1919] CLAYTON— FUMIGATION 491 



Exposure 5:30 to 7:30 p.m. 



Tomatoes Slightly injured 



Tradescantia 

 Leaves under surface coated . . Uninjured (stomatal surface closed) 

 Leaves upper surface coated . . Uninjured (stomatal surface open) 

 Leaves untreated. Slight injury (stomatal surface open) 



Average stomatal opening 5:30 p.m., almost all stomata were closed. 



Exposure 11:30 p.m. to i 130 a.m. 



Tomatoes Occasional slight injury 



Tradescantia 



Leaves under surface coated. .No injury (stomatal surface closed) 

 Leaves upper surface coated . . No injury (stomatal surface open) 

 Leaves untreated No injury (stomatal surface open) 



Average stomatal opening, none. 



It was evident that the closing of the stomata greatly increased 

 the resistance to fumigation. Approximately speaking, if 100 

 per cent were to represent the injury at 1:30 P.M., by 5:30 P.M. it 

 had dropped to 10 per cent, and by 11 : 30 to 2 per cent. Why the 

 plants were most resistant at the very late period was not entirely 

 apparent, the stomata at 5 : 30 p.m. being, with very few exceptions, 

 as tightly closed as at 1 1 : 30 p.m. Miss Eckerson, however, has 

 found that stomata in certain portions of some leaves lag behind 

 as to closing time. During sunny weather there are great variations 

 of light and temperature in the course of a day and night. This is 

 in contrast with conditions during dark weather. In a sunny 

 period, however, considerable work was done between 6:30 P.M. 

 and midnight. 



It was found that stomatal activity, as indicated by injury to 

 the hypostomatous Tradescantia, continued on a gradually diminish- 

 ing scale till 8 : 45 p.m. (about 2 hours after sundown). The stomata 

 at this time were found to be almost closed. A noticeable fact was 

 that at the later periods the correspondence between the amount of 

 stomatal opening and the injury was not absolute. The injury at 

 8 : 00 p.m. was not so great as would have been expected from the 

 size of the openings. In sunny weather, as in dark, there was a 

 gradual increase in resistance of the plants during the night. The 

 maximum of resistance was reached during the period between 

 1 1 : 30 P.M. and i : 00 a.m. 



