ii 6 
M. J. Lynn 
THE REVERSAL OF GEOTROPIC RESPONSE 
IN THE STEM 
I. THE EFFECTS OF VARIOUS PERCENTAGES 
OF CARBON DIOXIDE 
By M. J. LYNN, M.Sc. 
Senior Demonstrator, Botanical Department, Queen’s University 
of Belfast 
(With Plate I.) 
his work was undertaken with a view to obtaining further ex- 
X perimental evidence, bearing upon the Hydrion Differentiation 
Theory of Geotropism which was recently brought forward by Profes¬ 
sor Small (New Phytologist, 19, p. 49,1920). In particular, that aspect 
of the theory which relates to the carbon dioxide of respiration as a 
differentiating factor in the growth curvatures of geotropism was in 
mind (cf. Small and Rea, ibid. 19, p. 208, 1920). 
According to this theory it is the hydrion concentration of the 
continuous phase of the protoplasm in the cells of the region of 
gravity perception, which determines the direction of the curvature 
resulting from response to the stimulus. 
Under normal conditions the stem, according to this theory, is a 
relatively alkaline structure, because the carbon dioxide of respira¬ 
tion does not accumulate (most of it being used up by the plant during 
photosynthesis), and one result of the relative alkalinity of the con¬ 
tinuous phase of the protoplasm in the perceptive cells is that a 
horizontally placed stem under normal conditions turns upwards in 
response to the stimulus of gravity. 
If the conditions are changed so that the horizontally placed 
plant is growing in an atmosphere made acid by the addition of an 
excess of carbon dioxide, then, as a result of the accumulation of 
the carbon dioxide within the stem, the stem may be rendered less 
alkaline and the hydrion concentration of the continuous phase of 
the protoplasm in the perceptive cells may be raised to such a degree 
that the direction of the geotropic curvature will be affected. The 
experiments described below were carried out with the object of 
