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Show 148 SENSITIVENESS OF THE APEX CHAl'. III. of si.z e of an ob ~' ect which will act on the rha dibc'l e off the b ean. B u t I't I·s remarkable that when t e 1· tsk lo bristle did act, that they should have acted so qlilc y and efficiently. · h d As t h e apex of a radicle in penetratm. g t e groun must b e presse d on . all . sides ' we wish·el d . to 1e arn. whether it could distmgmsh between hare er or more resi.s t'm g, and softer substances. A squar.e off thet s.a nde]d 1 t as stiff as card and a square o ex Ierne y P:?er, a mos(too thin for w~·iting on), of exactly the ts amme psa1pzeer ( a b ou t lth of an inch) were fixed with "2U ' d d shellac on opposite sides of the apices of 12 susp~n ed ra d1. 0l es. 'rhe sanded card was between f0 ·1;::·, a1n) 0. 20 mm. (or between 0. 0059 and 0. 0079 o an me 1' and the thin paper only 0. 045 mm. (or 0 . 00176 of an m. eh ) I. n thl'C k n ess . In 8 out of the 12 casesd tfh ere could be no doubt that the radicle was deflecte rom the side to which the card-like paper was atta~hed, and towards the opposite side, bearing the very th~n paper. This occurred in some instances in 9 h., but m others not until 24 h. had elapsed. Moreover, some _of th~ four failures can hardly be considered as r~ally farlu.res i thus in one of them, in which the radicle remame~ quit~ straight, the square of thin paper was fo~nr' when both were removed from the apex, to have ecn so thickly coated with shellac that it was al~ost as stiff as the card : in the second case, the radrele ~as bent upwards m. to a semr· c·u c1 e , b ~1 t the deflectwn was not directly from the side bearmg the card, and this was explam. ed by t h e t wo squar.e s having becomifei' cemented laterally togeth~r, forming a .sort. 0~ s:he gable from which the rachcle was deflected · md b third ' case the square of card h a d 1) een fi"x e ti' Y mistake in' front and thong h t h ere wa s doflec on from it, this migh't have been due to S ac h 8, ' cmva' ture: CHAP. III. OF THE RADICLE OF THE BEAN. 149 in the fourth case alone no reason could be assigned why the radicle had not been at all deflected. These experiments suffice to prove that the apex of the radicle possesses the extraordinary power of discriminating between thin card and very thin paper, and is deflected from the side pressed by the more resisting or harder substance . Some trials were next made by irritating the tips without any object being left in contact with them. Nine radicles, suspended over water, had their tips rubbed, each six times with a needle, with suffi ·ient force to shake the whole bean ; the t emperature was favourable, viz. about 63° F. In 7 out of these cases no effect whatever was produced; in the eighth case the radicle became slightly deflected from, and in the ninth case slightly deflected towards, the rubbed side; but these two latter opposed curvatures were probably accidental, as radicles do not always grow perfectly straight downwards. The tips of two other radicles were rubbed in the same manner for 15 seconds with a little round twig, two others for 30 seconds, and two others for 1 minute, but without any effect being produced. We may therefore conclude from these 15 trials that the radicles are not sensitive to tom porary oontact, but are acted on only by prolonged, though very slight, pressure. We then tried the effects of cutting off a very thin slice parallel to one of the sloping sides of the apex, ~.we. thought that the wound would cause prolonged 1rritatwn, which might induce bending towards the opposite side, as in the case of an attached object. 'rwo preliminary trials were made: firstly, slices were c~t from the radicles of 6 beans suspended in damp a1r, with a pair of scissors which thouO'h sharp probably caused considerable ' crushing' , and b no curva-' |