56 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



it seems that the additional light was not sufficient to cause further 

 opening in either plant, and that the amount of opening found was 

 caused by moonlight alone. 



Another series was made August 23, 1919, to find the effect of 

 such illumination on a dark night. A plot of alfalfa and one of cow- 

 beet were illuminated from 10 p. m. to midnight and strips collected 

 every 30 minutes. The stomata of alfalfa were closed at the start 

 and remained closed until ll h 30 m p. m. in the dark plot. In the 

 illuminated plot the upper stomata showed slight opening at 1 1 p. m. 

 and by 12 had opened to 25 per cent. Night opening also occurred 

 in the dark plot, but was only 5 per cent at midnight. No opening 

 occurred in the lower stomata of either plot. In the cow-beet, the 

 stomata were 40 per cent open at the start of the experiment and 

 closed in the unilluminated plants to 25 per cent at midnight. They 

 closed slightly in the upper surface of the plant in the lighted plot 

 as well, since they were 35 per cent open at midnight. The lower 

 stomata of the lighted plants were like those of the unlighted ones. 

 Three hours later, from 3 to 4 h 30 m a. m. August 24, the experiment 

 was repeated on plots that had not been used previously. The 

 upper stomata of alfalfa were closed and remained closed in the dark 

 plot, while they opened 30 per cent in 90 minutes in the light plants, 

 the first slight appearance of opening being produced in half an hour. 

 The upper stomata of cow-beet were 20 per cent open at the outset 

 and closed to 15 per cent in the unlighted plants, but opened to 50 

 per cent in 90 minutes in the plants illuminated. 



These results indicate that night illumination is effective on a dark 

 night, but not on a night which is brightly lighted by the moon. 

 The additional light was not sufficient in the earlier experiment to 

 produce increased opening. The time of night likewise plays an 

 important part in the effectiveness of illumination, since in alfalfa 

 only 25 per cent opening occurred in 2 hours during the earlier part 

 of the night, while 30 per cent was produced in an hour and a half 

 toward morning. In cow-beet no opening occurred in the first part 

 of the night, but in the later part rapid opening was found. This 

 may be due either to fatigue or more probably to the fact that storing 

 of starch had to be reversed during the first part of the night, but 

 was practically complete and everything ready for the reverse process 

 during the later part of the night. 



TEMPERATURE. 



Temperature is manifest to a plant in three forms, radiant energy, 

 air-temperature, and soil-temperature. As radiant energy it tends to 

 raise the temperature of the leaf above that of the surrounding air, 

 and this must be met by the cooling action of transpiration. As air- 

 temperature, it affects the evaporating power of the air, and deter- 



