55 
A Theory of Geotropism. 
side. The turgor of both upper and under sides of the motor region 
will, therefore, be reduced, but that of the upper side will be reduced 
more. Growth will then be greatest on the underside and the stem 
will curve upwards. The distribution of these electrical effects in 
the perceptive and motor regions accounts for the fact that, although 
these two regions may be well defined, there is frequently a 
curvature which extends from the perceptive region to the 
comparatively distant region where the chief motor response takes 
place. 
These preliminary theories having been sketched out towards 
the end of the year 1918, some time was spent in considering which 
of the many experiments they suggested would be most critical and 
at the same time convincing; something that could easily be 
repeated was desired because of certain criticisms of the excessively 
careful and expert manipulation required for the repetition of the 
previous experiments (10) on the changes in conductivity under 
geotropic stimulation. 
It was finally decided that, if the reaction of the continuous 
phase of the plasma membranes in the perceptive region of the 
stem was really on the basic side of the isoelectric point of the 
disperse phase, and that of the corresponding medium in the root 
was really on the acidic side of the isoelectric point, it might be 
possible to reverse the relations of the reactions of these regions to 
the isoelectric point. To state the matter in more widely familiar 
but somewhat misleading^terms—taking the isolectric point of the 
proteins concerned as neutral, the stem growing up because it is of 
an alkaline nature should grow down if its reaction is changed, 
while the root growing down because it is of an acid nature should 
grow up if its reaction is changed. There was, of course, a 
considerable doubt as to whether the protoplast would continue to 
function and reversal of the actual curvature was as much as could 
be expected. 
This critical experiment was chosen because it seemed essential 
to the whole theory to suppose a real difference in the kind of 
charge on the particles of the disperse phase and at the same time 
an electropositive polarisation of the plasma membrane seemed to 
be the only point which was at variance with accepted views on the 
electrical conditions of the cell. These accepted views being based 
chiefly on animal cells, and no critical experiments having been 
carried out on the contrasted tissues of stem and root, there was a 
