THE 
New Phytologist 
Vol. XIX, Nos. 3 & 4. March & April, 1920. 
[Published May 18th, 1920.] 
A THEORY OF GEOTROPISM: 
WITH SOME EXPERIMENTS ON THE CHEMICAL REVERSAL OF 
GEOTROPIC RESPONSE IN STEM AND ROOT. 
By James Small, D.Sc. (Lond.) 
Professor of Botany, Queen's University , Belfast. 
[With Plate 1 and Five Figures in the Text.] 
Contents. 
Page. 
Preliminary Theories. ... 50 
Hypotheses . . . ... 50 
Explanation of the Difference Between the Upper 
and Under Sides of a Horizontally Placed Root 51 
Explanation of the Upward Curvature of the Stem 54 
Experimental :— 
Geotropic Response of the Root ... ... ... 56 
Geotropic Response of the Stem . 57 
Theoretical Conclusions . 58 
Explanation of Plate. 62 
Bibliography . . ... . 62 
C ERTAIN theoretical considerations have led to the conclusion 
that the difference in the responses of the stem and the root 
under the stimulus of gravity depends upon the relative 1 alkalinity 
of the continuous phase of the plasma membranes of the stem apex 
and the relative acidity 1 of the corresponding medium in the root 
apex. 
The experimental evidence for this view and the theoretical 
considerations involved, form the substance of the present 
contribution. 
1 “ Relative,” that is, to the isoelectric point of the protein or other 
particles of the disperse phase of the plasma membrane. 
