538 PLANT RESPONSE 



root-tip by means of gum or shellac varnish. The moderate 

 and constant irritation which was produced in this way 

 was usually found to induce a convexity on the same side of 

 the growing region. His second method was chemical. 

 He touched one side of the tip with silver nitrate. This 

 also, generally speaking, induced a convexity similar to the 

 last. The third and last method of unilateral stimulation 

 employed by Darwin was a slanting cut, resulting, in the 

 majority of cases, in the same responsive curvature as before. 

 All these cases, it will be understood, are illustrations of the 

 indirect effect of moderate stimulation. 



But we have seen — in the second law of responsive 

 growth-curvature — that under the long-continued action of 

 a stimulus sufficiently strong, this first, or negative, effect is 

 neutralised by the subsequent conduction of excitation (p. 52). 

 The degree of such conduction and the power of neutralisa- 

 tion will obviously depend on the conducting power of the 

 particular specimen. Now, it was found by Darwin, in 

 several of his experiments, that the negative effect was 

 followed by neutralisation. This he regarded as somewhat 

 exceptional. We have seen* however, that such a result, 

 under certain circumstances, was to be expected. 



We have also seen, in the same law of responsive growth- 

 curvature, that when the transmitted stimulus is strong, 

 it induces a reversed or positive curvature. And in some 

 of Darwin's experiments also, especially those in which he 

 used the strong stimulation of caustic, this was observed. 



Finally, in some six cases, Darwin found that on 

 applying unilateral stimulation to the tip there followed 

 an induced concavity. With regard to this it need only 

 be said that the sensibility of the tip to direct stimulation 

 is not specifically different from that of any other tissue. 

 All tissues contract, in response to direct stimulation. The 

 difference between the contractile powers of the tip and 

 the growing region is one merely of degree, that of the 

 latter being relatively greater, and for certain reasons, to 

 be described later, the more evident. Visible contraction 



