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THE HYDRION DIFFERENTIATION THEORY 
OF GEOTROPISM: 
A REPLY TO SOME CRITICISMS 
By JAMES SMALL 
aving read with interest a recent criticism 1 of the “creaming” 
I I part of the hydrion differentiation theory of geotropism, the 
writer feels indebted to Professor V. H. Blackman for affording him 
an opportunity of making clear certain points in the theory which 
seem obscure to several correspondents. The publication of this 
theory was really an honest attempt to provide a working hypo¬ 
thesis for the elucidation of the nature of geo tropic response. The 
theory is therefore open to considerable amendment and even to 
complete rejection, if and when experimental data or other known 
facts which are of sufficient weight are brought forward. But so 
far as purely theoretical considerations are concerned there is 
certainly no need to stumble over a particle as large as 0-0002 mm. 
in radius. 
Throughout his article Professor Blackman reasons from the 
assumption that “the protoplasmic [particles] in question are ultra- 
microscopic” (op. cit. p. 40). He even writes (loc. cit .): “As by the 
author's hypothesis the particles are ultra-microscopic,” and as this 
idea seems to be shared by a few others, the writer apologises for 
any ambiguity which may have given rise to this misunderstanding. 
So far as he is aware the term “ultra-microscopic” does not occur 
in any of the papers previously published. The hypothesis as origin¬ 
ally stated 2 is that: “The protoplasm was conceived to be an 
emulsion of protein or protein-lipoid particles in a colloidal protein 
medium.” 
Reference was also made to the Brownian movement of these 
particles and to the suspensoid structure of the plasma membrane. 
In an abstract 3 reference was again made to the emulsion struc¬ 
ture. The term “emulsion” rather than “colloidal solution” has 
been used always, as far as the writer is aware, because of the 
1 New Phytologist , 20, No. 1, p. 38. 1921. 
2 New Phytologist , 19 , Nos. 3 and 4, p. 50, § 3. 1920. 
3 Linnean Society , 18th March, 1920. 
