384 



Sir J, C. Bose and Mr. G. Das. 



curvature, its longitudinal transmission has the indirect effect of inducing a 

 negative curvature. 



9. Greotropism. 



No phenomenon of tropic response is so inexplicable as the opposite effects 

 of stimulus of gravity on the root and the shoot. The experiment that has 

 just been described will, I think, offer an explanation of these diametrically 

 opposite effects. In the root the stimulus is received at the tip and trans- 

 mitted to the region of growth at some distance ; stimulation here is there- 

 fore indirect. In contrast with this is the fact that the growing region of 

 the shoot is both sensitive and responsive. Hence the stimulation in this 

 case is direct. As the effects of direct and indirect stimulation on growth 

 are antithetic, the responses of shoot and root to the direct and indirect 

 actions of stimulus must be of opposite signs. 



I have carried out other experiments for determining the effects of 

 direct and indirect stimulation on the root itself. Unilateral stimulation of 

 the tip* by thermal radiation induced a negative curvature, while direct 

 stimulation of the growing region brought about a positive curvature.f 



These results indicate that there is no necessity for postulating two 

 different irritabilities for the shoot and the root, since tissues in general 

 exhibit positive or negative curvature accordingly as stimulation is direct or 

 indirect. 



10. Tropic Curvature with Transverse Transmission of Stii/iulus. 



We have next to consider a very large class of phenomena arising out of 

 the direct stimulation of one side, and its transversely transmitted effect on 

 the opposite site. The unilateral stimuli to which the plant is naturally 

 exposed are those of contact, of light, and of thermal radiation. There is 

 besides the stimulation of the electric current. I shall presently show that 

 these tropic curvatures are determined by the definite effects of direct and 

 indirect stimulations. The twining of tendrils by unilateral contact will be 

 taken first, as presenting fewest complications. 



11. Mechanotropism ; Tvnning of Tendrils. 



In experiments on the effect of mechanical friction on growth, I have 

 found that its direct effect is an incipient contraction and retardation of 

 growth ; the incipient contraction culminates in an actual contraction under 



* Darwin's experiments on the curvature brought about by unilateral contact or 

 injury of the root-tip show that other stimuli induce an effect similar to that of 

 gravitation. 



t For results of further investigation on the subject cf. 'Plant Eesponse,' p. 537. 



