S54 PLANT RESPONSE 



degree than the lower. This will be seen fully demon- 

 strated in heliotropic response, where the localised action of 

 moderate light on the upper surface will be found to induce 

 a movement upwards. In the present case, a given electrode, 

 carrying current, will be applied locally on the upper half of a 

 pulvinus, the second electrode being applied at a consider- 

 able distance on the stem, and the specific result recorded. 



For such local application, it will be understood that a 

 large pulvinus is an advantage, and therefore I used for my 

 experiment the pulvinus of Erythrina indica, the sensitive- 

 ness of which is not so marked as that of Mimosa. As 

 preventing the diffusion of stimulus to the lower side, this 

 lessened sensitiveness of the organ constitutes an added 

 advantage for the purposes of my experiment. The respon- 

 sive effect, it must be remembered, can be magnified to any 

 extent that is desired by the Optic Lever, and in the present 

 investigation a magnification of 200 times was employed. 

 If the effect of the given electrode (laid on the upper sur- 

 face), then, be to induce a responsive contraction, we can see 

 that the normally horizontal leaf will be moved upwards, 

 whereas a responsive expansion or relaxation would bring 

 about a movement downwards. In order, however, to show 

 the essential unity of all the different kinds of responses, 

 whether by mechanical or growth movements, we shall 

 regard them as cases respectively of the two fundamental 

 phenomena of expansion and contraction. Expansion thus 

 causes convexity or acceleration of growth, represented in 

 these records by up curves, while contraction, concavity, or 

 retardation of growth, is shown by down curves. 



In my experiment on the upper half of the pulvinus of 

 Erythrina, I first made it anode, the E.M.F. used being 

 twenty volts. In previous experiments on the anodic and 

 kathodic reactions of the pulvinus, it was the differential 

 effect that was observed, and the magnification employed for 

 the record was only slight. Hence the pure effect of anode- 

 make was found to be inconspicuous, whereas the anode- 

 break induced the usual marked contractile action. In the 



