25). The morning was cold, and the window had been accidentally left open for a short time, which must have chilled the plant; and this probably prevented it from moving quite as freely as on the previous day; for it rose only four and sank only four times during the day, one of the oscillations being very small. At 7.10 A.M., when the first dot was made, the cotyledons were not fully open or awake; they continued to open till about 9 A.M., by which time they had sunk a little beneath the horizon: by 9.30 A.M. they had risen, and then they oscillated up and down; but the upward and downward lines never quite coincided. At about 4.30 P.M. the great nocturnal rise commenced. At 7 A.M. on the following morning (Sept. 26th) they occupied nearly the same level as on the previous morning, as shown in the diagram: they then began to open or sink in the usual manner. The diagram leads to the belief that the great periodical daily rise and fall does not differ essentially, excepting in amplitude, from the oscillations during the middle of the day.

Lotus Jacoboeus (Leguminosae).--The cotyledons of this plant, after the few first days of their life, rise so as to stand almost, though rarely quite, vertically at night. They continue to act in this manner for a long time even after the development of some of the true leaves. With seedlings, 3 inches in height, and bearing five or six leaves, they rose at night about 45o. They continued to act thus for about an additional fortnight. Subsequently they remained horizontal at night, though still green [page 36] and at last dropped off. Their rising at night so as to stand almost vertically appears to depend largely on temperature; for when the seedlings were kept in a cool house, though they still continued to grow, the cotyledons did not become vertical at night. It is remarkable that the cotyledons do not generally rise at night to any conspicuous extent during the first four or five days after germination; but the period was extremely variable with seedlings kept under the same conditions; and many were observed. Glass filaments with minute triangles of paper were fixed to the cotyledons (1 mm. in breadth) of two seedlings, only 24 h. old, and the hypocotyl was secured to a stick; their movements greatly magnified were traced, and they certainly circumnutated the whole time on a small scale, but they did not exhibit any distinct nocturnal and diurnal movement. The hypocotyls, when left free, circumnutated over a large space.

Another and much older seedling, bearing a half-developed leaf, had its movements traced in a similar manner during the three first days and nights of June; but seedlings at this age appear to be very sensitive to a deficiency of light; they were observed under a rather dim skylight, at a temperature of between 16o to 17 1/2o C.' and apparently, in consequence of these conditions, the great daily movement of the cotyledons ceased on the third day. During the first two days they began rising in the early afternoon in a nearly straight line, until between 6 and 7 P.M., when they stood vertically. During the latter part of the night, or more probably in the early morning, they began to fall or open, so that by 6.45 A.M. they stood fully expanded and horizontal. They continued to fall slowly for some time, and during the second day described a single small ellipse, between 9 A.M. and 2 P.M., in addition to the great diurnal movement. The course pursued during the whole 24 h. was far less complex than in the foregoing case of Cassia. On the third morning they fell very much, and then circumnutated on a small scale round the same spot; by 8.20 P.M. they showed no tendency to rise at night. Nor did the cotyledons of any of the many other seedlings in the same pot rise; and so it was on the following night of June 5th. The pot was then taken back into the hot-house, where it was exposed to the sun, and on the succeeding night all the cotyledons rose again to a high angle, but did not stand quite vertically. On each of the above days the line representing the great nocturnal [page 37] rise did not coincide with that of the great diurnal fall, so that narrow ellipses were described, as is the usual rule with circumnutating organs. The cotyledons are provided with a pulvinus, and its development will hereafter be described.

The Power of Movement in Plants Page 19

19th Century English Literature

Charles Darwin

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Charles Darwin

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