INQUIRY INTO POSITIVE GEOTROPISM 537 



was in these cases subjected to strong unilateral stimulation 

 by the proximity of a heated platinum wire. Instead of 

 responding, however, by movement away, as would have 

 been the case had the advantage of the plant been the 

 primary object of its action, the root-tip gave only a 

 preliminary spasmodic twitch in the negative direction, and 

 then reversed its movement ; the organ now turned towards 

 the heating wire, fell upon it, and was burnt.^ There is thus 

 no protective adaptation here, any more than in the case of 

 the moth which is impelled to throw itself upon the destroy- 

 ing flame. No choice exists for either of these, for, in both 

 alike, the movement is due to the working of the inexorable 

 laws which govern the phenomenon of response. In the 

 case of the plant, that increased turgidity which is the in- 

 direct effect of a distant stimulus always induces an increased 

 rate of growth ; but when the stimulus is sufficiently strong or 

 long continued, its direct or true excitatory effect, being trans- 

 mitted, brings about retardation of growth. And each of 

 the diverse curvatures of growth, induced by the unilateral 

 application of stimulus, forms a particular case of the work- 

 ing out of these two laws, of the direct and indirect effects of 

 stimulation. 



Having now arrived at certain definite conclusions as 

 to the mechanism of the responsive curvature induced by 

 unilateral stimulation of the tip of root or shoot, it will be 

 well to pass in rapid review the exhaustive series of experi- 

 ments on the responsive behaviour of the tip of the radicle, 

 which were carried out by Darwin, more especially as some 

 of the cases which he noted as somewhat exceptional will be 

 found, in view of the investigation which I have described, 

 to be susceptible of very simple explanation. He pro- 

 duced unilateral stimulation in three different ways, first, by 

 attaching minute fragments of cardboard to one side of the 



' The same thing happens, in a stiU more striking manner, when the wire is 

 placed in front of the growing region, and suddenly heated. The excitatory effect 

 on the proximal side is then so great that the organ at once rushes upon the 

 stimulating wire and is scorched. This occurs before the distal side could be 

 excited by the transmitted effect of stimulus. 



