352 Dr. E. Drabble and Miss H. Lake. [May 12, 
short period, the subsequent rate of streaming was temporarily increased. 
Montemartini had, in 1892, observed that the growth in length of pea-roots 
was more rapid in 4 per cent. carbon dioxide than in air, and than in per- 
centages greater than 7, but the significance of this does not appear to 
have been very generally appreciated. Chapin (4), in 1902, determined the 
optimum percentage of carbon dioxide for the growth of higher plants to be 
1 to 2 per cent., and stated that, in small quantities, the effect is stimulative, 
whereas in large doses it acts as a poison. 
Brown and Escombe’s results have shown that the utilisation of carbon 
dioxide is, within limits, proportional to its partial pressure. In view of the 
facts that the weight of the plant does not increase in proportion to the 
absorption of the gas, it may, perhaps, be suggested that possibly a 
stimulating action is exercised on the protoplasm, resulting in increased 
photosynthesis and respiratory activity, a preponderance of the latter process 
explaining the absence of increase in weight. 
Effect of Carbon Dioxide on Geotropism. 
The fact that roots placed horizontally in boiled water do not respond to 
geotropic stimulus, owing to the absence of oxygen, has been known for 
some time. A similar failure to respond results when the roots are placed in 
hydrogen or other indifferent gas. That the stimulus is perceived in each 
case is clearly proved by the fact that, if the plant be removed from the 
boiled water or gas and placed vertically in air, the root-tip executes a 
movement out of the vertical in the direction of the previous stimulus. 
Czapek’s ammoniacal silver nitrate and guaiacum reactions (10) seem to 
offer a means of demonstrating such a perception, whether the stimulus be 
followed by curvature or no. | 
In view of the fact that carbon dioxide is a protoplasmic poison, and in 
small doses acts as a stimulant to streaming and to growth, it occurred to us 
that an investigation of its action on geotropic curvatures might be not 
without interest. With this object, experiments were set up as follows :— 
Method I.—FPeas, whose radicles had emerged to a length of about 3. cm., 
were fixed horizontally on a strip of cork in a glass vessel, through which 
gases could be passed. Through one such vessel carbon dioxide was passed 
for various lengths of time, a second vessel being employed as an air control. 
While horizontal no bending took place in either set. The peas were then 
placed with their roots in a vertical position in air, care being taken to keep 
the atmosphere saturated with water vapour, and the first appearance of 
curvature was noted. When the stimulus was allowed to act on the 
horizontally placed roots for 15 minutes, no appreciable difference could be 
