98 James Small on 



dioxide and to the control set of seedlings in fresh air. All Avere 

 placed horizontally ; and now you can see for yourselves that in 

 the control set all have turned upwards, while all the seedlings 

 of both sets in carbon dioxide have turned downwards. These 

 demonstration experiments were set up by Miss Lynn, and form 

 conclusive proof of the reversing action of excess of carbon 

 dioxide on geotropic curvature of the stem.^ 



It is interesting to note that the seedlings are able to 

 recover when brought into fresh air, or when the carbon dioxide 

 is removed. This occurs on practically every occasion when the 

 seedlings are removed from the bell-jars at the conclusion of the 

 reversing experiment. 



Another Cause of Erectness in Plants. 



Hitherto we have considered only gravity and its effects 

 upon the plant whereby normally the root grows downwards and 

 the stem upwards ; but there is another stimulus which acts in 

 nature. Roots turn away from light and stems turn towards 

 light ; so that the sunlight normally causes the stem to grow in 

 an upward direction. It seemed probable that excess of carbon 

 dioxide would also reverse this light-curvature in the stem' ; and 

 this phenomenon is being investigated by Miss I. Finnegan. The 

 first experiments with sunflower seedlings yielded remarkable 

 results. Miss Finnegan found that the stems not only curved 

 away from the light, but they also curved downwards ; so that 

 both heliotropic and geotropic responses were reversed in the 

 same plants. Further, these plants when removed to fresh air 

 recovered first their normal geotropic response and then their 

 normal heliotropic response, with the result that they formed a 

 complete S curve, as shown on the screen.^ 



•• Further details are giveM in a forthfiomingr communication by Miss M. J. Lynn on this 

 subject, which will probably appear in the New Phytologist, Vol. XX, No. 3, 1921. 



^ The theory of phototropism involved has been published in a brief form in the Ne\r 

 Phytolot?ist, Vol. XIX, p. 275, 1920 ; see also below p. 105. 



*A series of photof^-raphic lantern slides of this experiment was shown. 



