EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 79 



The ratio of 2,056 linear units in sunflower to 1,530 linear units in 

 corn is approximately 4 : 3, while the transpiration per unit surface 

 of sunflower is nearly twice that of corn. Only when the assumption 

 is made that the stomata of both species of plants are open all the 

 time, or are open equally for the same period, can this be taken as 

 evidence that the diffusion capacity of the stomata does not regulate 

 transpiration. Muenscher's plants were under far different condi- 

 tions of environment from those used in this investigation, but a 

 recalculation of results based upon the present findings is of interest. 

 A series containing both Zea mays and Helianthus annuus growing 

 under the same conditions showed the stomata of corn open approx- 

 imately 8 hours and those of sunflower 12 hours. If this was true 

 for Muenscher's plants, disregarding cuticular water-loss, the amount 

 transpired during a 24-hour period must have been lost largely 

 during 12 hours in Helianthus and 8 hours in Zea mays. The hourly 

 water-loss for Helianthus would then be 312 mg. and for Zea mays 

 240 mg. per hour per square decimeter. The ratio of linear units for 

 sunflower and corn is approximately 20 : 15 and the ratio of hourly 

 transpiration recalculated in this manner 312 : 240, both of which 

 reduce approximately to 4 : 3. Correction for cuticular water-loss 

 would undoubtedly make the agreement very close. However, this 

 explanation is only of theoretical interest; the preceding sections 

 have shown the futility of attempting to estimate the stomatal 

 behavior of a plant in an unknown environment. It does show, how- 

 ever, why Muenscher can not be considered to have produced valid 

 evidence to the effect that "the amount of transpiration is not gov- 

 erned entirely by stomatal regulation." 



In spite of the care with which Lloyd (1908) carried out his ex- 

 periments, and the clearness of his analysis of the problem, he can 

 not be said to have proved the case against stomatal regulation with 

 any greater success. He realized that the use of potometers to meas- 

 ure transpiration was the weak part of his investigation, but did 

 not check them sufficiently to ascertain where this weakness lay. 

 His discovery that the amount of water lost from the stem and leaves 

 was not the same as that absorbed possibly caused him to overlook 

 the very important fact that the stomatal movement in the leaves of 

 a cut stem is not at all like that of the leaves of a field plant or even 

 a potted plant. Hence Lloyd assumed inadvertently the very thing 

 he attempted to prove, namely, that stomatal movement had no 

 effect upon transpiration. 



To gain a clearer idea of the reliability of cut stems as a meas- 

 ure of water-loss from rooted plants, an alfalfa series was made on 

 August 8, 1916. A battery of potometers was set up and a pre- 

 liminary experiment carried out on August 5 to determine whether 

 alfalfa stems treated in this manner would check with one another 

 in regard to their transpiration (plate 11). The burette in each case 



