58 
Knight . — On the Use of the 
precautions were taken to obviate them. Accordingly, some preliminary 
experiments were carried out with a view to determining these possible 
errors and the steps to be taken to avoid them. The results of these 
investigations are embodied in the present paper. 
The majority of the experiments here quoted were carried out with 
the form of porometer described in a previous paper (6). The conclu- 
sions arrived at fall naturally into three main groups, and it is with these 
in view that the experiments have been classified, although many of them 
might be included in more than one group. 
Changes in Stomata. 
In dealing with such admittedly sensitive organs as stomata, it 
seemed possible that rough treatment might easily cause changes in the size 
of the pore, and some experiments were therefore carried out to determine 
whether the conditions of the porometer experiment affected the stomata. 
The main source of possible error appeared to be changes in stomatal size 
resulting from : 
(1) the mechanical strain on the leaf due to the reduced pressure ; 
(2) the passage of the air-stream through the leaf ; and 
(3) the effect of shock from the manipulation necessary for fixing the 
leaf-chamber. 
(1) One aspect of the question of mechanical strain has already been 
discussed in a previous paper, where it was pointed out that even a small 
pressure difference will cause a curvature of the portion of the leaf under 
observation, and this effect cannot therefore be neglected. Although 
there is no direct evidence on the question, it is possible that the stomatal 
pores in general will be affected by a change of curvature of the leaf. The 
acceleration (or retardation) of the air-current through the leaf, caused 
by this stomatal change, will obviously be incorporated in the porometer 
readings, and will constitute a regularly occurring error. If, however, the 
pressure difference employed to produce the air-current is maintained 
constant throughout an experiment, the leaf-curvature, and consequently the 
error resulting, will also be constant. Since this error will have a different 
value for every plant, perhaps for every leaf, and for each different pressure, 
it seems hopeless to attempt to determine it. 
As Darwin pointed out, the porometer gives comparative measurements 
of the direction and relative amount of stomatal changes, but not the 
actual dimensions of the pore ; this being so, a constant error, if small, 
will not detract from the comparative value of the readings, so that if the 
pressure differences used are as small as is convenient the curvature of the 
leaf is a factor which may be neglected. 1 
1 By supporting the leaf by means of a grid placed between it and the chamber the curvature 
might, no doubt, be avoided. 
