64 



EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



at 5 a. m. September 19, and ended at 6 a. m. September 20, 1919. 

 A heavy dew had fallen during the night before and the leaves were 

 wet, the upper surfaces much more than the lower. The humidity 

 was very high during the series, being 98 per cent at the start, de- 

 creasing slowly to a minimum of 49 per cent at 3 h 40 m p. m. and 

 increasing again to 80 per cent or more during the following night. 

 The day was only moderately warm, and, while there were no clouds, 

 the large amount of water-vapor and the time of the year prevented 

 sunlight from approaching maximum. The stomata of both sur- 

 faces were closed at the outset, but the upper opened 10 per cent at 



6 a. m. and 30 per cent the following hour. At this tune the lower 

 stomata, which had been closed, opened 50 per cent. By 8 a. m. 

 the dew had dried on the lower surfaces of the leaves and in response 

 the stomata closed to 20 per cent. They then began to reopen, the 

 maximum occurring at 1 p. m. The stomata of the upper surface 

 continued opening rather uniformly until 9 a. m., when they reached 

 90 per cent. In response to evaporation of the heavy dew upon the 

 surfaces, the stomata closed to 70 per cent the next hour. Maximum 

 opening was reached, however, at 11 a. m. The stomata of both 

 surfaces remained wide open until 5 p. m., when they started to close, 

 the lower closing only slightly more rapidly than the upper. Closure 

 was complete in the lower stomata and nearly so in the upper at 



7 p. m. At this time dew again began to form, and at 8 p. m. was 



X 



\ 



a 9 10 n NOON i 2 3 



5 6 7 8 S IO II MT. I 2 3 A 5 6 



FIG. 34. Series 35, showing stomatal movement in heavily watered plants of 

 alfalfa, the partial closure at 8 and 10 a. m. following disappearance 

 of dew; upper stomata (A), lower stomata (B), sunlight (C), tempera- 

 ture (D), humidity (E). 



distinctly noticeable on the upper surfaces of the leaves. In conse- 

 quence, the upper stomata began to reopen very slowly, reaching 

 25 per cent at 1 a. m. At this time a breeze sprang up, causing closure 

 the next hour, but this died away and the stomata again opened, 

 reaching 20 per cent at 4 a. m. The next hour they closed, and re- 

 mained in this condition until 6 a. m., when the series ended. The 

 lower stomata were closed all night, except for 5 per cent opening 

 at 11 p. m. and midnight, when the dew first affected the lower sur- 

 faces (fig. 34). When the leaves and stems of alfalfa, sweet clover, 



