122 
M. J. Lynn 
lated to a certain extent the carbon dioxide of respiration, and in 
such a case a smaller percentage of carbon dioxide in the atmosphere 
would alter the alkalinity of the stem sufficiently to cause a reversal 
of the normal geotropic curvature. Reference is made in the “ Notes 
on Experiments ” to cases in which such previous history may have 
affected the direction of curvature. 
In order to prove that the downward curvature of the Helianthus 
seedlings in the atmosphere containing carbon dioxide above a certain 
percentage really represents a reversal of geotropic curvature, experi¬ 
ments were carried out with a klinostat. 
A box containing seven Helianthus seedlings was attached hori¬ 
zontally to a revolving klinostat. The klinostat used in these experi¬ 
ments had a long horizontal rod as the revolving axis, and this 
axis was fitted through a hole in the cork of a jar which was supported 
horizontally and was similar to those used in all the above experi¬ 
ments. The box containing the seedlings was clamped horizontally 
at the end of the revolving axis of the klinostat and the jar was made 
water-tight by means of a glass plate and paraffin wax as before. 
Carbon dioxide was introduced by displacement of water as before 
to the extent of approximately 30 per cent. 
In the previous experiments in which the percentage of carbon 
dioxide was as high as 30 per cent, reversal of geotropic curvature 
was evident after a very short time (J to J hour), but when the 
seedlings were being revolved on a klinostat in an atmosphere con¬ 
taining 30 per cent. C 0 2 there was no curvature. In this first experi¬ 
ment with the klinostat the seedlings were watched carefully for 
6 hours, and at the end of that time they were still absolutely 
horizontal. 
This experiment was repeated on several successive days with 
different sets of seedlings, with the same result, i.e. no curvature 
on the klinostat. If the downward curvature of the seedlings in the 
atmosphere containing above 10 per cent, of C 0 2 were not a geotropic 
phenomenon, there would be curvature even when the seedlings 
were being revolved on the klinostat. It has been shown by these 
experiments with the klinostat that no curvature takes place under 
these circumstances, and therefore the downward curvature in all 
the experiments in which it occurred quite clearly represented a 
reversal of the normal geo tropic curvature. 
In conclusion, I should like to express my gratitude to Professor 
Small, under whose supervision this work has been carried out and 
whose suggestions and criticisms have been invaluable. 
