THE POSITIVE RESPONSE 195 



We have hitherto dealt with the question of the suppres- 

 sion of the positive by the negative. It is sometimes possible 

 to unmask this suppressed positive by separation of the 

 two impulses brought about by the differential effect of cold. 

 Stimulus is at first applied on the stem at a distance near 

 enough to give rise to the resultant negative response. A 

 strip of cloth is then wound round the intermediate conduct- 

 ing portion of the stem, and the region gradually cooled. 

 The result of this cooling is to reduce the velocity of trans- 

 mission of true excitation. 



I have in this manner often succeeded, by careful and 

 gradual cooling of the intermediate region, in converting 

 a resultant negative into a diphasic — positive followed by 

 negative ; further cooling gave rise to the positive alone. 

 In the case of the diphasic response, the result was brought 

 about by the lowering of the velocity of transmission of 

 excitation by cold, in consequence of which the negative 

 lagged behind the positive. In the case of the conversion 

 into positive, the effect was due to the arrest of the excitatory 

 negative impulse. 



Direct and Indirect Effects of Stimulus 



The effect induced by a given stimulus is modified, as 

 we have seen, by its point of application. A stimulus 

 may be applied directly on the responding organ, or it 

 may be applied at a distance from it. Following the usual 

 terminology we shall designate the former as Direct Stimu- 

 lation, and the latter as Indirect Stimulation. When 

 stimulus is applied directly on the responding organ, there 

 is induced an excitatory fall of the leaf concomitant with 

 contraction and negative turgidity variation. This parti- 

 cular reaction we shall designate as the Direct Effect of 

 stimulus. When, on the other hand, a feeble stimulus is 

 applied at a distance, there occurs only a positive response 



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