EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 89 



to 55.4 mg. per minute. Closure to 11 per cent was accompanied 

 by a fall in the rate to 35.9 mg. per minute. The stomata then opened 

 to 20 per cent in both surfaces, and the rate rose to 47.8 mg. Closure 

 to 12 per cent at 7 caused a falling off to 36.4 mg. From this time 

 there was a slow closure of stomata and a very slow falling off in the 

 transpiration-rate. A slight increase in opening from 2.6 per cent 

 to 4.3 per cent at midnight probably caused the slight increase in 

 the rate of water-loss from 8.5 mg. to 8.8 mg. at midnight. At 5 

 a. m. all the stomata were closed, and the minimum rate of 7.2 mg. 

 was recorded at that hour. At 7 a. m. the stomata opened 3 per cent 

 and the rate rose to 12.6 mg., due in part to the decrease in relative 

 humidity. At 8 a. m. average opening increased to 5 per cent and 

 the rate increased to 15 mg. At 9 a. m. the average increased to 7 

 per cent, the maximum for the morning, while the rate increased 

 to 20.1 mg. per minute. Between 9 and 10 a. m. the stomata closed 

 from 7 to 3.5 per cent, but because of the rapid fall in humidity and 

 of increased sunlight, the rate rose slightly, reaching 21.1 mg. per 

 minute. Reduction to 3 per cent average opening at 11 a. m. brought 

 on a slight decrease of water-loss to 21.0 mg. The stomata remained 

 in this condition another hour, but the rapid fall of relative humidity 

 caused an increase in the rate to 23.9 mg. Closure of the stomata at 

 1 p. m. brought on a reduction in the rate to 21.7 mg. per minute 

 (fig. 43). 



The two series described show that the potometer was useless as 

 an indicator of the water-loss from a field plant, or even a potted 

 plant of alfalfa or potato. One series with apple and peach twigs 

 in the potometers indicated that, with the variety of apple used the 

 water-loss from a twig may represent fairly well the transpiration of 

 the tree, since the stomata of the cut twigs and of the tree cor- 

 responded to some extent; but even in this case there was some 

 divergence. The great lack of correlation between transpiration and 

 stomatal movement in Verbena ciliata and Fouquiera spkndens as 

 found by Lloyd supported the view that no more correlation existed 

 between the stomatal movement of cut stems and rooted plants in 

 the case of these plants than in alfalfa or potato. Hence, when 

 the opportunity arose to test this view directly, a series was made with 

 these plants. 



This series, carried out at the Desert Laboratory, Tucson, Arizona, 

 was begun at 9 a. m. on March 15, 1918, and ended 9 a. m. the next 

 day. It was conducted in the same manner as the alfalfa series, 

 the water-loss from potometers being measured at hour intervals, 

 and sets of epiderm collected from cut stems, and watered and 

 un watered field plants. The day was fairly warm, clear, and very 

 dry. The humidity varied from 12 per cent at 3 p. m. to a maximum 

 of 47 per cent at 4 a. m. The temperature varied from 88 F. at noon 

 to 50.5 F. at 4 h 30 m a. m. The sunlight curve was rather regular, 



