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Show 572 SUMMARY AND CHAP. XII. astonishingly small stimulus suffices; and even with allied plants one may be highly sensitive to the slightest continued pressure, and another highly sensitive to a slight momentary touch. The habit of moving at certain periods is inherited both by plants and n.nimals; and several other points of similitude have been specified. But the most striking resemblance is the localisation of their sensitiveness, and the transmission of an influence from tho excitocl part to another which consequently moves. Yet plants clo not of course possess nerves or a central nervous system ; and we may infer that with animals such structures serve only for the more perfect transmission of impressions, and for the more complete intercommunication of the several parts. vV e believe that there is no structure in plants more wonderful, as far as its functions are coneerned, than the tip of the radicle. If the tip be lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side; and, what is more surprising, the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. If, however, the radicle is pressed by a similar object a little above the tip, the prossed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. If the tip perceives the air to be moister on one side than on the other, it likewise transmits an influence to the upper adjoining part, which bends towards the source of moisture. When the tip is excited by light (though bende Pflanzensnbstanz dernrt innerlich diffet·enzirt, dass einzelne Theile ruit specifischen Euergien ausgeriistet sind, ahn-licb wie die verschiedenen Sinnes· ner~en des 'l'hiere' (' Arbeiten des Bot. Inst. in W[irzburg,' Bd. ii. 1879, p. 282). CHAP. XII. CONCLUDING REMARKS. 573 in the case of radicles this was asce ·t · d . • c r ame m onl single mstance) the adjoining part b d f' Y a . h . en s rom th hght; but w en exerted by gravitat1· th e bends towards the centro of crravity onI e same pa 't 1 'r 0 • n a most eve case we can clearly perceive the final p . ry Uipose or advan-mge of the several movements. Two 0 . ·h of the exC· it·m g causes often act simu' lta1 per aps more 1 ' 'neous y on tho tip, and one conquers the other, no doubt · ~ ance wr· t h r· ts I· mportance for the life of trhn ea ccord- 1 1 1he course pursued by tho radicle i11 pen t t' P ant. era mg the ground must be determined by the tip. h . ha s acqu.u o d sue h dr' verse kincls of sensitiv, e ence Ir t . h dl . ness. t IS ar y an exaggeratwn to say that the ti f th radicle thus endowed, ancl having the pp 0 ef di . h ower o rectmg t e movements of the adJ'oining part t lik h b . f f s, ac s e t e ram o one o the lower animals . th b · b . 1 . h. h , e rarn em.g . seate. ( wit .I n t e anterior end of th e bo d y, recemng ImpressiOns from tho sense-oro·an d d. . h 1 o s, an 1rectmg t e severa movements. |