DAILY MOVEMENT IN CEREALS. 43 



ment by 9 p. m. As with the other cereals examined, the stomata 

 of corn remained closed throughout the night, but opened directly 

 after sunrise. The maximum opening of 42 per cent for the morn- 

 ing occurred at 9 a. m. They remained in this condition until 10 a. m. 

 and then became closed by noon. At 1 p. m. they had again opened 

 and were 80 per cent open at 2 p. m. when the series ended (fig. 25). 



The stomatal movement in corn during this series was remarkable 

 in several respects. One was the much greater opening compared 

 with that of the other cereals, and the comparatively little complete 

 closure during the hours of daylight. The stomata of corn differed 

 from those of alfalfa in opening for a much longer period, but never 

 to a maximum. Another important point is the difference shown 

 on the two successive days of the series. As the maximum opening 

 for the morning usually occurred about 9 a. m., the opening of 50 

 per cent observed at noon, June 19, indicates that the stomata 

 were more widely open some hours previous to the start of the series. 

 At all events, the stomata showed considerable opening during the 

 forenoon of June 19 and but slight opening on the afternoon of that 

 day. The next day this course was reversed, the stomata showing 

 but moderate opening during the forenoon and twice as great open- 

 ing in the early afternoon. This can not be fully explained by the 

 physical factors recorded, but was probably in part the result of 

 wind conditions. 



When conditions become more unfavorable, corn stomata close 

 for increasing periods during the day. Such midday closure is often 

 accompanied by rolling of the leaf and other evidences of wilting. 

 Under extreme conditions the stomata open with the first light of 

 dawn and close shortly after sunrise, to remain closed until the next 

 morning. If conditions are less extreme they stay open until late 

 morning and then close until evening, when they open for a few hours 

 before darkness sets in. Even under these circumstances, leaf-rolling 

 is very noticeable during the early afternoon, but when the stomata 

 are more or less open throughout the day, as in series 11, no trace of 

 rolling occurs. 



A collection of epiderm from potted plants of wheat growing in the 

 greenhouse at the University of Minnesota was made during the 

 course of series 26, on May 5, 1917. Owing to the remains of a 

 coat of whitewash and to soot from the trains passing just below, 

 only 17 per cent of the light outside penetrated through the glass. 

 This light was a diffused sunlight and much more than could pass 

 through a canvas covering, such as used by Gray and Peirce in their 

 experiments, or through a north window, such as used by Darwin. 

 The humidity was high, ranging from 45 per cent to 85 per cent and 

 averaging 70 per cent. The lowest temperature recorded was 55 F. 

 at 4 a. m. and the highest 72 F. at 2 h 30 m p. m. The day being clear 



