40 



THE DAILY MARCH OF STOMATAL MOVEMENT. 



9 10 II NOON I a 3 4 5 



Fio. 21. Series 2, showing movement in lower 

 stomata of oats (A) and of barley (B). 



opening and complete closure at 11 a. m., when the stomata of 

 alfalfa reached maximum. The stomata of barley remained closed 

 to the end of the series. The 12 per cent opening at the start of the 

 series is the average of 200 stomata, several of which were at max- 

 imum and a great many closed. It is not definitely known whether 

 a few stomata with more accessible water-supply do the opening on 

 days of unfavorable conditions, or whether groups of stomata open and 

 shut very rapidly and at different times. Direct observations on the 



same leaf would indicate the 

 former, but the fact that open 

 and closed stomata occur in 

 groups, and that the stomata of 

 cereals can open and close with 

 amazing rapidity, makes the 

 latter hypothesis possible. 



Oats and barley epiderms 

 were collected in series 2 as 

 well as that of alfalfa. The weather data of this series are shown 

 in figure 3. Because of the cool, cloudy forenoon, the barley stomata 

 were 17 per cent open at 9 a. m. and 10 per cent the following 



hour. As in the first series 

 made the day before, they 

 closed before 11 a. m. and 

 remained closed to the end 

 of the series. The stomata 

 of oats did not open as 

 widely as those of barley, 

 but remained open longer. 

 They were 10 per cent open 

 at 9 a. m. and remained in 

 this condition an hour. At 

 1 1 a. m. they closed to 4 per 

 cent and were completely 

 closed by noon. Like the 

 stomata of barley, they re- 

 mained closed to the end of 

 the series (fig. 21) . The dif- 

 ference in the weather con- 

 ditions in the two series, 



100 



Fio. 22. Series 3, showing movement in lower stomata 

 of oats (A) and of barley (B), and upper stomata of 

 alfalfa (C); sunlight (D), temperature (E). 



in 



although made on succes- 

 sive days, caused the stomata of barley as well as those of alfalfa to 

 act differently. The same plants were included in series 3, made 3 

 days later, May 12, 1916. It was a day of passing clouds, and was 

 distinctly cool, as the temperature did not rise above 63 F. Barley 

 had the same type of stomatal movement as in the first series. Oats 



