19 1 SUMMARY OF CHAPTER. CHAP. IIL 



very slight, and the adjoining upper part bends away 

 from the cauterised point, with more certainty in most 

 cases than from an object attached on one side. Here 

 it obviously is not the mere touch, but the effect 

 produced by the caustic, which induces the tip to 

 transmit some influence to the adjoining part, causing 

 it to bend away. If one side of the tip is badly 

 injured or killed by the caustic, it ceases to grow, 

 whilst the opposite side continues growing ; and the 

 result is that the tip itself bends towards the injured 

 side and often becomes completely hooked ; and it is 

 remarkable that in this case the adjoining upper part 

 does not bend. The stimulus is too powerful or the 

 shock too great for the proper influence to be trans- 

 mitted from the tip. We have strictly analogous cases 

 with Drosera, Dionaea and Pinguicula, with which 

 plants a too powerful stimulus does not excite the 

 tentacles to become incurved, or the lobes to close, or 

 the margin to be folded inwards. 



With respect to the degree of sensitiveness of the 

 apex to contact under favourable conditions, we have 

 seen that with Vicia faba a little square of writing- 

 paper affixed with shellac sufficed to cause move- 

 ment; as did on one occasion a square of merely 

 damped goldbeaters' skin, but it acted very slowly. 

 Short bits of moderately thick bristle (of which mea- 

 surements have been given) affixed with gum-water 

 acted in only three out of eleven trials, and beads of 

 dried shellac under j^yth of a grain in weight acted 

 only twice in nine cases ; so that here we have 

 nearly reached the minimum of necessary irrita- 

 tion. The apex, therefore, is much less sensitive to 

 pressure than the glands of Drosera, for these are 

 affected by far thinner objects than bits of bristla 

 and by a very much less weight than 3 Jjjth of a grain. 



