208 
J. Small and M. W. Rea. 
PRELIMINARY NOTES ON ADDITIONAL EVIDENCE 
FOR THE HYDRION DIFFERENTIATION 
THEORY OF GEOTROPISM. 
I.—On the Reversal of Geotropic Curvature in the Stem. 
By J. Small and M. W. Rea. 
S INCE the publication by one of us of a theory of geotropism 
supported by certain experiments (2) some considerable 
amount of additional evidence in favour of this theory has been 
obtained. The details of this evidence will not be published for 
some time, and therefore this preliminary account is being given. 
The theory of the experiments is, briefly, that the differentiation 
in hydrion concentration in stem and root is caused by the 
comparatively slow escape of the C0 2 of respiration from the root, 
which thus becomes relatively acid; and the rapid elimination of 
that C0 2 from the stem and leaves either by diffusion through the 
stomata or by its utilisation in photosynthesis or by both these 
methods. The shoot therefore becomes relatively alkaline and 
the hydrion differentiation is in this way connected with the normal 
metabolism of the plant. 
From this hypothesis it follows that when shoots are placed 
horizontally in the dark they may cease to react or may react in 
the reverse direction to the normal under geotropic stimulation. 
If these shoots are coated with vaseline the C0 2 of respiration 
should accumulate within and the geotropic response should be 
reversed. 
Shoots of a number of very different plants have been treated 
in this way, i.e. coated with vaseline and placed horizontally in the 
dark. Most of the shoots so treated showed downward curvatures, 
while shoots left in the dark unvaselined in a number of cases 
showed upward curvatures and in others showed either no curvature 
or downward curvatures. The plants which showed reversed 
curvature include maize, Pelargonium,Cerastium, sycamore seedlings, 
Myosotis inflorescence, Antirrhinum seedlings, groundsel and 
Anthemis. No upward curvature or positive downward curvature 
in the dark has been observed in unvaselined specimens of maize, 
Stellaria media and sycamore seedlings. Photographic records of 
these experiments have been kept and we intend to give a detailed 
account when the problem of the C0 2 balance has been more 
thoroughly investigated. One point, however, should be noted. If 
the plants are placed horizontally in the dark immediately after 
