The Erectness of Plants. 97 



The carbon dioxide of respiration, therefore, accumulates in 

 the root apex, because in that region there is no photosynthesis 

 and stomata (pores) do not occur in its outer layer. This acidic 

 respiratory gas, on the other hand, does not accumulate in the 

 stem and leaves because in these organs it is either used up in 

 photosynthesis or it diffuses through the stomata into the outer 

 air. It is now clear that the carbon dioxide of respiration may 

 be the substance which differentiates the root fi'om the stem in 

 so far as the relatively acidic or relatively alkaline reaction is 

 concerned. 



Using this extension of the theory as a working hypothesis, 

 the stems of seedlings or young plants of many different species 

 were induced to curve downwards. In the first experiments 

 carried out by Miss M. W. Kea and the writer^ the carbon 

 dioxide was caused to accumulate in the stems and leaves by 

 coating the whole of the green surface with vaseline and placing 

 the plants in a dark box. The stomata were thus rendered use- 

 less and photosynthesis was stopped. The plants which showed 

 downwaid curvature of the stem in these experiments included 

 maize, garden geranium (Pelargo7iium), mouse-ear chickweed 

 {Cerastium), sycamore seedlings, inflorescence of forget-me-not 

 (Myosotis), seedlings of snapdragon {AntivThiyimn) , young plants 

 of groundsel {Senecio vulgaris), and both flowering stems and 

 leafy stems of marguerite (Anthemis).- 



The effect of carbon dioxide has been further investigated by 

 Miss M. J. Lynn, who finds that in an atmosphere containing 

 about 30 per cent, of carbon dioxide reversal of the geotropic 

 curvatuic of the stem invariably takes place within about one 

 hour in the case of sunflower seedlings. This rapid curvature is 

 proljably due directly to turgor changes, not to actual growth 

 changes which usually take much longer to appear. 



At the beginning of this lecture your attention was directed 

 to the two sets of sunflower seedlings in 30 per cent, carbon 



1 See "The New Phytolot'ist," Vol. XIX., p. 208. 19-20. 

 ^Photograidiic lantern slides of these results were shown. 



