195 
The Mechanism of Root Pressure . 
Throughout all the mass of parenchyma, packed between the 
rigid xylem tubes of the centre and the relatively rigid (frequently 
strongly lignified) outer cylinder of endodermis, a strong hydrostatic 
pressure is developing owing to the entrance of water, governed by 
the existing osmotic gradient. Cells such as G (Text-fig. 1) are in 
the position of the basal cells of the hydathode (with concentration 
C4, Text-fig. 2), cells such as L (Text-fig. 1) are equivalent to the 
apical cells of highest osmotic concentration (C lt Text-fig. 2). If 
then, we make the same assumption as Lepeschkin, and assume that 
the protoplasmic membrane of L, as it abuts upon the xylem, is of 
relatively greater permeability than the protoplasmic layer of L 
where it abuts on other parenchymatous cells, 1 then we have the 
conditions existing for an excretion of water, plus solutes, into the 
xylem vessel so long as the osmotic pressure in L is greater than 
the osmotic pressure in K, and so long as the hydrostatic pressure 
generated within the core of tissue within the endodermis is strong 
enough to force water and solutes into the xylem vessel from L. 
When this stream of water and solutes enters the xylem 
vessel M, the only direction in which it is free to move is upwards 
in the cavity of the vessel. It is true it may leak through 
permeable cellulose walls, around the protoplasts, back as far as 
the endodermis, but at the endodermis it meets the barrier of the 
suberised region and cannot leak outwards any further (see 
page 4). 
At this point it is necessary to refer to a few simple 
experiments which support the hypothesis outlined in the preceding 
paragraphs. 
It would appear at first sight that even if water may not leak 
outwards from the xylem vessel to the surface because of the 
suberised walls of the endodermis, a slight leak might occur near 
the root tip where the endodermal cylinder is not yet differentiated. 
Such a leak would probably in any case be a minor factor, as the 
walls of the cells in the apical region, through which alone this 
leakage could take place, are very thin and the outward diffusion 
under the existing pressure through their total cross section would 
be small. But as the result of a number of experiments in this 
laboratory with various readily penetrating dyes, it is suggested 
that even this amount of leakage probably does not take place. 
As stated previously, de Rufz de Lavison’s observations that such 
1 For this difference in permeability good structural cause may well 
exist. The ectoplasm in one case will be supported by a similar layer opposed 
to it, to which it is attached by protoplasmic connexions, in the other case the 
ectoplasm faces a “ dead ” lignified layer. 
