34 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



cent opening at 5 a. m., when the upper had reached the maximum. 

 The next hour the lower opened swiftly to 60 per cent and then 

 gradually to 90 per cent at 10 a. m., which was the forenoon maxi- 

 mum of these stomata. The upper continued wide open until 11 

 a. m. and then closed to 90 per cent at noon, when the series ended. 

 The lower started to close after 10 a. m., reaching 65 per cent at 

 11 a. m. and 35 per cent at noon (fig. 17). 



The lower stomata opened 2 hours later in the morning than did 

 the upper, and closed 2 hours earlier in the evening. The smaller 

 amount of light received by them affords ready explanation of this. 

 However, the greater mid-day closure and much greater opening 

 found in these stomata at night can not be explained on the basis. 



JUNE 19- 



JUNE 20 



II 12 IRM 2 34. 56709*0 M IZ IAM2 3 4 5 6 7 8 9 10 II 12 



Fio. 16. Series 11, weather data for June 20-21, 1916; sunlight (A), humidity (B), 



temperature (C). 



II NOON I Z 3 * 5 7 6 9 K> II MT. I 23436 768 10 II NOON 



Fio. 17. Series 11, showing movement in upper (A) and lower (B) stomata of 



sugar-beet. 



of light intensity, in spite of the fact that this is the greatest differ- 

 ence in the environment of the two surfaces. In fact, the alfalfa 

 series would indicate that the upper stomata should show greater 

 night opening and day closure than the lower. On the other hand, 

 the shaded soil in the alfalfa plots did not give rise to convection 

 currents of great evaporating power striking the lower surfaces of 

 the leaves, such as rose from the hot, bare surface of the cultivated 

 soil of the sugar-beet field. 



