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Show 146 SENSITIVENESS OF THE ArEX CHAP. III. after an interval of about 24 or more hour.s, ?ent t ar·ds the bit of still <:"Lttached card,-that 1s, m a ow . ] . 1 direction exactly opposite to the prevwus .y me need curvature of the whole growing part for a le11gth of from 7 to 8 mm. This occurred chicfiy.whcn the first curvature was small, and when an obJect had ~oen affixed more than once to the apex of the arne rndtelc. The attachm nt of a bit of card by shellac to one side of the tender apex may som times mechanically pr· event its growth ; or the applicati.on of thick gum- 1 . . . water more than once to the sam s1c. c may ll1Jlll'O 1t; d then checked growth on this sirlc with continued agnro wth on the opposite and una.IT?ctecl 'l ld Sll c wou account for the reversed curvature ot the apex. Various trials were made for ascertaining, as far as we could, the nature and cl ()'reo of irritation to which the apex must be subjected, in orucr tha~ tho terminal growing part should b nd aw<cty, as. 1f to avoid the cause of irritation. vV c have seen m the numbered experiments, that a little square o.f ruther thick letter-paper gummed to the apex ~nduce(l, though slowly, considerable dcfic?tion. Juclgmg from several cases in whieh various obJcets had been affixe(l with gum, and had soon be ·ome scptwatou from ~he apex by a layer of fluid, as well as from some tnals in which drops of thick gum-water alone had been applied, this fluid never causes b.ending. We ~~,~~ also seen in the numbered expenmcnts that nan~ ~plintcrs of quill and of very thin glass, affix.od wlth shellac, caused only a slight degree of dc.flectwn, ~:ld this may perhaps have been clue to the s~1e ~c itself'. Little squares of goldbcaters' skin, whwh 18 oxct'ssively thin, were clamped, and th?s made t~ adh(•rc to one side of the tips of two rac.hcles ; .on~:e thoo~', after 24 h., produced no effect; nor chcl CHAP. III. OF '£liE RADICLE OF THE BEAN. 147 other in 8 h., within which time squares of card usually act; but after 24 h. there was slight deii ction. 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. '.rho e specks were removed, and both together weighed l ss than riT-oth of a grain; so that a weight of rather ]c. s than mth of a grain (0. 32 mgs.) sufficed to excite movement in two out of the nine radicles. Here th~n we h~ve apparently reached nearly the minimum weight whwh will act. A moderately thick bristle (which on measurement w.as found rather flattened, being 0 · 33 mm. in one diameter, and 0 · 20 mm. in the other) was cut into lengths of about '2 th of an inch. Th se aft r beinO' touched with .thick grum-watcr, were placed on the tip~ of .eleven radwles. rhroe of them were aili ctcd ; one bemg deflected in 8 h. 15 m. to an angle of about 90° from the perpendicular ; a second to the same amount '~hen looked at after 9 h. ; but after 24 h. from the hme of first attachment the deflection had decreased to only 19°; the third was only slightly deflected after 9 h., and the bit of bristle was then found not atod u~h .'m g t he apex; it was replaced, and after 15 h dltwnal hours the deflection amounted to 26° from t e perpendicular. The rcmainin()' eio·ht radicles were n0 t t ll o b a a acted on by the bits of bristle so that we her e appear to have nearly reached the m'i nimum L 2 |
