57 
A Theory of Geotropism. 
no smell of ammonia could be detected, a germinating bean pinned 
to the cork above the liquid had its testa blackened after two days. 
With this very slightly alkaline atmosphere various experiments 
were carried out by pinning the seedlings to the cork above the 
liquid. In all cases both with Vicia Faba and Zea Mays controls 
were grown pinned to cork above a jar of tap water, the whole 
under a bell jar. These showed the normal responses. The roots 
in the alkaline atmosphere grew up as shown in figures 1—5, 
Plate I and Text-Figs. 3-4. 
Many more roots were grown in an alkaline atmosphere and 
while many failed to show the upward curvature, none grew 
downwards. 
These experiments may be taken therefore as preliminary 
confirmation of hypothesis 6 in so far as the root is concerned. 
On account of the susceptibility of the plants to alkaline vapour and 
the growth of fungi after two or three weeks, the experiments with 
roots were not as satisfactory as those with stems. Direct 
determinations of the H-ion concentration of the protoplasm of the 
root apex and stem apex are being attempted, but it should be 
pointed out that the PH of the sap is not necessarily that of the 
plasma membrane. The reversal of curvature is considered to be 
the best test devised at present because in the case of reversed 
curvature we are dealing with that part of the mechanism which 
produces normal curvature. 
Geotropic Response of the Stem. 
In the first stages of experimentation bean seedlings with stems 
one to two inches long were pinned to cork above water containing 
about 1% of acetic acid, under a bell jar. No geotropic response 
was obtained and the stems excreted liquid in the same way as the 
rojQts did in an alkaline atmosphere, they afterwards became 
blackened also but not to the same extent as did the roots. 
The best results were obtained with the coleoptile of the maize 
seedling. The ordinary variety was used for most of the coleoptile 
experiments, but Sutton’s White Horse-tooth variety reacted with 
equal, if not greater, readiness. The seedlings were grown in moist 
sand in shallow porous dishes. They were transferred to the cork 
when the coleoptile was about an inch long and before the first leaf 
emerged. Controls were grown above tap water, and acetic acid 
was used as before to obtain an acid atmosphere. It was found 
that the concentration of the acid could be varied within wide 
limits and an atmosphere distinctly acid to litmus paper was easily 
