/. H. Priestley. 
189 
THE MECHANISM OF ROOT PRESSURE. 1 
By J. H. Priestley. 
[With Two Figures in the Text.] 
Introduction. 
N attempt is made in the following paper to give an 
explanation of the mechanism of root pressure that shall 
be in accord with the experimental evidence and with physics and 
chemistry. 
Most of the data presented below are drawn from the work 
of other investigators, and additional experimental work has only 
been referred to occasionally where it supplied further evidence 
for the views advanced. 
There is, therefore, little that is new presented below ; at the 
same time it is thought that most of these facts have not been 
marshalled before as a basis for an explanation of the mechanism 
of root pressure, and it is hoped that their treatment from this 
point of view is justified by the satisfactory basis upon which it 
places this subject. 
Arising out of the views stated below, a large amount of 
experimental work is in progress in this laboratory, and it may be 
said at once that the work, which it is hoped to publish later, in 
no way clashes with the theoretical considerations advanced below 
as to the mechanism of root pressure. 
Osmosis is obviously of primary importance in connection with 
the process by which sap is driven up the xylem from the roots, 
and, with the aid of Text-fig. 1, an attempt will be made to estimate 
the significance of osmosis in this process. 
In the diagrammatic section of a young root shown in Text-fig. 
1, it is permissible to consider all the cells in the series, from the 
root-hair A to the cell L, bordering upon the xylem vessel, as 
having cellulose walls, permeable to water and the majority of 
solutes, and as lined with a layer of living protoplasm, which 
functions as a semi-permeable membrane. These cells frequently 
have starch stored within them, and presumably, therefore, sugar 
is present dissolved in the cell sap. If this sugar is brought down 
from the leaves by the vascular strands it will tend to be more 
concentrated in the sap of L, but whatever may be the original 
concentration in these cells, owing to the entry of water from the 
1 The substance of this paper was communicated to Section K of the 
British Association, Bournemouth Meeting, September, 1919. 
