IRRITABILITY. 495 



injury, which is followed by a more active growth of that side 

 so that it becomes curved. The investigations of Detlefsen 

 and of Burgerstein have led them to conclusions similar to 

 those of Wiesner. They find that the root can grow, without 

 deflection, against a considerable resistance : for instance they 

 observed that radicles of Vicia Faba are capable of perforating 

 tin-foil of the thickness of O'O2 mm. ; and they attribute the 

 Darwinian curvature simply to the injury which is inflicted, 

 in making the experiment, on one side of the root-tip. 

 Detlefsen points out that the adhesive materials used by 

 Darwin, including gum-water, injure the root in greater or 

 less degree and induce curvature, and that the application 

 of a very thin film of glass to the root, without any adhesive 

 material, caused at first the Darwinian curvature and sub- 

 sequently the disorganisation of the tissue in the area covered 

 by it. He finds, too, that a Darwinian curvature can be in- 

 duced in other organs he instances the younger leaves of 

 the Onion by injury of one side of the growing region. He 

 shews, further, that the curvature depends upon injury of the 

 root-cap, and not upon irritation of the actual growing-point 

 of the radicle : injuries inflicted upon the radicle as far as 

 5 mm. from the apex, to which length the root-cap extended, 

 caused distinct curvature. 



Taking all these observations into consideration, and 

 especially the fact that the Darwinian curvature differs from 

 all other curvatures induced by contact or pressure in that 

 it is the irritated side of the organ which becomes convex, 

 it seems probable that Wiesner is right in regarding it as 

 a pathological phenomenon due to injury, or at least to a 

 disturbance of the normal conditions of growth. 



The curvatures which are the result of contact are, like 

 those produced by light, gravity, etc., to be regarded as phe- 

 nomena of induced heterauxesis. -A discussion of the mech- 

 anism by which they are effected will also be deferred to 

 a subsequent lecture. 



