LIGHT. 55 



The difference in the degree of reduction produced a difference in 

 the time necessary to show an observable response, as well as in the 

 degree of response. In the first half hour only the plants inside the 

 dark cabinet showed a response; at the end of an hour, the plants 

 in the muslin-covered cabinet showed a response as well; at the end 

 of an hour and a half the plants of all the cabinets except the first 

 showed a response. Hence, with light, at any rate, the difference 

 in time between the impact of a factor and the response decreases 

 as the degree of change increases. 



Three days later the cabinets were placed on the plots at 9 h 30 m 

 a. m. and removed at 11 a. m. The stomata of the plants outside 

 were wide open throughout the period. No change of behavior was 

 observed, except in the dark cabinet, in which some closure had be- 

 gun as the experiment ended. It seems evident that the time of 

 day also makes a difference in the response, and that stomatal 

 movement may be hastened more easily than reversed. This was 

 shown with even greater clearness in the experiments with night 

 illumination and in those carried on in April 1917 at the greenhouse of 

 the University of Minnesota. Potted plants of wheat and corn kept 

 38 hours in darkness showed stomatal opening in 40 minutes when 

 exposed to the light of a 40-watt mazda light hung 2 dm. above the 

 plants, while others kept in darkness 3 hours showed no opening 

 after 2 hours of such illumination. 



Potted plants of wheat which had been kept outside were taken 

 inside on the evening of May 4, 1917, and placed near those that had 

 been grown in the greenhouse for series 26. At the start of the series, 

 4 a. m. May 5, all the plants were stripped at hour intervals until 

 9 a. m. as were the plants of the series. The plants brought into the 

 greenhouse showed 5 per cent opening at 6 a. m., as did the plants 

 which had been grown inside. At 7 a. m. the greenhouse plants had 

 stomata half open, while those brought in were but 10 per cent. 

 The next hour the greenhouse plants showed stomata at maximum, 

 and the out-door plants 35 per cent opening. At 9 a. m. both groups 

 of plants had wide-open stomata. It is evident that the stomata of 

 wheat plants accustomed to strong light do not open as readily in 

 diffuse light as those of plants adjusted to such light conditions. 



To find the effect of night illumination upon alfalfa plants growing 

 in the field, a series was made on August 13, 1919, from midnight 

 to 2 a. m. A 250-watt mazda light was suspended 1 meter above a 

 plot of alfalfa and a similar one above a plot of cow-beet. The night 

 was clear and moonlight, the average temperature during the experi- 

 ment being 60 F. No difference in stomatal movement could be 

 detected between the lighted and unlighted plots, the stomata of 

 alfalfa being 40 per cent open throughout the experiment and those 

 of cow-beet remaining 75 per cent open during the 2 hours. Hence, 



