A Theory of Geotropism. 59 
direction of growth is influenced in these cases hy heliotropism, the 
action of which in the root system is negligible for present purposes. 
Any sound theory of geotropism should be able to yield an 
explanation of these phenomena with much the same type of 
protoplasm in the laterals as in the main axes. 
Considering first the secondary root B of the primary root A 
in Fig. 5. According to previous theorisings and experiments we 
can suppose that a current flows upwards in the cortex of the 
primary root A as a result of the creaming and potential differences 
in the apical meristem, which should be present even in the erect 
root according to the views expressed above. This current being 
symmetrically arranged produces no “action” current and no 
curvature. The PD on account of the resistance will decrease as 
it passes upwards, see Fig. 5, where the PD is supposed to be 
reduced from 4 to 1 at the base of the secondary root B. 
If the secondary r root B were placed horizontally it would 
develop asymmetric potential differences, an EMF and an action 
current as outlined for the horizontal primary root. Since the 
meristem is smaller the current would be smaller hut it would still 
have a greater effect in the lower side and the rootlet B would tend 
to grow vertically downwards. At the point where it joins the 
primary root A, however, there is a current flowing in the cortex 
which would enter the rootlet B both in the upper and the under 
sides, if the potential differences were suitable. This “leakage” 
of the normal polarity current from A would tend to flow towards 
the apex of B; in the under side it would meet an opposing 
current, while in the upper side it would flow or tend to flow 
in the same direction as the action current of B. The action 
current and its effects in the upper side of B would, therefore, be 
augmented hy the normal polarity current from A, while the action 
current in the under side of B would be more or less reduced 
by the normal polarity current from A, according to the relative 
potential differences of the current from A and the current in B 
at the point where it meets the primary root A. 
The general effect of the current from the primary root would 
thus be to neutralise more or less the asymmetric effects of the 
action current in B, and the angle at which the asymmetric effects of 
the action current in the secondary root are neutralised by the leakage 
of the normal polarity current from the primary root is the angle 
which the secondary root will make with the vertical. 
