276 Hydrion Differentiation Theory of Heliotropism. 
in some other way, increases the rate of respiration in the perceptive 
cells. This increase in respiration involves an increase in the 
carbon dioxide within the cells affected. Such an increase in 
carbon dioxide involves in “root” structures an increased hydrion 
concentration of the already relatively acid continuous phase of the 
plasma membranes, giving increased relative acidity with consequent 
increase in polarisation and in turgidity. Root structures, therefore 
become more turgid on the side which is more illuminated, and 
show negative heliotropism. 
The increase in carbon dioxide within the perceptive cells in 
“ stem ” structures involves an increased hydrion concentration of 
the already relatively alkaline continuous phase of the plasma 
membranes, giving decreased relative alkalinity with consequent 
decrease in polarisation and in turgidity. Stem structures, there¬ 
fore, become less turgid on the side which is more illuminated, and 
show positive heliotropism. 
Since this explanation of heliotropism holds for non-chloro- 
phyllous roots, fungi, sporogonium stalks (e.g., Pellia ), etc., as well 
as for chlorophyllous structures, it is more probable that light 
reacts directly on the amount of respiratory carbon dioxide, rather 
than indirectly by increasing the amount of respirable material. 
This aspect, as well as others, is now receiving attention, and 
reversed curvatures both geotropic and heliotropic have been 
obtained in atmospheres containing a large percentage of carbon 
dioxide. 
Botanical Department, 
October 28th, 1920. The Queen’s University of Belfast. 
1. Small, J. A Theory of Geotropism, New Phytologist, Vol. XIX., p. 49. 
2. Small, J. and Rea, M. W. Preliminary Notes I. On the Reversal of 
Geotropic Curvature of the Stem. Ibid. Vol. XIX., 
p. 208. 
