On the Use of the Porometer in Stomatal Investigation. 
V • 
BY 
R. C. KNIGHT, B.Sc., D.I.C. 
With seven Figures in the Text. 
I N the course of some investigations upon transpiration it was found 
necessary to record the changes taking place, under varying conditions, in 
the size of the stomatal pores of the plants under observation. 
The methods which have hitherto been employed for this purpose are 
various and widely divergent in principle. Some of the earlier observers 
assumed that the amount of transpiration from a plant was an index of the 
size of the stomatal apertures, and accordingly they expressed stomatal 
changes in terms of transpiration, as measured by experiments with the horn 
hygroscope (Darwin) (1) and cobalt chloride (Stahl) (2). Such methods 
must be rejected as untrustworthy in the absence of evidence in support of 
the assumption underlying them. 
At the other extreme is the direct method used by Lloyd (for Gossy- 
pium ) (3), in which the stomatal apertures are measured in situ in the living 
leaf by means of a microscope, a micrometer scale, and special illumination. 
The two methods now most commonly used are : 
(i) Lloyd’s earlier alcohol method (4) and 
( 2 ) the indirect porometer method of Darwin and Pertz (5). 
The relative values of these two methods have been discussed by 
the two latter authors (loc. cit.), the chief points considered being the 
advantage in Lloyd’s method of taking observations over many different 
parts of the plant, and the advantages in the porometer of automatically 
averaging many stomata and of dealing with the same stomata through- 
out the experiment. In addition, the manipulation of the porometer is 
very simple, while, as will be readily admitted by any one who has 
attempted it, the accurate measurement of stomatal aperture under 
a microscope is very difficult and would seem to be liable to serious 
error. 
In the light of these considerations, therefore, the porometer method 
was adopted for the work in question, but at the same time it was 
realized from an a priori analysis of the conditions of the experiment, 
that there was ample opportunity for errors to creep in unless suitable 
[Annals of Botany, Vol. XXX. No. CXVII. January, 1916.] 
