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Show 530 SENSITIVENESS TO GRA VI'l'A'l'ION. Cuw XI. some preliminary remarks must first be made. It may be objected that the caustic 'von ld injnre the raclicles and prevent them from bending; but mnplo evidence wns given in Chapter III. tlw.t touching the tips of vertically snRpenclcd radicles with caustic on one side, docs not stop their hen ding; on tho contrary, it cauf':OS tlwm to bcncl from the tonchcd side. We also tried touching both tho 11pper and the lower Rides of the tips of some radicles of tl1e bean, extended horizontally in damp friable earth. The tips of throe wore touched with cauRtic on their upper sides, and this "·onld aid their geotropic bonding'; the tips of three wore tonchod on their lower sides, which would tend to counteract tho hencling downwards; ancl three "·ore loft as controlR. Afl or 24 h. an indcpo11dent observer was asked. to pick out of the nin radicles, the two which were most and the two which were least bent; he selected as the latter, two of those which had been toucl10d. on their lower . ides, and as the most bent, two of those which had been touched on the upper side. Hereafter analogous and more striking experiments with J>isum /iativum and Ouctt?"uitu ovtj'e1·a will be p·iven. Wo may therefore safely conclude that tho mere application of caustic to the tip does not prevent the radiclos from bending. In the following experiments, the tips of young hori11onta~ly extended radiclos were just touched with a stick of dry caustJC; and this was held transversely, so that tbe tip might be cauteri.- ed all round as symmetrically as possible. The radicles were then suspended in a closed vessel over water, kcpL r~ther cool, viz., 55°-59° F. This was done because we had found that the tips were more sensitive to contact nncl r a low than under a high temperature; and we thought that the :::~me r~le might apply to geotropism. In one exceptional tnal, nme radicles (which were rather too old, for they hacl gro~vn to a length of from 3 to 5 em.), were extended h01·izontally m damp friable earth after their tips had been cauterised, and were kept at too ,h io·h a tompcratnre, . f £!8 ' F or 20u C The VJZ., o u ., · t result m. conseoq uence was not so ctn·1 n ·n g as 1· n the subscquen · f tl em were cases; for altl1ough when after 9 h. 4.0 m. SIX 0 . 1 t ft examined these did not exhibit any geotropic bendmg, ye a ~r 2'1 h. whe' n all nine were examm. ed, only t wo r.e mained hon- zoata' l two exhibited a trace of. geotropi.s m, a nd fived were slightl'y or moderately geotropi.c , yet not compar.a ble in egreef h d b made on seven o ''- ith the control specimens. Marks a eon . . 1 des these cauterised radiclcs at 10 mm. from the tips, which me u CuAP. XL TRANSl\II'I'TED EFFECTS : VIOlA. G31 tho whole growing portion; and after tho 24 h. this part had a mean length of 37 mm., so that it had increased to more than 3t times its original length; but it should be remembered that these beans had been exposed to a mther high temperature. Nineteen young radiclos with cauterised tips were extended at different times horizontally over water. In every trial an equal number of control specimens wore observed. In the first trial, the tips of three radicles were lightly touched with the caustic for 6 or 7 seconds, which was a longer application than usual. After 23 h. 30m. (temp. 55°-5G° F.) those three radiclos, Fig. 196. D. E. F. A. B. c. Vicia faba: state of radicles which had been extended horizontally for 2~h. 30m.: A, B, C, tips touched with caustic; D, E, F, tips uncautensed. Lengths of radicles reduced to one-half scale, but by an accident the beans themselves not reduced in the same degree. A, B,C (Fig. 196), were still horizontal, whilst the three control specimens had become within 8 h. slightly geotropic, and strongly so (D, E, F) in 23 h. 30 m. A dot had been made on all .six radicles at 10 mm. from their tips, when first placed honzont~lly. After the 23 h. 30m. this terminal part, originally 10 mm. m length, had increased in tho cautL:rised specimens to a ~ean length of 17·3 mm., and to 15·7 mm. in the control ~olea, as shown in the figures by the unbroken transverse :e; the dotted .line being at 10 mm. from the apex. The con-· ol or unc11.uterised radicles, therefore, had actually grown less 2M2 |