272 LIFE MOVEMENTS IN PLANTS 



organ, if the stimulus be feeble or if the intervening <lis- 

 tance be long or semi-conducting. Hence moderate stimulus 

 applied at a distance gives rise only to positive response ; 

 direct application of strong stimulus gives rise, on the 

 other hand, to the normal negative. By employing the 

 electric method of investigation, I have obtained with 

 ordinary tissues the positive, the diphasic, and the negative 

 electric response, in correspondence with the responses 

 given by a motile organ (p. 214). The mechanics of 

 propasation of the positive and the negative impulse are 

 different. It is therefore necessary to distinguish the 

 quick transmission of the positive impulse from tiie slow 

 rond'iicUon of the negative impulse due to the propagation 

 of excitatory protoplasmic change. 



It should be borne in mind in this connection that all 

 responsive movements are ultimately due to protoplasmic 

 changes which are beyond our scrutiny. We can infer the 

 nature of the change by the concomitant outward mani- 

 festations, which are of two kinds : the jjositii'e, associated 

 with increase of turgor, expansion, and galvanometric posi- 

 tivity, and the negative with concomitant decrease of turgor, 

 contraction, and galvanometric negativity. Thus positive 

 and negative reactions indicate the fundamental protoplas- 

 mic changes of opposite characters. 



The movement and curvature iniluced by stimulus 

 have, for convenience, been distinguished as positive curva- 

 ture. (movement towards stimulus), and negative cii,rv(tii>'r 

 (movement away from stimulus). Though these curvatures 

 result from protoplasmic reactions, yet the jjositive ciirvi- 

 ture is not necessarily associated with positive protopla^^mic 

 reaction. It will be shown that the curvature of an organ 

 is determined by the algebraical summation of effects 

 induced at the proximal and distal sides of the responding 

 orjjfan. 



