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Show 532 SENSITIVENESS 'rO GRAVITATION. OnAP. XI. tlutn the cauterised; but this no doubt was accidental, for radicles of different ages grow at different rates, and the growth of different individuals is likewise affected by unknown causes. Tl1e state of the tips of these three radicles, which bad been cauterised for a rather longer time than usual, was :1fl follows: the 'Llackened apex, or the pa.rt which had been actnally touched by the caustic, was succeeded by a yellowish zone, dnc probabl~· to the absorption of some of the c.austic; in A, hoth 7.0neR together were 1·1 mm. in length, and 1·4 mm. in diameter at the base of the yellowish zone; in B, the len gth of both was only 0·7 mm., and the diameter 0·7 mm.; inC, the length was O·R mm., and the diameter 1·2 rnm. Three other radicles, the tips of which had been touched with caustic during 2 or 3 seconds, remained (temp. 58°-59° F.) horizontal for 23 h.; the control radicl es having, of course, become geotropic within this time. The terminal growing part, 10 mm. in length, of the cauterised radicles had increased in this interval to a mean length of 24 · 5 mm., and of the controls to a mean of 26 mm. A section of one of the cauterised tips showed that the blackened part waR 0 · 5 mm. in length, of which 0 · 2 mm. extended into tht: vegetative point; and a faint discoloration could be detected even to 1 · 6 mm. from the apex of the root-cap. In another lot of six radicles (temp. 55°- 57° F.) tl1o three control specimens were plainly geotropic in 8~ h. ; and after 24 h. the mean length of theh· terminal part had increased from 10 mm. to 21 mm. When tho caustic was applied to the three cauterised specimens, it was held quito motionl ess during 5 seconds and the result was that the black marks were extremely ~inute. Therefore, caustic was again applied, after 8~ b., during which time no geotropic action had occ.~recl. 'When the specimens wore re-examined after an add1t10nal interval of 15~ h., one was horizontal and the other two showed, to our surprise, a trace of geotropism which in one of them soon afterwards became strongly marked; but in this latter specimen the discoloured tip was only -~ mm. in length. 'The growing part of these three radicles increased in 24 h. from 10 mm. to an average of 16 · 5 mm. . It would be superfluous to describe in detail the behavJ~ur of the 10 remaining cauterised radicles. The correspon~ng control specimens all became geotropic in 8 h. Of the cautensed, G were first looked at after 8 h., and one alone showed a trace CnAP. XI. TRANSl\II'rTED EFFEU'l'S: VICIA. f eotropism; 4 were first looked at after 14 h., and one alone ~f~hese was slightly geotropic. After ~3-24 h., 5 of th~ 10 were till horizontal, 4 slightly, and 1 dec1dodly, geotropw. After ~h. some of them became strrmgly geotropic. Tho cauterised radicles increased greatly in length, but the measurements arc not worth giving. As five of the last-mentioned cauterised radicles had become in Uh.somewhat geotropic, these (together with three which wore still horizontal) had their positions reversed, so that their tips were now a little upturned, and they were again touched with caustic. After 24 h. they showed no trace of geotropism; w heroas the eight corresponding control specimens, which had likewise been reversed, in which position the tips of several pointed to the zenith, all became geotropic; some having passed in tho 24h. through an angle of 180°, others through about 135°, and others through only 90°. The eight radicles, which had been twice cauterised, were observed for an additional day (i.e. for 48 h. after being reversed), and they still showed no signs of geotropism. Nevertheless, they continued to grow rapidly; four wero measured 24 h. after being reversed, and they had in this time increased in length between 8 and 11 mm. ; the other four were measured 48 h. after being reversed, and the. ·e had increased by 20, 18, 23, and 28 mm. Incoming to a conclusion with respect to the effects of cauterising the tips of these radicles, we should bear in mintl, firstly, that horizontally extended control radicles were always acted on by geotropism, and became somewhat bowed downwards in 8 or 9 h.; secondly, that the chief scat of tho curvature lies at a distance of from 3 to 6 mm. from the tip; thirdly, tha.t the tip was discolomed by the caustic rarely for more than lmm. in length; fourthly, that the greater number of tho ca.uterised radicles, although subjected to the full influence of geotropism during the whole time, remained horizontal for 24 h., and some for twice as long; and that those which did become bo~lld were so only in a slight degree; fifthly, that the cantensed radicles continued to grow almost, and sometimes quite, as well as the uninjured ones along the part which bends most. And lastly, that a touch on the tip with caustic, if on one ·ide, far from preventing curvature, actually induces it. Bearillg all !~ese facts ~n mind, we must infer that under normal conditions ~geotropic curvature of the root is duo to an influence transnutted from tho apex to the adjoining part where the bending |
