22 THE DAILY MARCH OF STOMATAL MOVEMENT. 



Mid-day closure had now come to be expected, but the speed with 

 which the stomata closed and opened was startling, as none of the 

 preceding series or my other investigations had given any hint of it. 

 Another surprise was the night opening at 1 and 2 a. m. in series 4. If 

 the stomatal mechanism depends upon light alone for its action, 

 neither night opening nor day closure should occur. The only con- 

 clusion possible was that some other factor than light ilso played 

 an important role in the behavior of the stomata. 



Some light is thrown upon this point by series 26, made at the 

 University of Minnesota, greenhouse at Minneapolis, Minnesota. 

 These plants were not growing in the field, but had been trans- 

 planted into pots in the greenhouse while dormant during the early 

 winter. On May 1, 1917, they were moved out of doors to a shel- 

 tered corner. The series was started at 4 a. m. on Miy 5 and con- 

 cluded at 3 a. m. the next day. On the whole, conditions for growth 

 were very good. The humidity was rather high, averaging 41 per 

 cent during the day and 72 per cent during the night. The sunlight 

 was almost normal, but the temperature was at times unfavorable. 

 The stomata began to open before 6 a. m. and continued very slowly 

 and uniformly to the maximum at noon. They remained open until 

 3 p. m. and then closed just as slowly and uniformly, closure being 

 completed between 8 and 9 p. m. There was no night opening (fig. 

 6). Such behavior of the stomata in the upper epiderm of alfalfa is 

 probably typical of the movement under favorable conditions, save 

 in one respect. The low morning temper itures had an inhibiting 

 effect upon the stom ita, and caused the very gradual opening which 

 occurred in the forenoon. The absence of both day closure and night 

 opening seemed to indicate that the two were related in some m inner. 

 The following series strengthens this hypothesis. 



Series 10, beginning June 8, 1916, at 4 i. m. and ending at 4 a. m. 

 June 9, wis the first 24-hour series made. It included, among others, 

 sets of upper and lower epiderm from the leaves of alfalfa and also 

 the stem epiderm. The day was cle ir and warm, the sunlight nearly 

 normal, and the temperature 88 F. at 2 p. m. The relative humidity 

 reached 74 per cent at night and dropped to 13 per cent in the 

 afternoon, an average of 65 per cent for most of the night and 20 

 per cent for the greater part of the day. Conditions were typical 

 of the usual clear warm summery day in the region (fig. 7). At 

 the start of the series the upper stomata were 20 per cent open 

 and the lower 15 per cent. At 5 i. m. the upper had opened to 90 

 per cent and the lower to 60 per cent. At 6 a. m. the upper were 

 wide open. The lower stomata remained at 60 per cent until 7 a. m., 

 after which they gradually closed, the process being completed by 

 noon. In the meantime, the upper stomata closed to 50 per cent at 

 7 a. m. and to 40 per cent at 8 a. m., and they opened to 60 per cent 



