120 CHAP. XII. MECHANISM OF PHOTOTROPIC CURVATURE 



positive response occurred after an interval of 3 • 5 seconds ; 

 the failure of the negative impulse to reach the pulvinus 

 may be ascribed to the feeble conductivity of the stem. 



Stimulus was now applied at Si at a distance of 

 20 mm. The response was positive, an erectile movement 

 of the leaf; this was followed by the excitatory negative, 

 a rapid fall of the leaf as indicated by the scratch-line 

 in the up-curve. The positive response was initiated 

 after an interval of 2 seconds, while the negative occurred 

 after 10 seconds. The velocity of the positive impulse under 

 indirect stimulation is thus greater than that of the excita- 

 tory negative impulse of 2 mm. per second, obtained by 

 dividing the distance of 20 mm. by the interval of time, 

 10 seconds. 



Finally, the stimulus was applied at S2 at a distance of 

 10 mm. from the pulvinus. The response was now only 

 excitatory negative, which was initiated after 5 seconds, 

 the velocity of conduction being the same as before, namely, 

 2 mm. per second. The positive response is here masked 

 by the more intense excitatory negative reaction. 



1 he results afford a basis for a rational explanation of 

 various tropic movements, which is only possible by taking 

 account of the two impulses, the existence of which is 

 fully established. 



Experiment 69. Impulse under indirect electric stimula- 

 tion. — Exactly parallel effects were obtained under electric 

 stimulation — that is to say, the response was positive when 

 the stimulus was applied at a relatively long distance ; a 

 diphasic response, positive followed by negative, occurred 

 when the stimulus was applied at a short distance. Applica- 

 tion of stimulus at or near the responding organ gave rise 

 only to negative response. 



I will next show that similar positive and negative re- 

 sponses are obtained when the stimulus is applied on one side 

 of the stem, the transmission being now in the transverse 

 instead of in the longitudinal direction. 



