The Power of Movement in Plants. Charles Darwin
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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 ⅕00th 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 ⅕0th 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.
Fig. 52. Zea mays: track left on inclined smoked glass-plate by tip of radicle in growing downwards.
Avena sativa (Gramineae).—A cotyledon, 1½ inch in height, was placed in front of a north-east window, and the movement of the apex was traced on a horizontal glass during two days. It moved towards the light in a slightly zigzag line from 9 to 11.30 A.m. on October 15th; it then moved a little backwards and zigzagged much until 5 P.m., after which hour, and curing the night, it continued to move towards the window. On the following morning the same movement was continued in a nearly straight line until 12.40 P.m., when the sky remained until 2.35 extraordinarily dark from thunder-clouds. During this interval of 1 h. 55 m., whilst the light was obscure, it was interesting to observe how circumnutation overcame heliotropism, for the apex, instead of continuing to move towards the window in a slightly zigzag line, reversed its course four times, making two small narrow ellipses. A diagram of this case will be given in the chapter on Heliotropism. [page 66]
A filament was next fixed to a cotyledon only ¼ of an inch in height, which was illuminated exclusively from above, and as it was kept in a warm greenhouse, it grew rapidly; and now there could be no doubt about its circumnutation, for it described a figure of 8 as well as two small ellipses in 5½ hours.
Nephrodium molle (Filices).—A seedling fern of this species came up by chance in a flowerpot near its parent. The frond, as yet only slightly lobed, was only .16 of an inch in length and .2 in breadth, and was supported on a rachis as fine as a hair and .23 of an inch in height. A very thin glass filament, which projected for a length of .36 of an inch, was fixed to the end of the frond. The movement was so highly magnified that the figure (Fig. 53) cannot be fully trusted; but the frond was constantly moving in a complex manner, and the bead greatly changed its course eighteen times in the 12 hours of observation. Within half an hour it often returned in a line almost parallel to its former course. The greatest amount of movement occurred between 4 and 6 P.m. The circumnutation of this plant is interesting, because the species in the genus Lygodium are well known to circumnutate conspicuously and to twine round any neighbouring object.
Fig. 53. Nephrodium molle: circumnutation of very young frond, traced in darkness on horizontal glass, from 9 A.m. to 9 P.m. Oct. 30th. Movement of bead magnified 48 times.
Selaginella Kraussii (?) (Lycopodiaceae).—A very young plant, only .4 of an inch in height, had sprung up in a pot in the hot-house. An extremely fine glass filament was fixed to the end of the frond-like stem, and the movement of the bead traced on a horizontal glass. It changed its course several times, as shown in Fig. 54, whilst observed during 13 h. 15 m., and returned at night to a point not far distant from that whence it had started in the morning. There can be no doubt that this little plant circumnutated.
Fig. 54. Selaginella Kraussii (?): circumnutation of young plant, kept in darkness, traced from 8.45 A.m. to 10 P.m. Oct. 31st. [page 67]
CHAPTER II. GENERAL CONSIDERATIONS ON THE MOVEMENTS AND GROWTH OF SEEDLING PLANTS.
Generality of the circumnutating movement—Radicles, their circumnutation
of service—Manner in which they penetrate the ground—Manner in which
hypocotyls and other organs break through the ground by being arched—
Singular manner of germination in Megarrhiza, etc.—Abortion of cotyledons-
-Circumnutation of hypocotyls and epicotyls whilst still buried and arched-
-Their power of straightening themselves—Bursting of the seed-coats—
Inherited effect of the arching process in hypogean hypocotyls—
Circumnutation of hypocotyls and epicotyls when erect—Circumnutation of
cotyledons—Pulvini or joints of cotyledons, duration of their activity,
rudimentary in Oxalis corniculata, their development—Sensitiveness of
cotyledons to light and consequent disturbance of their periodic movements-
-Sensitiveness of cotyledons to contact.
THE circumnutating movements of the several parts or organs of a considerable number of seedling plants have been described in the last chapter. A list is here appended of the Families, Cohorts, Sub-classes, etc., to which they belong, arranged and numbered according to the classification adopted by Hooker.* Any one who will consider this list will see that the young plants selected for observation, fairly represent the whole vegetable series excepting the lowest cryptogams, and the movements of some of the latter when mature will hereafter be described. As all the seedlings which