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Show 526 SENSITIVENESS TO GRA Vrt'A'l'ION. CnAP. XI. were perfectly straight, and tl10se were 11lacod horizm1tall the peat, being covered hy a thin layer of it. They w01 . 0 ihon left for an averu.ge period of 1 h. 37m. Tho iips were then c~~ off trunsvm·sely for a length of 1·5 mm., and immc<liatcly afterwards they were embedded verticn.lly in tho peat. In th is position geotropism would not tend to induce nny curvature, hut if some infl.uence had alreu.<1y bo?n tran mittcd from tho tip to tho part which bends moRt, we m1ght expect iJHtt thi ::; part wonld become curved in ~he direction in which g otropil'>m had previously acted; for It shonl<1 be noted that th cF>e radi clos being now destitute of their sensitive tips, wonld not be prevented hy geotropism from curving in any direction. 1'hc result was that of the sixteen vertically embedded radiclcs, four continued for several days to grow straight downward::;, whilst twelve became more or less bowed latemlly. In two of tho twelve, a trace of curvature waR perceptible in 3 h. 30m., counting from tho time when they had first boon laid horizontally; and all twelve were plainly bowed in 6 h., a.nd still more plainly in 9 h. In every one of them tho curvature was directed towards tho side which had been downwards whilst the radicl os remained l10rizontal. The curvature extended for a length of from 5 to, in one instance, B mm., measured from the cut-off end. Of tho twelve bowed radiclos five became permanently bent i11to a right angle; the other seven were at fi1·st much less bent, and their curvature generally decreased after 24 h., but did not wholly diRappear. This decrease of curvature would naturally follow, if an exposure of only 1 h. 37 m. to gooti·opism, served to modify the turgescence of the cells, but not their subsequent growth to the full extent. The five radiclos which were rectallgularly bent became fixed in this position, and they continued to grow out horizontally in tho peat for a length of about linch during from 4 to 6 days. By this time new tips had been formed; and it should be remarked that this regeneration occurred slower in the peat than in water, owing perhaps to the mdicles being often looked at and thus disturbed. After the tips had been regenerated, geotropism was able to act on them, so that they now became bowed vertically downwards. An accurate draw· ing (Fig. 195) is given on the opposite page of one of these five radicles, reduced to half tho natural size. . We next tried whether a shorter exposure to gcotroplsiD would suffice to produce an after-eftoct. Seven racliclo~ were extended horizontally for an hour, instead of 1 h. 37m. as 111 the CuAP. XI. TRANSMITTED EFFEC'rS: VICIA. 527 former trial; and after their tips (1·5 mm. in length) had been amputated, they were pla.ced vertically in damp peat. Of those, three were not in tho least affected and continued for days to grow straight downwards. Four showed after 8 h. 30m. a mere trace of curvature in tho direction in which they had been acted on by geotropism; and in this respect they differed much from those which had been exposed for ,. 1 h. 37m., for many of tho latter l' lg. 195. were plainly curved in 6 h. The curvature of one of those four radicles almost disappeared after 2! h. In the second, the curvature increased during two days and then decreased. The third ndicle became permanently bent, - so that its terminal part made an r-= dA~T' angleofabout45°withits original ~~ vet1ical direction. The fourth radicle became horizontal. Theso two latter radicles continued during two more days to grow in the peat in the same directions, c that is, at an angle of 45o be- Vicia f aba : radicle, rectangularly neath the horizon and horizon- bent at A, after the amputation tally. Bythefourthmorningnew of the tip, uue to the previous tips had been re-formed, and now influence of geotropism. L, siue of bean which lay on the peat, geotropism was able to act on whilst geotropism acted on the them again, and they became radicle. A, point of chief curbent perpendicularly downwards, vature of the radicle, whilst exactly as in the caso of tho standing vertically downwards. B, point of chief curvature after five radicles described in tho the regeneration of the tip, when lllSt paragraph and as is shown in geotropism again acted. C, re-the figure (Fig. 195) here given. generated tip. Lastly, five other radicles were similarly treated, but wore exposed to geotropism during only 45 m. After 8 h. 30m. only one _was doubtfully affected; after 24 h. two were just perceptibl~ curved towards the side which had been acted on by gootrop1sm; after 48 h. the one first mentioned had a, mdius of curvature of 60 mm. That this curvature was due to the action of geotropism during the horizontal position of the radicle, was shown after 4 days, when a new tip had been reformed for it then grew perpendicularly downwards. vVe learn fro~ this |