Researches on Growth and Movement in Plants. 



387 



here insist once more on the two opposite effects of heat, the thermal effect 

 which induces an acceleration and the radiation effect which causes a retarda- 

 tion of growth. In order to demonstrate this, I subjected this flower to 

 thermal radiation acting from all sides. The result is a movement which is 

 of opposite sign to the effect of rising temperature (fig. 11). 



14. Positive Plwtotropism. 



In the study of the responsive curvature induced by unilateral action of 

 light, we have to deal with the joint effects of the contraction of the proximal 

 and the expansion of the opposite side, which would, under normal condi- 

 tions, give rise to a positive curvature. 



By following the electric mode of investigation I have obtained inde- 

 pendent corroboration of the characteristic effects of direct and indirect 

 stimulation on the proximal and opposite sides.* The following Table gives 

 the responsive effects induced in pulvini and in growing tissues which act as 

 pulvinoids : — 



Table III. — Showing Responsive Effects Common to Pulvini and Growing 

 Organs under Unilateral Stimulation. 



Effect of direct stimulation on proximal side. 



Effect of indirect stimulation on distal side. 



Diminution of turgor 

 Galvanometric negativity 

 Contraction and concaTity 



Increase of turgor. 

 ' Galvanometric positivity. 



Expansion and convexity. 



When stimulus is strong or long continued, the true excitatory effect is conducted to the 

 distal side neutralising or reversing the first response. 



When a radial organ is subjected to the unilateral action of light, it 

 exhibits a positive curvature. Fig. 12 gives the positive response of the stem 

 of Dregea voluhilis to light of short duration from an arc-lamp ; the recovery 

 on cessation of light was complete. When the stimulation was stronger, and 

 continued for a longer time, the curvature was greater, and the after-effect 

 was more persistent. The positive curvature does not undergo any modifi- 

 cation in the case of thick organs with feeble transverse conductivity, for 

 neutralisation can only take place by the conduction of excitation to the 

 opposite side. 



I have said that the normal positive curvature was brought about by the 

 contraction of the proximal and the expansion of the opposite side. I made 



* 'Plant Response,' p. 515. 



