Hydrion Differentiation Theory of Geotropism. 209 
being vaselined they curve upwards as a rule. They must he 
placed vertically in the dark for two to seven days, to allow the 
C0 2 to accumulate, if downward curvature is to be demonstrated. 
In several cases recovery from downward curvature to upward 
curvature was obtained by placing the plants in the light, to enable 
them to remove the C0 2 by photosynthesis. The treatment with 
vaseline and darkness also inhibits the growth of the stem (cp. 1). 
A practical application of the phenomenon which has just been 
described occurs in the “ earthing-up ” of potatoes; the “ earthing- 
up ” increases the amount of C0 2 in and around the lower stem 
branches with the result that they grow down into the soil and 
ultimately produce tubers, which may be regarded as physiological 
malformations due to the disturbance of the normal condition of 
the protoplasm. This explanation of the production of potato 
tubers may be extended to include the experience by farmers in the 
north of a loss in the potato crop, instead of a gain as with barley, 
when the plants are treated with high tension currents. To put 
the matter briefly—whereas the positivity of the high tension 
currents induces a negative charge in the already negatively 
polarised sub-aerial parts, that same positivity decreases the 
polarisation in the already positively polarised subterranean parts 
with consequent increase of permeability, loss of turgor, loss 
of growing power and decrease in tuber-production. 
II.—On the Angle of Balance in Roots , Stems and Leaves. 
By J. Small and M. J. Lynn. 
In the above mentioned paper (2, p. 58, Fig. 5) an explanation, 
based upon the new theory of geotropism, is given of the orientation 
of secondary roots and stem branches. Certain deductions from 
that explanation are obvious. (1.) Provided that the distance of the 
point of junction from the tip of the main axis remains constant, 
the angle of balance will increase with the growth in length of the 
of the lateral organ, because the strength of the action current in it 
at the point of junction will decrease, and therefore a greater angle, 
giving a larger action current, will be required to balance the leak 
of the normal polarity current from the main axis. Since it has 
been shown (3, 4, 5) that the strength of the response (or action 
current) varies as the sine of the angle, it follows that when the 
angle of balance is plotted against the length of the lateral organ, 
the graph should be a sine curve. (2.) Provided that the length 
of the lateral organ remains constant, the angle of balance will 
