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 Aesculus.  The curvature often amounts to a rectangle,—­that is, the terminal part bends upwards until the tip, which is but little curved, projects almost horizontally.  Occasionally 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 downwards, 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 prolonged, for the tips of 15 radicles were rubbed for a short 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 exquisitely sensitive to the slightest pressure if prolonged, but not to two or three rough touches.

When the tip of a radicle is lightly touched on one side with dry nitrate of silver, the injury caused is [page 194] 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 transmitted 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.