July, 1921] OVERTON — 'MECHANISM OF ROOT PRESSURE 
thodes, and that there exists the unilateral secretion of water and solutes 
from the root parenchyma into the vessels, which is an osmotic phenom- 
enon. The question arises as to how such a unilateral excretion can take 
place from a turgescent cell. 
Pfeffer first pointed out the necessity of investigating experimentally 
the secretion of water and solutes in one-celled plants as a basis for under- 
standing the phenomena in hydathodes. This Lepeschkin has done, and 
on the basis of experiments he explains the continuous secretion of water 
and solutes by Pilobolus by assuming a different semi-permeability on the 
upper and lower sides of the cell. He finds that the lower or absorbing 
part of the sporangiophore may possess a greater osmotic pressure than the 
upper or secreting portion, and adopts Pfeffer's assumption that, if the cell 
sap at different points in the cell has different concentrations, the inflow 
must exceed the outflow until an equality is reached between the sides, 
so that there results a unilateral exudation of water through the upper, 
more permeable, membrane, brought about by a pressure corresponding 
to the difference in concentration on the two sides of the cell. The same 
holds true for the epidermal secreting structures of phanerogams and ferns. 
Pfeffer's scheme assumes an unequal distribution of osmotic material 
in the protoplasm, the concentration being greatest in that part of the cell 
which is richest in osmotic substances. Lepeschkin has actually deter- 
mined that such a condition obtains in Pilobolus, which indicates in this 
case that the two opposite ends of the cell are chemically and physically 
different. We should, therefore, expect that the water-absorbing and the 
water-holding capacity of the two sides of the cell would be different. 
Copeland actually constructed a piece of apparatus by which a current of 
water is maintained through an artificial cell by using membranes unequally 
permeable, thus proving Pfeffer's assumption, and holds that it is possible 
for the osmotically active substances in a cell to exert different pressures 
in different directions if the protoplasm is permeable to these substances 
in different degrees in different parts. Copeland points out that root 
pressure must be due to the same process, and that in order for root pressure 
to be caused in this way the protoplasm must be permeable to the osmotically 
active matter of the cell sap to a different degree in different parts of itself. 
On the basis of Lepeschkin's results and of some of his own. Priestly 
recently attempts to explain the movement of water and solutes through 
the root and their excretion into the xylem vessels. He points out, as 
has been suggested by Atkins, that on account of sugar being brought down 
from the leaves by the vascular elements, the sap of the cells bordering on 
the vessels will tend to be more concentrated than that of the cells further 
out, and an osmotic gradient will be established from these cells through 
the root parenchyma to the root periphery. As a result of this osmotic 
gradient, water will enter at the periphery and pass toward the center, 
gradually distending the cells of the parenchyma of the vascular cylinder. 
