58 
A Theory of Geotropism. 
(see hypothesis 6). In the case of the vertical root there would be 
not only a reversed normal EMP, but a definite EMF developed as 
a result of the greater positive polarisation of the upper surface of 
each cell in the perceptive region, giving a difference of potential 
between the upper and lower ends of the cell. For one cell this 
EMF would be small, but the cells may be regarded as connected 
in parallel (laterally) and in series (longitudinally); the EMF would, 
therefore, be something quite appreciable (several hundred 
millivolts). The arrangement of these concentration cells and 
consequently of any current developed is symmetrical about the 
axis of the vertical root and no curvature would be produced. 
When the root is placed horizontally the potential differences 
are no longer arranged symmetrically about the axis of the root 9 
There is a higher potential (because of the “ creaming ” and greater 
polarisation) on the upper side of each cell and therefore an EMF is 
developed which produces a current flowing from the upper side 
to the lower side of the perceptive region of the root (Fig. 1); This 
may be described as an “ action ” current 1 and, on account of the 
resistance in the circuit, it will produce its maximum effect in the 
under side of the root. 
One effect of the current is naturally the lowering of the 
polarisation of the cells giving a greater increase in permeability to 
ions and in conductivity in the under side of the root than in the 
upper side. 
This current is developed in the sensitive, perceptive root apex 
and may be considered to be the cause of the stimulation of the 
cells of the motor regions. This electrical stimulation of the cells 
of the motor region would be, as shown above, greater in the 
under side; this would produce a greater increase in the permea¬ 
bility of the under side than of the upper side, giving great&r 
relative turgor in the upper side and therefore greater growth of 
that side. The downward curvature of the root and the decrease 
in the rate of growth under geotropic stimulation are, therefore, 
explained, together with the variations in conductivity found. 
It should be noted that for the effect to be greatest in the 
under side the outer surface of the plasma membrane is supposed 
to be electropositive, because the particles of the disperse phase 
which produce the polarisation are enclosed by a continuous phase 
which is acidic beyond the isoelectric point of the protein particles. 
Bose (1) has recorded data which prove that potential differences, such 
as would occur in this case, do exist in the stimulated root and stem. 
