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Show 536 SENSI'l'IVENESS '1'0 GRAVITATION. CHAr. XI. that is to say, rather more than 6 mm. as measured from the a . of tho_ root-cap. Nino otJ~or. radiclos w~ro similarly oxtenle~~ throe m damp peat and SIX m damp air, and dry caustic was hold transversely to their tips during 4 or 5 , cconds. 'l'hree of their tips wore afterwards examined: in (1) a length of 0 · 68mm. was discoloured, of which tho basal 0 ·136 mm. was yellow tho npir.al part being black; in (2) tho discoloration was 0·65 ~1m. in length, of which tho basal 0 · 04 mm. was yellow; in (3) the di .. coloration was 0 · 6 mm. in length, of which tho ha. at O·l3mm. was yellow. Therefore less than 1 mm. was affected by the caustic hut this sufficed almost wholly to prevent geotropic action ; fo; after 24 h. one alone of the nino cauterised radieles became ::;lightly geotropic, being now inclined at 10° beneath the horizon · tho eight others remained horizontal, though one was curved~ 1i ttl o lt.t tcrally. The terminal part (10 mm. in length) of the six cauterised radicles in the damp air, had more than doubled in length in the 24 h., for this part was now on an average 20 ·7 mm.long. The increase in length within the same time was greater in tho control specimens, for the terminal part had grown on an average from 10 mm. to 26 · 6 mm. But as the cauterised mdicles had more than doubled their length in the 24 h., it is manifest that they had not been seriously injured by the caustic. W c may hero add that when experimenting on the effects of touching one side of the tip with caustic, too much \\·as applied at first, and the whole tip (but we believe 11ot more than 1 mm. in length) of six horizontally extended radicles was killed, and these continued for two or three days to grow out horizontally. Many trials were made, by coating the tips of horizontally uxtendcd radicles with the before described thick grease. The geotropic curvature of 12 radicles, which were thus coateu for <t length of 2 mm., was delayed during the first 8 or 9 h., but after 24 h. was nearly as great as that of the control specimens. The tips of nine radicles wore coated for a length of 3mm., and after 7 h. 10 m. these stood at an average anglo of 30° beneath the horizon, whilst tho controls stood at au average of 54°. After 24 h. the two lots differed but little in their degree of curvature. In some other trials, however, there was a fairly well-marked difference after 24 h. between those with greased tips and tho controls. 'I'he terminal part of eight control speci-. mons increased in 24 h. from 10 mm. to a mean length of CHAP. XI. 'l'RANSMI'l''l'ED EFFECTS; CUCURB£T..A. G37 2!•3 mm., whilst the mean increase of thos 'tl . 20 7 Tl c WJ 1 groasctl t1ps was . mm. 1? grease, therefore, slightly checked tho growth of tho tormmal part but this pa 't t • ' < r wus no much injured; for several rad10los which had been d f' length of 2 mm. con tm. ucd to grow during seven dg rease or a . ' ays, anc1 wore then only a httle shorter than the controls 'r1 . · 1c appearance presented by these radwlos after the seven day" h bl 1 · < o was very curious, fort e ac c grease had been d rawn ont into tl fi t longitu dm. a1 s t n. w, WJ' tl1 d ots and reticulations which lce no. sd their sur f•a ces f' or a 1e ng tl 1 o f' from 26 to 44 m' m., or oofv e1r ct o 1·7 inch .. Wo m.ay t.herofore conclude that grease on the tips of the rad1c~os ot this Phas~olus somewhat delays and los ·ons the geotropic curvature of the part which ought to lJcnd most. Gossypium he?·bacettm.-Tho ra.liclcs of this plant bond through the action of geotropism, for a length of about 6 mm~ Five radicl.es, place~ horizontally ~n damp air, had their tips touched w1th caustic, and the d1. ·coloration extended for a length of from ·~ to 1 mm. 'I'hcy showed, after 7 h. 45 m. and again after 23 h., not a trace of geotropism; yet the terminal portion, 9 mm. in length, had increased on an average to 15·9 mm. Six controlmdicles, after 7 h. 45 m., wore all plainly geotropic, two of them being vertically dependent, and after 23 h. all were vm·tical, or nearly so. Oucurbita OV'~fera.-A largo number of trials proved almost useless, from the three following causes: l! ir. tly, the tips of radicles which have grown somewhat old arc only feebly gootropic if kept in damp air; nor did we succeed well in our experiments, until the gem1inati11g seeds were placed in peat and kept at a rather high temperature. Secondly, the hypocotyls of the seeds which were pinned to the lids of the jars gradually became arched; and, as the cotyledons were fixed, tho movement of the hypocotyl affected tho position of tho radicle, and caused confusion. Thirdly, the point of the radicle is so fino that it is difficult not to cauterise it either too much or too little. But we managed gonera.lly to overcome this latter difficulty, as the following experiments show, which are given to provo that a touch with caustic on one side of the tip docs not prevent the upper part of the radicle from bonding. Ten radicles were laid horizontally beneath and on damp friable peat, and their tips were touched with caustic on the upper side. After 8 h. all were plainly geotropic, three of them rectangularly; after 19 b. |