CHAP. XI. TKANSMITTED EFFECTS : CONCLUSION. 513 



that after the tip has been destroyed this part goes on 

 growing at such a rate, that its length was often doubled 

 in a day. We have also seen that the destruction of the 

 tip does not prevent the adjoining part from bending, 

 if this part has already received some influence from 

 the tip. As with horizontally extended radicles, of 

 which the tip has been cut off or destroyed, the part 

 which ought to bend most remains motionless for 

 many hours or days, although exposed at right angles 

 to the full influence of geotropism, we must conclude 

 that the tip alone is sensitive to this power, and trans- 

 mits some influence or stimulus to the adjoining parts, 

 causing them to bend. We have direct evidence of 

 such transmission ; for when a radicle was left extended 

 horizontally for an hour or an hour and a half, by 

 which time the supposed influence will have travelled 

 a little distance from the tip, and the tip was then 

 cut off, the radicle afterwards became bent, although 

 placed perpendicularly. The terminal portions of 

 several radicles thus treated continued for some time 

 to grow in the direction of their newly-acquired curva- 

 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 were formed, the 

 radicles were again acted on by geotropism, and now 

 they curved themselves perpendicularly downwards. 

 To see anything of the above kind in the animal 

 kingdom, we should have to suppose that an animal 

 whilst lying down determined to rise up in some par- 

 ticular direction ; and that after its head had been cut 

 off, an impulse continued to travel very slowly along 

 the nerves to the proper muscles ; so that after several 

 hours the headless animal rose up in the predeter- 

 mined direction. 



As tho tip of the radicle has been found to be the 

 24 



