654 Report of a Discussion on 
abstraction of water, when unsupported by osmotic pressure, to 
largely increase its resistance to the passage of water and at the same 
time diminish its effectiveness as an evaporating area, not only because 
of the diminution of this area, but also because of its increased 
fineness of grain and consequently increased capillary retentiveness. 
The stretching of the wall by the osmotic pressure, on the other hand, 
increases both its permeability and its evaporating effectiveness. This 
result of osmotic pressure in the cell permits us to regard this force as 
maintaining the water upon the outer surface of the turgescent cell 
against the elastic force of the cell-wall and the tensile pull of the sap. 
When the steady state is reached, it is however none the less a static force. 
While something of what I have already sa : d is, I hope, a contri- 
bution to this discussion, and over and above personal matter, I turn 
with pleasure to the unalloyed scientific interest of some points 
mentioned in Mr. Darwin’s paper. 
Mr. Darwin points out the want there is for observations on the 
osmotic pressure of the cells of leaves of high trees, in order to 
account for the high tractive forces required to elevate the water 
against gravity, and the viscous resistance of the ascending column of 
liquid. I am glad to say that Dixon has already done something 
towards supplying the want and is actively engaged in pushing the 
matter further. 
In a paper of his read June 8, 1896, before the Royal Irish Academy, 
he gives an account of his preliminary researches. This paper is not 
yet published : I have the proof sheets here, however. 
The mode of experimenting is already described in our full paper 
(pp. 564-66) and need not be re-described. The method is based 
on the assumption that a collapse of the leaf under a high external 
gaseous pressure will indicate the limit of the osmotic resistance of 
the turgescent cell ; the water is then supposed to be forced back 
into the conduits and ejected from the cut end of the branch, which, 
from the nature of the arrangements, is not exposed to the high 
gaseous pressure. 
In Dixon’s experiments it is important to note, first, the pressure 
at which what we may call the saturated osmotic cell begins to eject 
water into the conduits ; and secondly, the higher pressure at which 
the leaves begin to collapse and lose their turgescence. 
Similarly to the manner described in our paper in the Philosophical 
Transactions, the first phenomenon is observed by observation, at 
