20 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



comparison with total closure found the preceding day. In series 1 

 maximum opening occurred in the forenoon, and the widest opening 

 in the afternoon was 80 per cent, while in series 2 maximum opening 

 occurred during the afternoon and only 85 per cent was reached in 

 the forenoon. The conclusion could not be escaped that stomatal 

 movement was not uniform each day and that weather conditions 

 probably caused great variation in the movement from day to day. 

 It seemed significant that the stomata did not open fully during the 

 cloudy morning of May 9, and not until an hour after the maximum 

 of sunlight had occurred. 



Another series was made of the same plants on May 12 in order to 

 discover whether the mid-day drop in the curve was due to the failure 

 of the stripping method or was an actual occurrence. The day was 



8 9 10 II NOON 123-45 



FIG. 4. Series 3, showing movement in upper 

 stomata of alfalfa (A), sunlight (B), 

 temperature (C). 



one of passing clouds, and a certain amount of haze prevented the 

 light from reaching maximum at any time. A severe frost on the 

 10th had injured the vegetation to some extent, but a preliminary 

 examination showed that the stomata of the alfalfa plants were 

 functioning in spite of the low temperatures. The upper stomata 

 were 40 per cent open when the series started and opened gradually 

 to the morning maximum of 80 per cent at 11 a. m. Again mid-day 

 closure was found, the stomata closing to 70 per cent at noon and to 35 

 per cent at 1 p. m., reaching a minimum of 20 per cent at 2 p. m. At 

 3 p. m. they had opened to 55 per cent and to 65 per cent at 4 p. m., 

 in which condition they remained until the end of the series (fig. 4). 

 At 9 a. m., 11 a. m., 2 p. m., and 4 p. m. a direct examination of the 



