CaAi III. SUMMARY OF CHAPTER. 193 



The curvature of the radicle sometimes occurs 

 within from 6 to 8 hours after the tip has been irritated, 

 and almost always within 24 h., excepting in the 

 case of the massive radicles of ^sculus. The curva- 

 ture often amounts to a rectangle, — that is, the ter- 

 minal part bends upwards until the tip, which is but 

 little curved, projects almost horizontally. Occa- 

 sionally the tip, from the continued irritation of the 

 attached object, continues to bend up until it forms a 

 hook with the point directed towards the zenith, or 

 a loop, or even a spire. After a time the radicle 

 apparently becomes accustomed to the irritation, as 

 occurs in the case of tendrils, for it again grows down- 

 wards, although the bit of card or other object may 

 remain attached to the tip. 



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 upwards 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. The weight of' 

 the object, though quite insignificant, is opposed 

 to the upward curvature. We may therefore conclude 

 that it is the irritation due to contact which excites 

 the movement. The contact, however, must be pro- 

 longed, for the tips of 15 radicles were rubbed for a 

 "hort time, and this did not cause them to bend. Here 

 then we have a case of specialised sensibility, like 

 that of the glands of Drosera ; for these are ex- 

 quisitely sensitive to the slightest pressure if prolonged, 

 bnt not to two or three rough touches. 



When the tip of a radicle is lightly touched on one 

 fiide with dry nitrate oi' silver, the injury caused is 



