94 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



The relationship between the water-loss and stomatal movement 

 of heavily watered plants on the verge of wilting from lack of water, 

 and cut stems, was investigated in alfalfa series 32, begun 7 p. m. 

 on August 25 and ending 7 p. m. August 26, 1919. The night was 

 clear and starlit, no moon showing. During the first part of the 

 evening there was no wind, but one rose toward midnight, char- 

 acterized by short gusts and intervals of calm. Between 12 and 



I a. m. it rose to a maximum of 3,700 feet for the hour, and then 

 slowly died down, disappearing between 3 and 4 a. m. Relative 

 humidity was not high, but rather variable, the mean rising from 38 

 per cent at 7 p. m. to a maximum of 63 per cent at 3 a. m. and again 

 at 7 a. m. The temperature fell from 79 F. at 7 p. m. to 63.5 at 



II p. m., rose again to 70 F. at 1 a. m. and then fell again to 64 F. 

 at 3 and 4 a. m. The next day was slightly hazy and with large 

 drifting clouds, which four times during the day reduced the sun- 

 light. Although the sun rose before 6 a. m., it was not until after 

 7 a. m. that the sunlight reached 10 per cent. The maximum for the 

 day was reached at 2 h 30 m p. m., when the light was 85 per cent. 

 Sunset occurred shortly after the series ended. The maximum tem- 

 perature was 89 F. at 2 p. m. ; hence the day was very warm. Be- 

 tween 8 and 9 a. m. the wind rose suddenly to 12,200 feet for the 

 hour, fell to 9,800 feet the next hour, and then averaged 3,000 feet 

 per hour during the rest of the day, dying away at sunset. During 

 the period the barometric pressure fluctuated from 25.4 inches to 

 25.7 inches from 7 p. m. on the 25th to noon the next day (fig. 49). 



130 



ieo 

 no 

 100 

 90 



80 

 70 

 60 

 50 



40 

 30 

 20 

 10 



D- 



7 8 9 10 II MT I 234- 56789 10 II NOON I Z 3 4 5 6 7 



FIG. 49. Series 32, weather data for August 25-26, 1919; sunlight (A), 

 temperature (B), humidity (C), wind velocity (D). 



The water-loss from heavily watered plants was determined by 

 weighing small plants in metallic containers at 2-hour intervals 

 upon a precision balance, weighing them to the nearest milligram. 

 The total weight of each plant and container was approximately 150 

 grams. The plants stripped were somewhat larger, but of the same 



