197 
The Mechanism of Root Pressure. 
If a section is cut across the root some 24 hours later, the 
walls of the xylem will be found to be stained, and possibly the dye 
will have diffused via the walls as far as the endodermis, any patches 
of thick walled sclerenchyma within the endodermis being thus 
quite likely to show their walls well stained. But the protoplasts 
within the cells will still be quite colourless, except for some cells 
close to the xylem, but not always immediately adjacent. Frequently 
these cells form a group lying between protoxylem and endodermis. 
These cells will stain deeply and show up most conspicuously, owing 
to the fact that the dye has in many cases penetrated them and 
accumulated within them. The effect has been particularly well 
seen in this laboratory, using acid green and seedlings of Vicia 
Faba. 
It is submitted then that the hypothesis outlined above, based 
on the work of other investigators and supported by a small 
amount of new experimental evidence, provides an adequate basis 
for the interpretation of the phenomenon of root pressure. It is 
not pretended, however, that the hypothesis is free from difficulties 
and one or two of these will now be briefly considered. 
One outstanding difficulty in the case of a hydathode which 
functions for some considerable period, is accentuated in the case 
of the root where a very considerable stream of water may pass 
upwards. It is difficult to understand how the necessary solutes 
can be provided in sufficient quantity to permit of the constant 
leakage of the solute across the inner membrane of cells such as 
L. (Text-fig. I). 
From Lepeschkin’s work, such solutes might be either organic 
or inorganic. It is suggested tentatively that in the root they are 
organic, and either sugars, or, more probably, organic acids derived 
from the breaking down of sugars. Of these sugars probably a 
constant supply is available from hydrolysis of the stored starch 
or from the sugars arriving by translocation from the leaf. In 
favour of the view that the solute is a sugar may be quoted the 
data given by Gelston Atkins (I, loc. cit. Chap. XI) as to the presence 
of sugar in the sap rising in the xylem, and the interesting data 
as to the permeability of blood corpuscles to glucose, discussed by 
Bayliss (2, loc. cit. pp. 126 and 127) which leads him to the 
conclusion that “The facts suggest the possibility that the normal 
semi-permeability of the membrane to glucose is connected with a 
particular difference of concentration on the two sides, but that the 
actual value of this difference may be changed by other influences.” 
