclusive at all times. In barley and 

 other plants with leaves that may have either side up, the sto-nata 

 on both surfaces behaved alike for the most part. In the cases where 

 they did not, the difference was clearly due to difference in lighting, 

 and had no relation to the side on which the stomata were found. 



Another point of importance was the behavior of individual stomata. 

 Direct observation showed that individual stomata may become 

 functionless from one cause or another and remain permanently 

 closed. Some seemed to have the ability to open much more widely 

 than others. Moreover, those that opened very widely also opened 

 earlier, reached maximum opening sooner than the others, and re- 



