A Self-Recording Porometer and Potometer. 353 
A SELF-RECORDING POROMETER AND POTOMETER. 
By W. Neilson Jones, M.A. 
[With Seven Figures in the Text.] 
I.—A Self-Recording Porometer (Figs. 1-6). 
f |HE researches of Darwin 1 and of Lloyd 2 have emphasised the 
fact that variations, in the rate of transpiration do not 
necessarily show so simple a relation with the size of stomatal 
aperture as was formally supposed. 
It is evident that if a knowledge of the size of stomatal aperture 
under varying conditions is required, determinations must be made 
directly without reference to transpiration rates. 
For this purpose two methods are in general use—(1) the 
alcohol method of Lloyd, (2) the porometer method of Darwin. 
The former has an advantage in that a direct measurement is made 
of the size of the stomatal openings; on the other hand, if successive 
readings are required, the porometer method of Darwin is to be 
preferred in that the same group of stomata remains under obser¬ 
vation during the experiment. The estimation of stomatal aperture 
in this case is indirect—the unit of measurement being the time 
taken for a given volume of air to pass through the stomata on a 
given area of leaf-surface under a given average pressure. Since 
the results are usually required for comparative purposes, the 
indirectness of this method is not a serious disadvantage. 
A more serious drawback to the use of the porometer as 
described by Darwin lies in the fact that each observation must be 
made and recorded by the worker—a procedure which in a lengthy 
experiment becomes very tedious. To overcome this drawback 
Balls 3 devised a self-recording instrument of very compact form and 
of great convenience when it is desirable to make the observations 
some distance from the laboratory. Indeed, the only disadvantage 
of this instrument—which he has called the Stomatograph—is its 
cost, which renders it almost prohibitive for mostgeneral laboratories. 
The self-recording instrument about to be described requires 
only a small outlay to supplement the ordinary equipment of a 
physiological laboratory, and it is hoped may be of value where 
1 Darwin, F., and Pertz, D. J. M. “On a New Method of Estimating the 
Aperture of Stomata.” Proc. Roy. Soc., B, Vol. 84, 1912, pp. 136-154. 
3 Lloyd, F. E. “The Physiology of Stomata.” Carnegie Inst., Wash., 
Publ. 82, 1908. 
3 Balls, W. L. “ The Stomatograph.” Proc. Roy. Soc., B, Vol. 85, 1913, 
pp. 33-44. 
