The Power of Movement in Plants. Charles DarwinЧитать онлайн книгу.
this was probably not the case, for it was placed close to a north-east window and left there for 25 m., after which time, instead of moving still more quickly towards the light, as might have been expected, it travelled only at the rate of 12 m. 30 s. for five divisions. It was then again left in complete darkness for 1 h., and the point now travelled in the same direction as before, but at the rate of 3 m. 18 s. for five divisions.
We shall have to recur to the cotyledons of the cabbage in a future chapter, when we treat of their sleep-movements. The circumnutation, also, of the leaves of fully-developed plants will hereafter be described.
Fig. 11. Githago segetum: circumnutation of hypocotyl, traced on a horizontal glass, by means of a filament fixed transversely across its summit, from 8.15 A.m. to 12.15 P.m. on the following day. Movement of bead of filament magnified about 13 times, here reduced to one-half the original scale.
Githago segetum (Caryophylleae).—A young seedling was dimly illuminated from above, and the circumnutation of the hypo- [page 22] cotyl was observed during 28 h., as shown in Fig. 11. It moved in all directions; the lines from right and to left in the figure being parallel to the blades of the cotyledons. The actual distance travelled from side to side by the summit of the hypocotyl was about .2 of an inch; but it was impossible to be accurate on this head, as the more obliquely the plant was viewed, after it had moved for some time, the more the distances were exaggerated.
We endeavoured to observe the circumnutation of the cotyledons, but as they close together unless kept exposed to a moderately bright light, and as the hypocotyl is extremely heliotropic, the necessary arrangements were too troublesome. We shall recur to the nocturnal or sleep-movements of the cotyledons in a future chapter.
Fig. 12. Gossypium: circumnutation of hypocotyl, traced on a horizontal glass, from 10.30 A.m. to 9.30 A.m. on following morning, by means of a filament fixed across its summit. Movement of bead of filament magnified about twice; seedling illuminated from above.
Gossypium (var. Nankin cotton) (Malvaceae).—The circumnutation of a
hypocotyl was observed in the hot-house, but the movement was so much
exaggerated that the bead twice passed for a time out of view. It was,
however, manifest that two somewhat irregular ellipses were nearly
completed in 9 h. Another seedling, 1½ in. in height, was then observed
during 23 h.; but the observations were not made at sufficiently short
intervals, as shown by the few dots in Fig. 12, and the tracing was not now
sufficiently enlarged. Nevertheless there could be no doubt about the
circumnutation of the hypocotyl, which described in 12 h. a figure
representing three irregular ellipses of unequal sizes.
The cotyledons are in constant movement up and down during the whole day,
and as they offer the unusual case of moving downwards late in the evening
and in the early part of the night, many observations were made on them. A
filament was fixed along the middle of one, and its movement traced on a
vertical glass; but the tracing is not given, as the hypocotyl was not
secured, so that it was impossible to distinguish clearly between its
movement and that of the cotyledon. The cotyledons rose from 10.30 A.m. to
about 3 P.m.; they then sank till 10 P.m., rising, however, greatly in the
latter part of the night.
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The angles above the horizon at which the cotyledons of another seedling
stood at different hours is recorded in the following short table:—
Oct. 20 2.50 P.M … 25o above horizon. Oct. 20 4.20 P.M … 22o above horizon. Oct. 20 5.20 P.M … 15o above horizon. Oct. 20 10.40 P.M … 8o above horizon. Oct. 21 8.40 A.M … 28o above horizon. Oct. 21 11.15 A.M … 35o above horizon. Oct. 21 9.11 P.M … 10o below horizon.
The position of the two cotyledons was roughly sketched at various hours with the same general result.
In the following summer, the hypocotyl of a fourth seedling was secured to a little stick, and a glass filament with triangles of paper having been fixed to one of the cotyledons, its movements were traced on a vertical glass under a double skylight in the house. The first dot was made at 4.20 P.m. June 20th; and the cotyledon fell till 10.15 P.m. in a nearly straight line. Just past midnight it was found a little lower and somewhat to one side. By the early morning, at 3.45 A.m., it had risen greatly, but by 6.20 A.m. had fallen a little. During the whole of this day (21st) it fell in a slightly zigzag line, but its normal course was disturbed by the want of sufficient illumination, for during the night it rose only a little, and travelled irregularly during the whole of the following day and night of June 22nd. The ascending and descending lines traced during the three days did not coincide, so that the movement was one of circumnutation. This seedling was then taken back to the hot-house, and after five days was inspected at 10 P.m., when the cotyledons were found hanging so nearly vertically down, that they might justly be said to have been asleep. On the following morning they had resumed their usual horizontal position.
Oxalis rosea (Oxalideae).—The hypocotyl was secured to a little stick, and an extremely thin glass filament, with two triangles of paper, was attached to one of the cotyledons, which was .15 inch in length. In this and the following species the end of the petiole, where united to the blade, is developed into a pulvinus. The apex of the cotyledon stood only 5 inches from the vertical glass, so that its movement was not greatly exaggerated as long as it remained nearly horizontal; but in the course of the day it both rose considerably above and fell beneath a horizontal position, and then of course the movement was much exaggerated. [page 24] In Fig. 13 its course is shown from 6.45 A.m. on June 17th, to 7.40 A.m. on the following morning; and we see that during the daytime, in the course of 11 h. 15 m., it travelled thrice down and twice up. After 5.45 P.m. it moved rapidly downwards, and in an hour or two depended vertically; it thus remained all night asleep. This position could not be represented on the vertical glass nor in the figure here given. By 6.40 A.m. on the following morning (18th) both cotyledons had risen greatly, and they continued to rise until 8 A.m., when they stood almost horizontally. Their movement was traced during the whole of this day and until the next morning; but a tracing is not given, as it was closely similar to Fig. 13, excepting that the lines were more zigzag. The cotyledons moved 7 times, either upwards or downwards; and at about 4 P.m. the great nocturnal sinking movement commenced.
Fig. 13. Oxalis rosea: circumnutation of cotyledons, the hypocotyl being secured to a stick; illuminated from above. Figure here given one-half of original scale.
Another seedling was observed in a similar manner during nearly 24 h., but with the difference that the hypocotyl was left free. The movement also was less magnified. Between 8.12 A.m. and 5 P.m. on the 18th, the apex of the cotyledon moved 7 times upwards or downwards (Fig. 14). The nocturnal sinking movement, which is merely a great increase of one of the diurnal oscillations, commenced about 4 P.m.
Oxalis Valdiviana.—This species is interesting, as the coty- [page 25] ledons rise perpendicularly upwards at night so as to come into close contact, instead of sinking vertically downwards, as in the case of O. rosea. A glass filament was fixed to a cotyledon, .17 of an inch in length, and the hypocotyl was left free. On
Fig. 14. Oxalis rosea: conjoint circumnutation of the cotyledons and hypocotyl, traced from 8.12 A.m. on June 18th to 7.30 A.m. 19th. The apex of the cotyledon stood only 3¾ inches from the vertical glass. Figure here given one-half of original scale.
Fig. 15. Oxalis Valdiviana: conjoint circumnutation of a cotyledon and the hypocotyl, traced on vertical glass, during 24 hours. Figure here given one-half of original scale; seedling illuminated from above.
the first day the seedling was placed too far from the vertical glass; so that the tracing was enormously exaggerated and the movement could not be traced when the cotyledon either rose or sank much; but it