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Show 536 SENSITIVENESS '1'0 GRAVITATION. CHAP. XI. that is to say, rather .m ore than 6 m.m . as measu. red from tllo nn pcx of the. root-cap. Nme otl:or. radiCios w?ro similarly extended, throe m damp pon.t n.nd SIX m damp an·, and dry cau, tic was hold transversely to their tips during 4 or 5 seconds. 'l'brec of their ~ips wore n.ftorwn.r~s examined: in (1) a length of 0·6Smm. wa~ discolouro.d, of wlucl: the bn.sal 0 ·136 mm. was yellow, the ll]_nral part bemg black; m (2) the discoloration wa· 0·65 rum. in length, of which the basal 0 · 04 mm. was yellow; in (3) tho discoloration was 0·6 mm. in length, of which the l>a.al O·l3mm. was yellow. Therefore less than 1 mm. was affected by tho caust.ic hut this sufficed almost wholly to prevent geotropic action; fo; after 24 h. one alone of tho nino cauterised radiclcs became slightly geotropic, being now inclined at 10° beneath the horizon· tho eight others remained horizontal, though one was curved~ little laterally. 'l'he terminal part (10 mm. in length) of tho six cauterised radicles in tho damp air, had more than doubled in length in tho 24 h., for this part was now on an average 20·7 mm.long. Tho increase in length within tho same time was greater in tho control specimens, for t110 terminal part bad grown on an a.verage from 10 mm. to 26 · 6 mm. But as the cauterised radicles had more than doubled their length in the 24 h., it is manifest that they hn.d not been seriously injured by the eaustic. W c may hero add that when experimenting on the oticcts of touching one side of t.bo tip with caustic, too much was applied at first, and tho whole tip (but we believe not more than 1 mm. in length) of six horizontally extended radiclos was killed, and these continued for two or throe days to grow out hori:wn tally. Many trials were made, by coating the tips of horizontally <;xtended radicles with the before described thick grease. 1'he geotropic curvature of 12 mdicles, which wore thus coated for it length of 2 mm., was delayed during the first S or 9 h., lll~t after 24 h. was nearly as great as that of the control specimens. The tips of nine radicles wore coated for a lm1gth of 3 mm., ;md after 7 h. 10 m. these stood at an average anglo of 30° beneath the horizon, whilst tho controls stood at an average of 54°. After 24 h. tho two lots differed but little in their degree of curvature. In some other trials, however, there was a fairly well-marked differenco after 24 h. between those with greased tips and tho controls. The terminal part of eight control specimens increased in 24 h. from 10 mm. to a moan length of CHAP. XI. TRANSMI'l'TED EFFECTS; CUCUll13ITA. 537 2t·3 rom., whilst the mean increase of those with grcasocl tips was 20·7 mm. Tho grease, therefor~, slightly checked the th Of the terminal part, but tlus part wus not much g·r o·w ed· . . d f for several radJClos which had boun grease or a !DJUf , · l length of 2 mm. continued to grow dtmng seven days, n.nc were then only a little shorter than the controls. The appearance presented by these radicles after the seven d~ys was very curious, for the bln.ck grease hacl boon. clraw_n ont m~o the finest longitudinal striro, with dots n.nd retJCnlatwns, wh1ch cover d their surfaces for a length of from 26 to 44 mm., or of 1 t.o 1·7 inch. We may therefore conclude that grease on tho tips of the radiclos of this Phaseolus somewhat delays and lcssonA • the geotropic curvature of tho part which ought to llcnd most. IJo~ypium herbaceum.-The ra licles of this plant bend, through the action of geotropi. m, for a lono-th of ahout G mm. Five radicles, placed horizontally in cbmp air, had their tips touched with caustic, and the discoloration extend d for a length of from ~ to 1 mm. They showo<l, after 7 h. 45 m. and again after 23 h., not a trace of geotropism ; yet tho terminal portion, 9 mm. in length, had incr a od on an average to 15·9 mm. Six control radiclos, after 7 h. 45 m., w re all plainly geotropic, two of them being vertically d. p ndont, and after 23h. all were vertical, or nearly so. Ot1curbita wifera.-A largo number of trials proved almost useless, from the throe following can cs: Fjrstly, tho tip of mdicles which have grown somewhat old ar only foehly gootropic if kept in damp air; nor diu wo Fmcceod w ll in our experiments, until the genninatiug seeds wore plac d in p at and kept at a rather high temperature. S 'conclly, the hypocotyls of the seeds which wore pinned to the lids of tho jars gradually became arched; and, as the cotylC'dom; were fixed, tho movement of the bypocotyl affected tho po ition of tho radi lo, and cau eel confusion. Thirdly, the point of tho racli lo is so fuw that jt is difficult not to cauterise it either too much or too little. But we m~nagcd generally to overcome this latter difficulty, a the followmg experiments show, which aro given to provo that a touch with caustic on one sido of tho tip clocs not pr v nt tho up~r part of the radicle from bomling. Ton radicles wore laid honzontally beneath and on damp friable peat, and their tips were touched with caustic on the upper side. After B h. all were plainly geotropic, three of them rectangularly; after 19 h. |
