88 EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 



well with those found in the other, especially when the probable 

 effect of sunlight and relative humidity are taken into consideration. 

 The agreement, however, is not so close as in the alfalfa experiment 

 for several reasons, in addition to those already enumerated. Later 

 experiments as well indicate that changes in the degree of stomatal 

 opening are not nearly so effective in producing changes in trans- 

 piration when near the maximum opening as when approaching 

 closure. Moreover, in this as well as in other experiments, the 

 stomata of the two surfaces do not have an equal effect upon the 

 control of transpiration in a leaf having unlike stomata upon the 

 upper and lower surfaces. Hence, the rate of water-loss corresponds 

 more nearly at times to the movement found in the lower surface 

 than to the average stomatal movement. As the ratio of upper 

 stomata to lower was 3 : 20 per unit area in this variety, the effect 

 of the upper stomata upon the calculated average was not large. 

 If, in addition to the use of this ratio, all changes of opening above 

 50 per cent of the maximum had been ignored, the effectiveness of 

 stomatal movement upon transpiration would probably have been 

 shown with greater accuracy (fig. 43). 



The water-loss from noon to 1 p. m. on August 25 was 91.6 mg. 

 per minute. Stomatal opening during this time changed from 100 

 to 91 per cent. A sudden reduction to 15 per cent in the lower 



100 

 90 

 80 

 70 

 60 

 SO 

 40 

 30 

 EO 

 10 



V 



\7 



NOON I 234 5 6 78 9 10 II MT. I Z 34 567 89 10 It NOON I 



Fio. 43. Series 20, showing average stomatal movement (A) and transpiration (B) in cut 



stems of potato. 



stomata at 2 p. m. brought average opening to 26 per cent and de- 

 creased the water-loss to 52.5 mg. The next hour the lower stomata 

 opened to 25 per cent, while the average of opening rose only to 

 28 per cent, since the upper stomata closed 50 per cent, The trans- 

 piration-rate rose to 64.7 mg., which would indicate that the change 

 from maximum to half opening in the upper stomata did not have 

 nearly the effect upon the rate of water-loss that it had upon the 

 calculated average stomatal movement. At 4 p. m. the average 

 dropped to 22.6 per cent, while during the hour transpiration fell 





