100 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



per minute. Further opening to 25 per cent at 1 p. m. resulted in 

 an increase of water-loss to 88.33 mg. per minute. At 2 p. m. the 

 stomata closed to 7 per cent and the rate fell to 66.67 mg. per minute. 

 At 3 p. m. reopening to 17 per cent was accompanied by a rise in the 

 rate to 72.73 mg. per minute. From this time to the end of the 

 series the stomata closed slowly and the rate of water-loss fell in 

 proportion (fig. 53). 



NOON I 2 3 4 5 4 



FIG. 53. Series 33, showing average movement of upper and lower stomata 

 (A) and transpiration (B) of cow-beets in dry pots. 



In this series, as well as in the alfalfa, Fouquiera, and Verbena 

 series, direct evidence was found that as the stomatal movement 

 of cut stems and rooted plants differed considerably, so did their 

 rates of water-loss. In this series the leaves in the potometers 

 showed little or no wilting, but their stomata were closed or nearly 

 closed throughout the experiment. At the start the stomata were 

 completely closed and opened to 7 per cent at 7 p. m. At 8 they 

 closed to 1 per cent and at 9 were completely closed. Thus a com- 

 parison shows that the stomatal movement of the cut leaves and 

 the plants in the dry pots was identical at 8 and 9 p. m. and differed 

 not at all in the rates of water-loss at this time. At 1 and 2 a. m. 

 the stomata of the potted plants were practically closed, while those 

 of the cut stems were 2 per cent open; hence at this time the rate 

 of water-loss for the cut leaves was slightly higher than that of the 

 plants. The stomata of the potted plants closed again at 5 and 6 

 a. m., while the cut leaves showed 4 per cent opening. At this time 

 the transpiration-rate of the potted plants again fell to less than that 

 of the cut leaves. At all other times the transpiration-rate of the 

 cut stems was lower and during the day considerably lower than 

 that of the potted plants. This must be attributed to the closed 

 stomata during the day in the cut leaves (fig. 54). 



The experiments presented show that stomata regulate the water- 

 loss from plants and contradict the results of Lloyd (1908). The 

 reasons for this lack of agreement have been demonstrated, and 



