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Show 542 SENSI'riVENESS TO GHAVIT'ATIO:N. OuAP. XL \V.ith Pis·urn sat1:vwn, five r;.ulielcs harl t heir tips touched. with caustic, anll after 32 h. four wore still horir.ontal. 'J'he control specimens were slightly geotropic in 7 h. 20 m., n.nd strongly so in 24 h. The tips of 9 other radiclcs of this plant were toncheJ. only on the lower siclc, and G of them romn.incd horizontal for 2± h., or were uptnrn u in opposition to geotropi sm; 2 were slightly, ancl 1 plainly geotropic. \Vith Phaseol1is multijlorus, 15 nul.iclcs wuro cauterised, and 8 re· mainod horizontal for 24 h.; whereas nll the controls were plainly g otropic in 8 h. 30 m. Of 5 cauterised radiclos of Gos~;yp£tt 'Jn hm·baceum, 4 romaino<l horizontal for 23 h. and 1 became slightly o·eotropic ; 6 control radicles were distinctly geotropic in 7 h. 45 m. Five radicles of Oucurbita ovifera remained hori11ontal in peat-earth during 25 h., and !) remainc<l so in damp air during t)} h.; whilst the controls bccm110 slightly geotropic in 4 h. 10 m. Tho tips of 10 radieals of this plant were touched on their lower sides, and 6 of them remained. horizontal or were npturnod after 19 h., 1 being slightly and 3 strongly geotropic. Lastly, the tips of several radiclos of Vicia f aba and Phaseolus mtdtijlorus wore thicldy coated with grease for a length of 3 mm. 'rhis matter, which is highly injurious to most plants, <lid. not kill or stop tho growth of the tips, and only slightly lessoned the rate of growth of the whole ra.diclc ; but it generally delayed. a little the geotropic bending of the upper part. The several foregoing cases would tell us nothing, if the tip itself was the part which became most bent; but we know that it is a part distant from the tip by some millimeters which grows quickest, and which under the influence of O'eotropism, bends most. "'\Ve h~ve no reason to suppose that this part is injur~d by the death or iniury of the tip; and it is certam OnAP. XI. TRAN MITT ED EFFECTS : OONOL US! ON. 543 that ~fter the tip has boon ~ostroyoJ. this part goes 011 growmg at such a rate, that 1ts length wtts often doubled ina day. We have also soun that tho <1estruction of the tip does not prevent tho adjoining part from bendinoif this part has already received some influence fro~~ the tip. As with horizontally extended radicles of wh~ch the tip has been cnt off or ~cstroyed, tho ~art winch ought to bend most ronutms motionless for many hours or days, although exposed at right ano·les to the full influence of geotropism, we must concl~1de that the tip alone is sensitive to this power, anti transmits some influence or stimulus to the adjoinino- parts causing them to bend. W c have direct cvicl~nce of such transmission ; for when a radicle was loft extended ho~izon~ally for an hour .or an hour and a half, by whwh time the supposed ulfluence will have travelled a little . distance from the tip, and the tip was then cut off; the radicle afterwards became bent, although placed perpendicularly. Tho terminal portions of several radlCles thus treated continued for some time to grow in the direction of th ir newly-acquired cnrva.ture; for as they were destitute of tips, they were no longer acted on by geotropism. But after three or four days when new vegetative points wore formed, the radicles were again acted. on by geotropism, and now ~hey curved t~emsel vos perpendicularly uown wards. r? see anythmg of the above kind in the animal ki~gdom~ wo shoulU have to suppose that an animal ~hllst ly~ng ~own determined to rise np in some particular duectwn; and that after its head had been cut off, an impulse continued to travel vorv slowly alon()' the nerves to the proper muscles; so th~t after seven~ h~nrs the headless aninml rose up in the prcdotcrmmed direction. As the tip of the radicle has been found to be tho |
