492 LECTURE XIX. 



of the radicle is sensitive to contact, and that the stimulus is 

 transmitted from this, the sensory organ, to the growing 

 zones behind it in which the necessary curvature was then 

 effected. In order to ascertain whether or not this was the 

 case, he made a number of experiments by attaching small 

 objects to one side of the tip of the radicle, in various plants, 

 by means of shellac or gum-water, or by touching one side 

 with dry caustic, or by cutting a thin slice off one side. He 

 vindicates his method of experimentation by pointing out, in 

 the first place, that it is evident that a small object attached 

 to the free point of a vertically suspended radicle can offer 

 no mechanical resistance to its growth as a whole, for the 

 object is carried downwards as the radicle elongates, or up- 

 wards as the radicle curves upwards. Nor can the growth of 

 the tip itself be mechanically checked by an object attached 

 to it by gum-water, which remains all the time perfectly soft. 

 Finally, when the tip is lightly touched on one side with dry 

 caustic (nitrate of silver) the injury caused is very slight. 

 The general result which Darwin obtained in a very large 

 number of experiments of this kind was that the radicle was 

 sensitive to one or other of these stimuli, the response being 

 that it curved in such a way that the side to which the small 

 object (a small piece of card) was attached, or which had 

 been touched with caustic, or from which a small slice had 

 been cut off, became convex. This result is a remarkable 

 one ; it appears that the response of the root to stimulation 

 is different from that of all other sensitive plant-organs. In 

 all the instances of sensitiveness given above, the induced 

 curvature was such that the stimulated side of the organ 

 became the concave side. Sachs, as already mentioned, found 

 that roots offer no exception to this rule, whereas, according to 

 Darwin, they do. The peculiar curvature observed by Darwin 

 in roots may be conveniently spoken of as the "Darwinian 

 curvature." 



The following details will illustrate the general nature of Darwin's 

 experimental results : 



Vicia Faba; tip of radicle sensitive to attached objects, to caustic, to 

 lateral slicing. 



