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Show 30 CIRCUMNUTATION OF SEEDLINGS. CHAP. I. b uptly then made a small loop and then a larger zig~ag ~u~·ve. 'During the night and till 11 A.M. on the followmg Fig. 20. 10 . ~------· Vicia faba: circumnutation of a radicle, at first pointing ':crtically up- , wards kept in darkness, traced Qll a horizontal glass, dunng 14 hours. Move~ent of bend of filament magnified 23 times, here reduced to one-half of original scale. morning, the bead moved to a great distance in ~ ne~rly straight line in the direction indicated by the broken line m the figur?. Thi~ resulted from the tip bending quickly downwards, ~s. 1t had now become much declined, and had thus gained a pos1hon highly favourable for the action of geotropism. Fig. 21. A. B. D. E. Vicia faba: tracks left on inclined smoked glass-plates, by tips of radicl~ in growing downwards. Plate C was inclined at 63°, plates A and h nt 71°, plate B at 75°, and plate E at a few degrees beneath t e horizon. CHAP. I. VI CIA. 31 We next experimented on nearly a score of radicles by allowing them to grow downwards over inclined plates of smoked glass, in exactly the same manner as with lEsculus and Phascolus. Some of the plates were inclined only a few degrees beneath the horizon, but most of them between 60° and 75°. In tho latter cases tho radicles in growing downwards were deflected only a little from tho direction which they had followed whilst gorminating in sawdust, and they p1·essed lightly on the glassplates (Fig. 21). Five of the most distinct tracks arc here copied, and they arc all slightly sinuous, showing circumnutati0n. Moreover, a close examination of almost every one of the tracks clearly showed that the tips in their downwnrd course had alternately pressed with greater or less force on the plates, and had'sometimes risen up so as nearly to leave them for short intervals. The distance between tho extreme right nnd left positions of the radicle A was 0·7 mm., ascertained in the same manner as in the case of Phascolus. Epicotvl.-At the point where the radicle had p1·otrudcd from a bean laid on its side, a flattened solid lump projected ·1 of an inch, in the Ramo horizontal plane with the bean. This protuberance consisted of the convex summit of the arched epicotyl; and as it became developed the two legs of the arch curved themselves laterally upwards, owing to apogeotropism, at such a mte that the arch stood highly inclined after 14 h., and vertically in 48 h. A :filament was fixed to the crown of the protuberance before any arch was visible, but the basal half grew so quickly that on tho second morning the end of the filament was bowed greatly downwards. It was therefore removed and fixocllower down. The line traced during these twn days extended in the same general direction, and was in parts nearly straight, and in others plainly zigzag, thus giving some evidenco of circumnntation. As tho arched epicotyl;1'n whatever position it may be placed, bends quickly upwards through apogcotropism, and as the two legs tend at a very early age to separate from one another, as soon as they are relieved from the pressure of tho surrounding earth, it was difficult to ascertain positively whether the cpicotyl, whilst remaining arched, circumnutated. Therefore some rather deeply buried beans wore uncovered, and the two legs of tho arches were tied together, as had been done with the cpicotyl of 'l'ropmolum and tho hypocotyl of the Cabbage. The movements of the tied arches were traced in the usual manner on |