1911.] New Method of Estimating the Aperture of Stomata. 141 



§ 4. Stomatal Aperture in Relation to Transpiration. 

 The object of the present paper is to give an account of the new method, 

 and to illustrate its applicability to some of the problems of stomatal 

 movement. Nevertheless, we propose to give a single instance of the 

 resemblance which exists between variations in the transpiration rate, on 

 the one hand, and the variation in the size of the stomata as indicated by the 

 porometer. This resemblance, which forms the subject of a future paper, 

 can only be established by the average result of a considerable number of 

 experiments,* partly, no doubt, because of the irregularity in the stomatal 

 behaviour of individual leaves, but probably for other reasons as well. The 

 following experiment is merely intended to show that the correspondence 

 between the two curves may be fairly close. But variations from this degree 

 of parallelism are both common and great. 



The curve S in fig. 3 is constructed not from the rate of air-flow through 

 the leaf, but from the square root of the rate (-v/E). Our experience is that 

 a curve so constructed follows the transpiration curve more closely than 

 curves built from any other function of the rate of flow. 



We hope elsewhere to give an account of the theoretical considerations 

 bearing on the problem, and for these we are indebted to the kindness of 

 Sir Joseph Larmor. It is not certain that we shall ever be able to deduce 

 the size of the stomata from the readings of the porometer ; but it seems well 

 to make use of the square root of the air-flow in recording our results, and 

 to do so even in the experiments in which the relation between stomatal 

 aperture and transpiration is not directly investigated. And this rule has 

 been generally followed. 



Experiment 75 ; October 10, 1910. — Laurel, P. laiorocerasus. Temp. 15-16°. 

 i/r (i.e., psychrometer), 70-79 per cent. 



Branch of laurel having 17 of the current year's leaves, cut about 9.30 A.M., 

 surface of stem vaselined and fitted to a postometer and a porometer. 



The plant was exposed to bright diffused light when not in the dark room. 



The postometer readings have been roughly corrected to a constant degree 

 of relative humidity. 



* In our judgment the experiments already made, but not published, do establish a 

 relation between stomatal aperture and transpiration. 



