A Theory of Geotropism. 51 
4. The semi-permeable membrane was assumed to be polarised. 
This is a generally recognised phenomenon (see 12, pp. 106-120). 
5. The polarisation was assumed to be due, in part at least, to 
the electric charges on the protein particles (see 12, pp. 123-124). 
Proteins are known to be amphoteric colloids which are electro¬ 
positive in acid media and electronegative in alkaline media. But 
many proteins, such as albumins and globulins, are more acid 
(electronegative) in themselves than basic; to* render such proteins 
electropositive, requires therefore, a H-ion concentration (or acidity) 
greater than that of the protein itself. 
6. The difference in the geotropic response of stem and root 
was supposed to be due to a difference in the charges of the protein 
particles in these two organs. The continuous phase of the plasma 
membrane in the root was supposed to be acidic beyond the PH 
of the isoelectric pqint of the protein or protein-lipoid particles of 
the disperse phase of the membrane. The corresponding phase of 
stem plasma membranes was supposed to be of an acidity below 
the PH of the protein particles, i.e ., on the basic side of the 
isoelectric point of the protein. The particles of the disperse 
phase of the plasma membranes in the perceptive region of the stem 
would then be electronegative ; while the particles of the disperse 
phase of the plasma membranes in the perceptive Region of the root 
would be electropositive. 1 
7. “ There is abundant evidence to show that the plasma 
membrane in the resting condition is electrically polarised, and 
that increase in permeability reduces this polarisation ” (12, p. 149). 
That reduction of the pojarisation as such gives an increase 
in permeability seems probable from many investigations on the 
effect of acids, alkalies, salts and other substances on the 
permeability of both plant and animal cells. 
Explanation of the Difference between the Upper and 
Under Sides of a Horizontally Placed Root. 
With these hypotheses an explanation is suggested for the 
differential action of gravity on the upper and under sides of a root 
when it is placed horizontal. Considering the diagram (Pig. l)the 
change in the polarisation of the cells in the perceptive region is 
supposed to affect each cell to the same extent but in an unequal 
or one-sided manner, i.e. f the amount of the change is the same in 
1 Haas (9) has shown that no acid or acid-forming substance other than 
C0 2 is excreted by the living root, and it seems probable from preliminary 
experiments that the C0 2 balance in stem and root is the chief factor govern¬ 
ing the differentiation in hydrogen ion concentration. 
