Fig. 50. Phalaris Canariensis: circumnutation of a very young cotyledon, with a mark placed below the apex, traced on a horizontal glass, from 11.37 A.M. to 9.30 P.M. Dec. 13th. Movement of apex greatly magnified, here reduced to one-fourth of original scale.

Lastly, the soil in the same pot was searched with the aid of a lens, and the white knife-like apex of a seedling was found on an exact level with that of the surrounding surface. The soil was removed all round the apex to the depth of a quarter of an inch, the seed itself remaining covered. The pot, protected from lateral light, was placed under the micro- [page 64] scope with a micrometer eye-piece, so arranged that each division equalled 1/500th of an inch. After an interval of 30 m. the apex was observed, and it was seen to cross a little obliquely two divisions of the micrometer in 9 m. 15 s.; and after a few minutes it crossed the same space in 8 m. 50s. The seedling was again observed after an interval of three-quarters of an hour, and now the apex crossed rather obliquely two divisions in 10 m. We may therefore conclude that it was travelling at about the rate of 1/50th of an inch in 45 minutes. We may also conclude from these and the previous observations, that the 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 the case before given of the asparagus) for a circular, narrow, open space or crack being distinctly visible round several seedlings which had risen through very fine argillaceous sand, kept uniformly damp.

Fig. 51. Zea mays: circumnutation of cotyledon, traced on horizontal glass, from 8.30 A.M. Feb. 4th to 8 A.M. 6th. Movement of bead magnified on an average about 25 times.

Zea mays (Gramineae).--A glass filament was fixed obliquely to the summit of a cotyledon, rising .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 the tracing (Fig. 51) was much more magnified on the first than on the second day. The upper part of the cotyledon changed its course by at least as much as a rectangle six times on each of the two days. The plant was illuminated by an obscure light from vertically above. This was a necessary precaution, as on the previous day we had traced the 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 held for a minute or two on the same side; and the result was that the cotyledons travelled all day long to this side, though making in their course some conspicuous flexures, from which fact alone we might have [page 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 bending downwards through geotropism their courses were strongly zigzag; from this latter circumstance circumnutation might have been inferred, had not their tips become slightly withered after the first 24 h., 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 angles between 45o and 80o 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) there 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.

Charles Darwin

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