no 
V. H. Blackman 
pressure of the fluids at C and at B, though of course the resistance 
of the two membranes will retard the rate of entry of the water. 
The osmotic pressure of the solution in A plays no part in the process, 
for A merely acts as a carrier of water and a reservoir of solute 
which can diffuse into the fluid above C. 
In Pilobolus the excreted fluid is a weak solution containing no 
organic matter and with a concentration of mineral substances only 
one quarter that of the cell contents, so that unless the tension of 
the membrane comes into play the water from these drops will be 
drawn back into the cell—a difficulty by the way which Lepeschkin 
never meets. When the upper membrane of the sporangiophore be¬ 
comes permeable some of the osmotic substances present in the cell 
sap will diffuse out into the water imbibed in the cell-wall. This weak 
solution would then draw water through the turgid cell in the manner 
just described for the model, and so the drops would be formed. 
That drops of fluid might arise on the turgid sporangiophore of 
Pilobolus in this way seems clear, but the view that such a mechanism 
plays any important part in the actual process of exudation seems, 
on the other hand, very doubtful. The high temperature coefficient 
for exudation observed by Lepeschkin is rather against this view, 
though the effect might be due to the increased permeability of the 
protoplasm to water (see Delf, 1918). More definitely against the 
view is the fact that at a high temperature and under the action of 
alcohol the exudation may go on up to a point at which the turgor 
of the cells is markedly reduced, and it may even go on when the 
cell is partly plasmolysed by a 0-5 per cent, solution of NaCl. One 
would expect the exudation of fluid to stop before the turgor had been 
markedly reduced. The salt solution would certainly bring the ab¬ 
sorbing power of the cell beyond the osmotic force of the drops 
(which is less than 0-35 per cent. NaCl), so that the continuance of 
the exudation, though at a diminished rate, is definitely against 
such a simple explanation of the mechanism. The process is probably 
a more complex one, depending on special energy relations of the 
living cell. 
The hydathodes of Phaseolus and the secreting systems acting in 
root-pressure are essentially similar, but differ from Pilobolus in that 
the secreting cell or cells are separated from their water supply by 
a chain of intermediate cells. Now Lepeschkin claims that in order 
that water may pass to the apical secreting hairs of the Phaseolus 
hydathodes an osmotic gradient must exist with the apical secreting 
cells exhibiting the highest osmotic pressure; and Priestley appears 
