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 sensi- 

 tive to a slight momentary touch. The habit of moving 

 at certain periods is inherited both by plants and 

 animals ; and several other points of similitude have 

 been specified. But the most striking resemblance is 

 the localisation of their sensitiveness, and the transmis- 

 sion of an influence from the excited part to another 

 which consequently moves. Yet plants do not of coui'se 

 possess nerves or a central nervous system; and we 

 may infer that with animitls such structures serve only 

 for the more perfect transmission of impressions, and 

 for the more complete intercommunication of the 

 several parts. 



We believe that there is no structure in plants more 

 wonderful, as far as its functions are concerned, 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, M'hat is more surprising, the tip 

 can distinguish between a slightly harder and softer 

 object, by which it is simultaneously pressed on oppo- 

 site sides. If, however, the radicle is pressed by a 

 similar object a little above the tip, the pressed 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 



beude Pflanzensubstanz derart lioh,wiedieversohiedenen Sinnee- 



iniieilich differenziit, dass ein- nerven des Tliiere' ('Arbeiten 



zelae Theile niit specifischpn des Bot. Inst, in Wiirzburg,' Bd. 



Eiiergien ansgeiiistet siud, iihn- ii. 1879, p. 2S2). 



