66o 
Report of a Disctcssion on 
one to the gas bubble, and one which locks and bars itself against the 
exit of an imprisoned bubble. In a word, it is a structure semi- 
permeable towards matter in its three states, the solid, the liquid, and 
the gaseous. Taking the nature of this structure into account; the 
observations on the swamped condition of a great part of it ; the 
experimentally proved fact that water containing air and in presence 
of wet wood is stable under high tension ; and that the phenomena 
occurring in the leaf are capable of giving rise to the necessary 
traction, it is not too much to say that one who questions the existence 
of tensile stress in the sap (when root pressure is inactive), because it 
has not been directly demonstrated, is in the same position as one who 
questions the existence of pressure in the deepest part of the ocean 
because observation there had never demonstrated it. 
I may perhaps be permitted to remark more clearly here — in view 
of a misconception referred to early in this paper — that the mention 
of revision at the head of our complete paper in the Phil. Trans., 
refers to no change of opinion or modification of views expressed in 
our original manuscript. The term refers solely to excisions. 
In conclusion I desire to express how cordially Mr. Dixon and 
I concur in Mr. Darwin’s remarks upon the work of Strasburger. 
Whatever the fate of our hypothesis — and the clearest evidence may 
deceive ; the strongest convictions err — his name must ultimately be 
for ever associated with the true theory of the ascent of sap. 
Professor G. F. FitzGerald, in continuing the discussion, spoke 
of the difficulty of drawing distinctions between the physical nature of 
4 imbibition 7 and capillary forces ; and the risk the use of the first 
term involved of substituting the word for the idea. 
He did not consider that the supporters of Messrs. Dixon and 
Joly’s theory required necessarily to prove the existence of tractive 
forces in the leaf equivalent to a hydrostatic head five times the 
height of the tree, in order to meet the viscous resistance of the sap. 
The estimate of the viscous resistance to be accounted for must be 
based on the velocity of the transpiration current in the higher parts of 
the tree under normal conditions of supply ; and this, probably, was 
not ascertained. The actual movement of water in the higher parts 
of the tree might be very slow under normal conditions, and hence 
the viscous resistance very small. 
The principles determining the permeability of a septum by the 
