The Reversal of Geotropic Response in the Stem 117 
discovering whether the curvature was affected under such conditions. 
The method of procedure in the experiments was to place the plants 
horizontally in an atmosphere to which known percentages of carbon 
dioxide had been added, and to determine the effect of such abnormal 
conditions on the geotropic curvature of the stem. 
In the course of the work it became apparent ttiat, with added 
amounts of carbon dioxide above a certain percentage, there was a 
very decided reversal of geo tropic response, the stem structures being 
actually positively geotropic. 
It was also equally apparent that with added quantities of carbon 
dioxide lower than this “certain percentage/' the response to the 
stimulus of gravity resulted in quite a normal negatively geotropic 
curvature. 
It therefore became necessary to determine, as closely as possible, 
the minimum percentage of carbon dioxide which had to be added 
so that there would be reversal of the normal curvature. 
The different stages in the development of the work are described 
below. 
A number of preliminary experiments were carried out in October 
1920. In the first one conducted, two healthy seedlings of Clarkia 
elegans, growing in garden-soil in a box, were placed horizontally in 
a glass jar into which carbon dioxide was being passed at the top 
and from which there was an outlet at the base. 
After the lapse of one day no change was noted in the seedlings, 
i.e. the stems were still quite horizontal. At the end of two days 
both seedlings showed a slight downward curvature of the stem and 
on the third day there was a very strong downward curvature. 
On the fourth day the seedlings had an upward turn. This last 
change was probably due to the fact that, after the end of the second 
day, the supply of carbon dioxide was cut off from the jar, and since 
the jar was not darkened in any way the seedlings were able to carry 
on photosynthesis, thus using up the carbon dioxide from the 
atmosphere in the jar (which in addition was not air-tight), so that 
on the fourth day the atmosphere in the jar was almost back to 
normal. The stems of the seedlings would then become relatively 
alkaline again and so show the normal negatively geotropic curvature. 
This case of recovery may be compared with other instances to 
which reference will be made below. 
Further preliminary experiments were carried out with seedlings 
of Clarkia elegans, Antirrhinum and Helianthus ; from these ex¬ 
periments it became quite clear that reversal of normal curvature 
