Ij 
364 Knight.— Further Observations on the Transpiration , 
stomata on that ring-shaped portion of the leaf which is covered by the 
luting material which attaches the leaf to the apparatus, are shut off from 
light from below and from external air, but these are not the stomata whose 
changes are being measured. The porometer does not deal with the 
stomata covered by the luting material, but with those enclosed by the leaf- 
chamber and those on the rest of the leaf, and it is apparently the stomata 
enclosed by the leaf-chamber which are referred to as being shut off from 
both light and air. With a clear glass leaf-chamber it is true that some of 
the reflected light from below is prevented from reaching the leaf, but the 
diffuse light transmitted through the leaf is unaffected. Further, in a poro- 
meter experiment, the stomata, far from being shut off from air, are given 
freer access to it, in that air passes through the leaf, under the influence of 
the suction applied, far more rapidly than it would normally do by diffusion. 
There are, however, grounds for an objection to the porometer method 
which has not yet been urged. Neger ( 18 ) has distinguished between two 
definite types of leaves. In one type the intercellular spaces are in free 
communication throughout the leaf and in the other type this intrafoliar 
communication is interrupted, the system of spaces being divided into more 
or less numerous units. The former type of leaf Neger calls homobaric 
and the latter heterobaric. Plainly it would be theoretically possible 
to attach a leaf-chamber to a heterobaric leaf in such a manner that it 
would be impossible to draw any air through at all, even though the 
stomata were wide open. It is evident that in comparing stomata of 
different plants by the porometer method, it is necessary to consider the 
condition of the intercellular spaces, a precaution which has already been 
advocated by the writer ( 14 ). 
As the result of his experiences with the determinations of stomatal 
behaviour during wilting ( 16 ), the present writer felt convinced that the rise 
of transpiring power which Bakke found to accompany permanent wilting, 
could not possibly be associated with the stomatal opening first demon- 
strated by Darwin’s experiments. At the outset the time factor appears to 
preclude the possibility of any such correlation. Thus, the rise of trans- 
spiring power referred to by Bakke is considered to be closely associated 
with the point at which the wilting process becomes irreversible. This 
point was reached in practice some days after the transpiration rate 
had begun to fall as the result of insufficient supply of water to the 
leaves. On the other hand, Darwin and Pertz, and Laidlaw and Knight, 
have given the periods which elapse, under different conditions, between the 
cessation of the supply of water to the leaves and the attainment of the 
maximum stomatal aperture consequent upon wilting. The longest period 
recorded by Darwin and Pertz is about 90 minutes, whilst the periods 
quoted by Laidlaw and Knight varied from 9 to 35 minutes. It is recog- 
nized, of course, that comparison of mere time intervals in a matter of 
