The Hydrion Differentiation Theory of Geotropism 81 
charged particles which become redistributed when the organ is 
displaced. Then, if the obvious is admitted, and the root is taken as 
acidic and the stem as relatively alkaline, these particles must move 
upwards in both organs, not downwards; otherwise the current 
would pass in the other direction both in stem and root, and the 
stem would normally grow down, as it does practically every day 
in our laboratory under the reversing action of excess of carbon di¬ 
oxide 1 . If, however, it can be shown that starch grains or other 
heavy bodies in the cell aid in the upward movement of these other 
particles, or that such starch grains are electro -positive in alkaline 
media and electro -negative in acid media, then they may be corre¬ 
lated with geotropism, but this type of “amphoteric” electrolyte 
is as yet unknown to science. 
As a contrast it is quite a logical deduction from the well- 
known influence of hydrion concentration on enzymic and other 
biochemical reactions 2 , that the P H of the protoplasm is important 
in cell processes. Haas 3 , for example, writes : “The reaction of proto¬ 
plasm is one of the most important factors of metabolism.” But 
the further deduction that this hydrion differentiation is the cause 
of the difference in geotropic response in stem and root is admittedly 
only a very plausible theory; nevertheless the experimental evidence 
in its favour rapidly accumulates. 
In concluding it must be pointed out that all the steps which 
are supposed to occur in geotropic response are capable of experi¬ 
mental demonstration, if they do occur, and it is this aspect of 
the theory and its applications which is now receiving attention. 
I would also like to express my gratitude to Professor V. H. 
Blackman for the kindly and helpful interest he has taken in the 
theory and for giving me this opportunity of dilating upon certain 
aspects of it which, like other points, received only brief notice in 
the original condensed account. 
Botanical Department, 
Queen’s University of Belfast. 
April, 1921. 
1 See a forthcoming communication by Miss M. J. Lynn. 
2 See The Determination of Hydrogen Ions by W. M. Clarke, Baltimore, 1920; 
also I. Aggazzotti in Archiv f. Entwicklnngsmechanik, 37 , p. 1. 1913; W. J. 
Crozier in Jour. Gen. Physiol. 1 , p. 581. 1919; and E. O. Schley in Bot. Gaz. 56 , 
p. 480. 1913. 3 Bot. Gaz. 63 , p. 232. 1917. 
