318 BOTANICAL GAZETTE [APRIL 
the evaporating surface, and the vapor tension (and hence the 
evaporation) of a 3-molecular solution of potassium nitrate is only 
about 8 per cent lower than that of pure water. A comparison of 
this figure with 49 per cent, the approximated retardation in tran- 
spiration, leads us to conclude that an increased concentration and 
the accompanying lowerings of vapor tension of the foliar solutions 
cannot possibly be directly related to the great fall in relative 
transpiration which is observed.” That such variations in concen- 
tration may have some indirect effect is highly improbable, and 
there is no evidence at hand to enable us to consider this possibility 
here. That a slight diurnal increase in the concentration of solu- 
tions bathing the protoplasmic membranes might bring about 
marked changes in the colloidal state of the latter, and hence in 
their permeability to water, is of course possible; but if such were 
the case, the effect would be manifest simply as a lowering of the 
possible rate of supply to the cell walls, resulting in a partial drying 
out of these, so that the phenomenon would appear as incipient 
drying. 
We conclude, therefore, that the hypothesis of incipient drying, 
due to partial drying of exposed membranes, is the only adequately 
possible explanation of the diurnal fall in relative transpiration. 
If this process actually occurs, it should be exhibited in a sensible 
decrease in the actual moisture content of the leaves concerned. 
We should thus expect to find the moisture content of sun- 
illuminated foliage to become less as the day advances, attaining 
a minimum some time in the afternoon and then rising again. 
Whether or not this is actually manifest, as a diurnal fall in 
the percentage of foliar moisture, is the question which we have 
experimentally attacked. 
It needs to be added here that the hypothesis of incipient dry- 
ing, while apparently the only logically possible explanation of the 
non-stomatal hindrance to water loss, is not the only logically pos- 
sible cause of a marked diurnal fall in the relative water content of 
green leaves. With the actual water content of the tissues remain- 
% This conclusion is quite in accord with that reached by DraBBLe and DRABBLE. 
See DrapB ie, E., and Drassce, H., The relation between the osmotic strength of cell 
sap in plants and their physical environment. Biochem. Jour. 2: 117-132. 1907- 
