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Show ()4 CIROUMNUTATION OF SEEDLINGS. 0HAI'. I, scope with a micrometer ey~-picce, so arra~1gcd that. each division equalled 6 ri 0 th of an mch. After an mt?rval o.f.30 m. the apex was observed, and it was seen to crosl-3 n, httlc obliquely two divisions of the micrometer in 9 m. 15 .. ; and after a f ·w minutes it crossed tho same space in 8 m. 50 s. Tho soo<1ling was acrain obs'ervod after an intorvalof throo-qun.rtors of an hour, n.nd n~w tho apex crossed rather obliquely two divisions in 10m. We may therefore conclude tbu.t it was travelling at. about tl1o rate of -L.th of an inch in 45 minutes. We may also conclude from th~~e and the previous observations, that tho seedlings of Phalaris in breaking through the surface of the soil circumnutate as much as the surrounding pressure will permit. This fact accounts (as in tho case before given of tho asparagus) for a circular, narrow, open space or crack being distinctly visible round several seedlings which bad rison through very fino argillaceous sand, kept uniformly damp. Zea mays (Graminero).-A glass filament \vas fixed obliquely Fig. 51. ~) ) ____ ...__ .. _____ / -- -._.- Zeu, mays: circumnutatiou of cotyledon, tracect on horizontal ~las;;, from 8.30 A.M. Feb. 4th to 8 A.M. 6th. Movement of bead magnifieJ. on an average about 25 time.>. to tho summit of a cotyledon, risin:..; ·2 of an inch above the ground; but by the third morning it had grown to exactly thrice this height, so that the distance of the bead from the mark below was greatly increased, consequently tho tracing (Fig. 51) was much more magnified on tho first than on the second day. 'fhe upper part of tho eoty lcdou changed its course by at least as much as a rectangle six times on each of the two clays. Tho plant was illuminated by an obscure light from vertically above. This was a necessary precau-tion, as on the previous day we had traced tho movements of cotyledons placed in a deep box, the inner side of which was feebly illuminated on one side from a distant north-east window, and at each observation by a wax taper hold for a minute or two on the same side; and the result was that tho cotyledons travelled all day long to this side, though making in their course SOlllO conspicuous flexures, from which fact alone we might have OuAP. I. PHALARIS. 65 concluded that they were circumnutating; but we thought it advisable to make the tracing above given. Radicles.-Glass filaments were fixed to two short radicles, placed so as to stand almost upright, and whilst bonding downwards through geotropism their courses wore strongly zigzag; from this latter circumstance circumnutation might have been inferred, had not their tips become slightly withered after the first 24 b., though they were watered and the air kept very damp. Nine radicles were next arranged in the manner formerly described, so that in growing downwards they left tracks on smoked glass-plates, inclined at various n,ngles between 45° and 80° beneath the horizon. Almost every one of these tracks offered evidence in their greater or less breadth in different parts, or in little bridges of soot being left, that the apex had come alternately into more and less close contact with the glass. In the accompanying figure (Fig. 52) we have an accurate copy of one such track. In two instances alone (and in these the plates were highly inclined) thoro was some evidence of slight lateral movement. We presume therefore that the friction of the apex on the smoked surface, little as this could have been, sufficed to check the movement from side to side of these delicate radicles. Fig. 52. , J Ze ~ mays: track left on inclined smoked glassplate by tip of radicle in growing downwards. Avena sativa (Graminero).-A cotyledon, H inch in height, was placed in front ·of a northeast window, and the movement of the apex Was traced on a horizontal glass during two d~ys. It moved towards the light in a slightly ZI~zag line from 9 to 11.30 A.M. on October 15th; it then moved a httle backwards and zigzagged much until 5 P.M., after which h~ur, and during the night, it continued to move towards the wmdow. On the following morning the same movement was conti~ued in a nearly straight line untill2.40 P.M., when the sky romamed until 2.35 extraordinarily dark from thunder-clouds. ?uring ~his int~rval of 1 h. 55 m., whilst the light was obscure, It :vas mterestmg to observe how circumnutation overcame hehoh:opism, for the apex, instead of continuing to move towards ~e wmdow in a slightly zigzag line, reversed its course four ti~es, making two small narrow ellipses. A diagram of this case Wlll be given in the chapter on Heliotropism. F |
