The Reversal of Geotropic Response in the Stem 121 
Exp. 24. In this experiment, response to the stimulus was much 
slower than the abnormal response in the higher percentages. When 
the six seedlings were showing normal geotropic curvature, an addi¬ 
tional amount of carbon dioxide was passed into the jar, and the 
jar was replaced in darkness. After two days the hypocotyls of the 
seedlings (which with 7*9 per cent, carbon dioxide in the atmosphere 
had shown a normal curvature) had turned downwards, because the 
addition of the extra carbon dioxide to the jar had rendered the 
atmosphere sufficiently acid to raise the hydrion concentration in the 
perceptive cells to an extent sufficient to cause reversal of the normal 
curvature. 
Exp. 27. These seedlings had been brought on in fairly dull light, 
so that there would be little photosynthesis, and therefore C 0 2 would 
have accumulated to a certain extent. This would probably account 
for the reversal with the small percentage (see below). 
From the results tabulated above it is quite clear that reversal 
of geotropic curvature actually does occur in the hypocotyls of 
seedlings of Helianthus annuus, when the seedlings are fixed hori¬ 
zontally in an atmosphere containing from about 9 per cent, to 
30 per cent, of carbon dioxide. 
With the apparatus employed in the analysis of the gas it was 
found that an error of o-i c.c. in the reading of the volume of the 
gas remaining after the C 0 2 had been absorbed from the sample, 
resulted in an error of 1 per cent, either way in the percentage of 
carbon dioxide as analysed; when allowance is made for such an 
error [i.e. 1 per cent, in either direction) it seems reasonable to con¬ 
clude from the results tabulated that the percentage of carbon 
dioxide necessary to produce reversal of the normal geotropic curva¬ 
ture in the hypocotyls of Helianthus annuus is approximately 7 to 
10 per cent. 
The minimum percentage of carbon dioxide necessary to produce 
such abnormal response would no doubt be slightly different even 
for different plants of the same species, e.g. in cases where the previous 
history of the plants used in the experiments had differed. 
The factor in the previous history which would have the greatest 
effect would be light. If the plants were brought on in bright light 
then the C 0 2 of respiration would be used up in photosynthesis, 
and, therefore, would not have accumulated to any extent in the 
plant. On the other hand, photosynthesis would be much less active 
in plants brought on in dull light. Such plants would have accumu- 
