
1905. ] Effect of Carbon Dioxide on Roots. — 353 
observed between the rates of curvature in the two sets when both were 
replaced in air at the close of the application of the stimulus. When, | 
however, the stimulus acted for 20 minutes, in every case the curvature 
commenced appreciably sooner in the plants treated with carbon dioxide. 
The difference between the excess of bending in the plants stimulated in 
carbon dioxide for 15 and 20 minutes over that in their respective air 
controls is, perhaps, to be attributed to the slow penetration of the cells by 
the gas. 
After 25 minutes’ stimulation, the excess of curvature in the plants 
treated with carbon dioxide over that of the air-control plants was dis- 
tinctly observable, though it was markedly less than in the plants treated for 
20 minutes. It is probable that, during the additional 5 minutes, the 
carbon dioxide penetrates the cells in sufficient quantity to produce a 
solution in the cell-sap of supra-optimal strength, whereas in 20 minutes 
the optimal strength may be just attained. 
Method II—The above results seemed to be of sufficient interest to 
warrant a continuation of the investigation under more rigid experimental 
conditions. Plants, after stimulation in a horizontal position, were placed on 
a klinostat in such a manner that the axis of the root was parallel to the 
axis of rotation. The klinostat performed one revolution in 19 minutes—a 
sufficiently slow rate, according to Mr. Francis Darwin’s (11) researches, 
to allow movement of the starch grains from side to side of the cell. Under 
these conditions, it was again found that no appreciable difference between 
the plants stimulated for 15 minutes in carbon dioxide and in air could be 
detected. After 20 minutes’ stimulation, however, as before, the curvature of 
the roots treated with carbon dioxide was much more rapid. 
Method I[1I—The plants were at once immersed in pure carbon dioxide, 
and were subsequently transferred to the klinostat. After stimulation for 
20 minutes in carbon dioxide, the subsequent geotropic response set in 
distinctly earlier than in the air-control plants. 
Method IV.—Mixtures of different percentages of carbon dioxide in air 
were employed with a view to determining the optimal partial pressure for 
geotropic response. The mixed gases were continuously passed over the 
peas, which were kept in a horizontal position throughout the experiment. 
The amount of curvature was recorded after stated intervals. By this 
means comparison is instituted between the curvatures due to continuous 
geotropic stimulus in atmospheres exerting different partial pressures of 
carbon dioxide. In the preceding methods the curvature was that following 
stimulation for limited and short periods, during which alone the carbon 
dioxide was allowed to act. 
