14(3 SENSITIVENESS OF THE APEX Cjiap. HI. 



after an interval of about 24 or more hours, bent 

 towards the bit of still attached card, — that is, in a 

 direction exactly oiDposite to the previously induced 

 curvature of the whole growing part for a length of 

 from 7 to 8 mm. This occurred chiefly when the first 

 curvature was small, and when an object had been 

 affixed more than once to the apex of the same radicle. 

 The attachment of a bit of card by shellac to one 

 >side of the tender apex may sometimes mechanically 

 prevent its growth ; or the application of thick gum- 

 water more than once to the same side may injure it ; 

 and then checked growth on tliis side with continued 

 growth on the opposite and unaffected side would 

 account for the reversed curvature of the apex. 



Various trials were made for ascertaining, as far 

 as we could, the nature and degree of irritation to 

 which the apex must be subjected, in order that the 

 terminal growing part should bend a^^'ay, as if to 

 avoid the cause of irritation. \\^g have seen in the 

 numbered experiments, that a little square of I'ather 

 thick letter-paper gummed to the apex induced, 

 though slowly, considerable deflection. Judging from 

 several cases in which various objects had been afHxed 

 with gum, and had soon become separated from the 

 apex by a layer of fluid, as well as from some trials 

 in whicli drops of thick gum-water alone had been 

 applied, this fluid never causes bending. We have 

 also seen in the numbered ex])eriments that narrow 

 splinters of quill and of very thin glass, affixed with 

 shellac, caused only a slight degree of deflection, and 

 this may perhaps have been due to the shellac 

 itself. Little squares of goldbeaters' skin, which is 

 excessively thin, were damped, and thus made to 

 adhere to one side of the tips of two radicles ; one of 

 these, after 24 h., produced no effect ; nor did tho 



