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Show 540 RENSITIVENESS '1'0 GHAVI'l'ATlON. CHAP. XI. the l) h. 30 m. to a mean length of 13 mm. ; and in the controls to 14·3 mm. In n. third trial the tips of five radicles (exposed to a temp. of 70°-71 °) were touched with the caustic only once and very slightly; they were afterwards examined under tl1c microscope, and the part which was h1 any way cbi::;colonred was on an average ·76 mm. in length. After 4 h. 10m. none.were b~nt ; after 5 h. 4:5 m., and again after 23 h. 30 m., they still rornamcd horizontal, excopting one which waR now iHclinecl 20° beneath tl1e horizon. Tho terminal part, 10 mm. in length, had increased o·reatly in length during the 23 h. 30 m., viz., to an average of 26 mm. Four control m~li c l es became slightly geotropic after the 4 h. 10 m., and plamly so aftm: the 5 h. 4? m. Their mean leno-th after the 23 h. 30 m. had mcreasecl from 10 mm. to 31 m~. Therefore a light cauterisation of the tip checks slightly the growth of tho whole radicle, and manifestly stops the bending of that part which ou ~ht to ~)end m~st ~nder tho influence of geotropism, and which shU contmues to increase greatly in length. Ooncz~~ding Remarks.-Abnndant evidence has n~w been given, showing that with various pLtnt~ the tlp of the radicle is alone sensitive to geotropism; and that when thus oxcitod, it causes the adjoining parts to bend. Tho exact l ngth of the sensitive part seems to be somewhat variable, depending in part on t~e nge of the radicle . but tho destruction of a length o~ from less than 1 to' 1·5 mm. (about !21oth of an inch), m the several species observed., gonm·ally sufficed to prevent any part of the radicle from bondm· g w1· t ]u ·n 2. 4 h. , or even for a longer pon.o d . Tll e f e:tc· t of tho tip alon.e beino· sons1· t1· ve 1· s so romar1 m ble a fact ' that w. e WI 11 here bg 1. ve a bn.e f summary of t h e f or.e bcr ol• n g expornne. nts. rrhe tins were cut off 20 hon.z ontal l. y ex t en dod ra.d w 1e s of V-.w ~~a faba, and w1. th a few excep t1' 0.ns they chd. not 1 t d become geotropw. 1. n 22 or 23· ll ., wb il.s t unm. uti a eIt radiclos were always bowed downwards m 8 or 9 h. · should be borne 1. n m1. n d t h a t tho nl er·e act of cuttmg Ci!AP. XI • 'l'RA~N SMI'I"fED El~FECTS : CONCLUSION. 541 ff the tip of a hori~ontally extended radicle does n(1t ~revent the adjoining parts from bonding, if the tip has been previously exposed for an hour or two to tho 'nfluence of geotropism. The tip after amputation is ~ometimes completely reg nerato<l in three days; and it is possible that it may b • <.tble to transmit an impulse to the adjo~ning pt~rts lm.foro its complo.te re()'eneration. rrhe tips of SlX rachclrs of Oucurb'lta ovlfera wore amputated lil~e those of Vic~a f~ba; and these radicles showed no signs of gootrop1sm 1n 24 h. ; whereas the control specimen· were slightly affecte<l in 5 h., and strongly in 9 h. With plants belonging to six genera, the tips of th -• radicles were touched transvers -ly with dry caustic ; and the injury thus caused rarely oxteu(lod for a greater length than 1 mm., and sometimes to a less distance, a · judged by even the faintest discoloration. We thought that this would be a better method of destroying th vegetative point than cutting it off; for we knew, from many previous experiments and from some given in the present chapter, that a tonch with caustic on one side of the apex, far from pr venting the adjoining part from bending, caused it to bond. In all tJw following cases, radicles with uncantcrised tips wer<' observed at the same time and mH.lor similar circumstances, and they became, in almost every instance, plainly bowed downwards in one-half or one-third of the time during which the can tori eel specimens wore observed. With Vicia faba 10 rudielos wore cauterised; 12 remained horizont<'tl during 23-24 h.; 6 became slightly and 1 strongly geotropic. Eight of these radicles were afterwards r eversed, and again touched with caustic, and none of them became geotropic in 24 h., whilst the r vors d control specimens became strongly bowed <lowuwanls within this time. |