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Show 192 SUMMARY OF CHAPTER. CnAP. III. fact that they would not have been scnsiti vo to somewhat greater continued pressure, if this could have been applied. The peculiar form of sensitiveness which we arc here considering, is confined to the tip of tho radicle for a length of from 1 mm. to 1 · 5 mm. vVhen this part is irritated by contact with any object, by caustic, or by a thin slice being cut of-f, the upper adjoining part of the ntdicle, for a 1 ngth of from G or 7 to even 12 mm., is excited to bend aw<ty from the side which has been irritated. Some influ nee must there· fore be transmitted from the tip along the radicle for this length. The curvature thus canseu is generally symmetrical. 'fhe part which bends most apparently coincides with that of the most rapid growth. ~J.1he tip and the basal part grow very slowly and. they bend very little. Considering the widely se.paratcu position in the veO'etable series of the several above-named genera, b we may conclude that the tips of the nuliclcs of all, or almost all, plants arc similarly scnsitiv , and transmit an influence causing the upper part to bend. With respect to the tips of the secondary radie.les, those of Vicia faba, Pis~~m sativum, and Zea mays were alone .observed, and they were found similarly sensitive. In order that these movements should be properly displayed, it appears necessary that tho radicles should grow at their normal rate. If subjected. to a high temperature and made to grow rapid.ly, the tips seem either to lose their sensitiveness, or th.e upper part to lose the power of bending. So. It appears to be if they grow very slowly from not bemg vigorous, or from being kept a.t too low a tempera~ure; also when they are forced to germinate in the m1ddle of the winter. CHAP. Ill SUMMARY OF CHAPTER. 193 The curvature of the radicle sometimes occ . h' f 6 urs w1t m rom to 8 hour~ a~ter the tip has been irritated, and almost always within 24 h., excepting in the case of the massive radicles of 1Esculus. The curvatn: e often amounts to a Tectangle,-that is, the ter~ mal part bends ~pwards until the tip, which is but l~ttle11curved,. proJects almost horizontally. OccaSlona y the tip, from the continued irritation of th attache~ object, c~ntinues to bend up until it forms : hook with the pOint directed towards the zenith a lo op, or even a spu. e. After a time the radi' coler appare~tly becomes accustomeu to the irritation, as occurs m the case of tendrils, for it again grows downward~, although the bit of card or other object may rema~n at~ached to the tip. !t lS evident that a small object attached to the free pomt o~ a ver~icall y suspended radicle can offer no m~han~cal r~sistance to its growth as a whole, for the obJect Is earned down wards as the radicle elono-ates or upwards as the radicle curves upwards. No~ ca~ ~he growt~ of the tip itself be mechanically checked y a_n obJect attached to it by gum-water which ~mal~~ all the time perfectly soft. The w:ight of to ~h 0 ~ect, though quite insignificant, is opposed that ~tupwahrd ?u~vat~re. We may therefore concluue th Is t e untatwn due to contact which excites Ione emd ovement . .T he con t act, h owever, must be.. pro-sh gt t~ for the tips of 15 radicles were rubbed fOl' a thor Ime, and this did not cause them to bend. Here th:~ wfe hhave a case of specialised sensibility like 0 t e glands of D . .1! h ' quisitel . . rosem.' lOr t ese are ex .. but ty sensitive to the slightest pressure if prolonged w~ to two. or three rough touches. ' side ~~the tip ?f a radicle is lightly touched on one WI dry mtrate of silver, the injury caused is 0 |