EFFECT OF STOMATAL MOVEMENT UPON TRANSPIRATION. 93 



stems and rooted plants. But in a later paper (1912), he checked 

 the rate of loss against the rate of absorption of water in the cut 

 stem of Fouquiera and at the same time apparently measured the 

 stomatal movement in one of the cut stems. As a result, the stomatal 

 movement illustrated is like that found here to be typical of the 

 movement in cut stems of Fouquiera. Throughout the period 

 shown in his graph the stomata fluctuated between 50 and 90 per 

 cent of maximum, at no time closing to a point where they would 

 closely regulate the water-loss from the potometer. He states, how- 

 ever, that the stomata were 2.4 microns open at midnight preceding the 

 experiment, but evidently the stem had just been removed, since in 

 none of the present series have the stomata of a cut stem of Fouquiera 

 closed to this degree, i. e., 28 per cent. Regarding the failure of the 

 stomata to close during the afternoon, he states: "the behavior dur- 

 ing the latter part of the day can not with certainty be regarded as 

 wholly normal, as it does not accord with my previous results." 



9 IO II NOON 1 2 3 



9 10 II MT I 23*56789 



FIG. 48. Series 29, showing stomatal movement (A) and transpiration 

 (B) in cut stems of Verbena ciliata. 



Knight's evidence against stomatal regulation is largely based upon 

 "relative transpiration" readings as compared with the stomatal 

 aperture index as measured by the porometer. As previously stated, 

 it is to be doubted whether anyone really knows what this index 

 represents, since apparently no one has compared it with direct 

 observations of the stomatal apertures which it is supposed to meas- 

 ure. Moreover, "relative transpiration" is meaningless until some 

 instrument is devised that will represent with some degree of accuracy 

 the effect upon the plant of each factor concerned in evaporation. 

 This objection applies to Trelease and Livingston's investigation 

 (1916) as well. 



