* ‘Arbeiten Bot.  Instit., Würzburg,’ Heft iii. p. 456. [page 146]

after an interval of about 24 or more hours, bent towards the bit of still attached card,—­that is, in a direction exactly opposite 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 this 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 away, as if to avoid the cause of irritation.  We have seen in the numbered experiments, that a little square of rather thick letter-paper gummed to the apex induced, though slowly, considerable deflection.  Judging from several cases in which various objects had been affixed with gum, and had soon become separated from the apex by a layer of fluid, as well as from some trials in which drops of thick gum-water alone had been applied, this fluid never causes bending.  We have also seen in the numbered experiments 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 the [page 147] other in 8 h., within which time squares of card usually act; but after 24 h. there was slight deflection.

An oval bead, or rather cake, of dried shellac, 1.01 mm. in length and 0.63 in breadth, caused a radicle to become deflected at nearly right angles in the course of only 6 h.; but after 23 h. it had nearly straightened itself.  A very small quantity of dissolved shellac was spread over a bit of card, and the tips of 9 radicles were touched laterally with it; only two of them became slightly deflected to the side opposite to that bearing the speck of dried shellac, and they afterwards straightened themselves.  These specks were removed, and both together weighed less than 1/100th of a grain; so that a weight of rather less than 1/200th of a grain (0.32 mg.) sufficed to excite movement in two out of the nine radicles.  Here then we have apparently reached nearly the minimum weight which will act.