CHEMO-TROPISM AND GALVANO-TROPISM 547 



has doubtless been due to the action of these sources of varia- 

 tion that the observations made by investigators have so often 

 been contradictory. 



All these uncertainties disappear, however, when we 

 employ the high magnification, and the method of continuous 

 record, which have been described ; for by these means we 

 are enabled to follow the different phases of effect which occur 

 immediately on the application of stimulus, and during its 

 continuation. As these curvatures, moreover, are the results 

 of the unilateral application of stimulus, expressed in the 

 one-sided modification of longitudinal growth, the previous 

 demonstration of the effect of the diffuse application of the 

 same agent on growth itself enables us to infer the result 

 to be looked for. The experiment is thus resolved into a 

 verification of the inference. 



Again, I have shown that the growing region acts like a 

 diffuse pulvinoid. Experiments then, by unilateral applica- 

 tion of the particular stimulus to a true pulvinus, offer us 

 another and independent means of testing the results arrived 

 at. Thus we have no fewer than three distinct methods of 

 testing the action of a given agent in the induction of growth- 

 curvature, which, if they corroborate each other, may be 

 regarded as affording a rigorous demonstration of the results. 

 These are : first, the variation of longitudinal growth under 

 the diffuse action ; second, the responsive movement of a 

 pulvinus under the unilateral action ; and, third, the curvature 

 which represents the modification of growth induced by the 

 unilateral action of a given stimulus or agent. 



Method of application of chemical reag'ent. — In dealing 

 with problems involving the unilateral application of chemical 

 reagents particularly, the experimental arrangements which I 

 am about to describe will be found suitable. In order to 

 obviate the complications which might result from the indirect 

 effect of stimulus applied at a distance, the chemical reagent 

 should be applied directly on the growing region. For this 

 purpose the petals of various flowers, while in an active state 

 of growth, are very appropriate, and I have used particularly 



