The Power of Movement in Plants. Charles Darwin
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began to sink about noon, instead of late in the evening; and those of another seedling were almost paralysed by having been similarly kept during two whole days. The cotyledons of several other species of Ipomoea likewise sink downwards late in the evening.
Cerinthe major (Boragineae).—The circumnutation of the hypocotyl of a young seedling with the cotyledons hardly
Fig. 36. Cerinthe major: circumnutation of hypocotyl, with filament fixed across its summit, illuminated from above, traced on horizontal glass, from 9.26 A.m. to 9.53 P.m. on Oct. 25th. Movement of the bead magnified 30 times, here reduced to one-third of original scale.
expanded, is shown in the annexed figure (Fig. 36), which apparently represents four or five irregular ellipses, described in the course of a little over 12 hours. Two older seedlings were similarly observed, excepting that one of them was kept in the dark; their hypocotyls also circumnutated, but in a more simple manner. The cotyledons on a seedling exposed to the light fell from the early morning until a little after noon, and then continued to rise until 10.30 P.m. or later. The cotyledons of this same seedling acted in the same general manner during the two following days. It had previously been tried in the dark, and after being thus kept for only 1 h. 40 m. the cotyledons began at 4.30 P.m. to sink, instead of continuing to rise till late at night. [page 50]
Nolana prostrata (Nolaneae).—The movements were not traced, but a pot with seedlings, which had been kept in the dark for an hour, was placed under the microscope, with the micrometer eye-piece so adjusted that each division equalled ⅕00th of an inch. The apex of one of the cotyledons crossed rather obliquely four divisions in 13 minutes; it was also sinking, as shown by getting out of focus. The seedlings were again placed in darkness for another hour, and the apex now crossed two divisions in 6 m. 18 s.; that is, at very nearly the same rate as before. After another interval of an hour in darkness, it crossed two divisions in 4 m. 15 s., therefore at a quicker rate. In the afternoon, after a longer interval in the dark, the apex was motionless, but after a time it recommenced moving, though slowly; perhaps the room was too cold. Judging from previous cases, there can hardly be a doubt that this seedling was circumnutating.
Fig. 37. Solanum lycopersicum: circumnutation of hypocotyl, with filament fixed across its summit, traced on horizontal glass, from 10 A.m. to 5 P.m. Oct. 24th. Illuminated obliquely from above. Movement of bead magnified about 35 times, here reduced to one-third of original scale.
Solanum lycopersicum (Solaneae).—The movements of the hypocotyls of two seedling tomatoes were observed during seven hours, and there could be no doubt that both circumnutated. They were illuminated from above, but by an accident a little light entered on one side, and in the accompanying figure (Fig. 37) it may be seen that the hypocotyl moved to this side (the upper one in the figure), making small loops and zigzagging in its course. The movements of the cotyledons were also traced both on vertical and horizontal glasses; their angles with the horizon were likewise measured at various hours. They fell from 8.30 A.m. (October 17th) to about noon; then moved laterally in a zigzag line, and at about 4 P.m. began to rise; they continued to do so until 10.30 P.m., by which hour they stood vertically and were asleep. At what hour of the night or early morning they began to fall was not ascertained. Owing to the lateral movement shortly after mid-day, the descending and ascending lines did not coincide, and irregular ellipses were described during each 24 h. The regular periodicity of these movements is destroyed, as we shall hereafter see, if the seedlings are kept in the dark. [page 51]
Solanum palinacanthum.—Several arched hypocotyls rising nearly .2 of an inch above the ground, but with the cotyledons still buried beneath the surface, were observed, and the tracings showed that they circumnutated. Moreover, in several cases little open circular spaces or cracks in the argillaceous sand which surrounded the arched hypocotyls were visible, and these appeared to have been made by the hypocotyls having bent first to one and then to another side whilst growing upwards. In two instances the vertical arches were observed to move to a considerable distance backwards from the point where the cotyledons lay buried; this movement, which has been noticed in some other cases, and which seems to aid in extracting the cotyledons from the buried seed-coats, is due to the commencement of the straightening of the hypocotyl. In order to prevent this latter movement, the two legs of an arch, the
Fig. 38. Solanum palinacanthum: circumnutation of an arched hypocotyl, just emerging from the ground, with the two legs tied together, traced in darkness on a horizontal glass, from 9.20 A.m. Dec. 17th to 8.30 A.m. 19th. Movement of bead magnified 13 times; but the filament, which was affixed obliquely to the crown of the arch, was of unusual length.
summit of which was on a level with the surface of the soil, were tied together; the earth having been previously removed to a little depth all round. The movement of the arch during 47 hours under these unnatural circumstances is exhibited in the annexed figure.
The cotyledons of some seedlings in the hot-house were horizontal about noon on December 13th; and at 10 P.m. had risen to an angle of 27o above the horizon; at 7 A.m. on the following [page 52] morning, before it was light, they had risen to 59o above the horizon; in the afternoon of the same day they were found again horizontal.
Beta vulgaris (Chenopodeae).—The seedlings are excessively sensitive to light, so that although on the first day they were uncovered only during two or three minutes at each observation, they all moved steadily towards the side of the room whence the light proceeded, and the tracings consisted only of slightly zigzag lines directed towards the light. On the next day the plants were placed in a completely darkened room, and at each observation were illuminated as much as possible from vertically above by a small wax taper. The annexed figure (Fig. 39) shows the movement of the hypocotyl during 9 h. under these circumstances. A second seedling was similarly observed at the same time, and the tracing had the same peculiar character, due to the hypocotyl often moving and returning in nearly parallel lines. The movement of a third hypocotyl differed greatly.
Fig. 39. Beta vulgaris: circumnutation of hypocotyl, with filament fixed obliquely across its summit, traced in darkness on horizontal glass, from 8.25 A.m. to 5.30 P.m. Nov. 4th. Movement of bead magnified 23 times, here reduced to one-third of original scale.
We endeavoured to trace the movements of the cotyledons, and for this purpose some seedlings were kept in the dark, but they moved in an abnormal manner; they continued rising from 8.45 A.m. to 2 P.m., then moved laterally, and from 3 to 6 P.m. descended; whereas cotyledons which have been exposed all the day to the light rise in the evening so as to stand vertically at night; but this statement applies only to young seedlings. For instance, six seedlings in the greenhouse had their cotyledons partially open for the first time on the morning of November 15th, and at 8.45 P.m. all were completely closed, so that they might properly be said to be asleep. Again, on the morning of November 27th, the cotyledons of four other seedlings, which were surrounded by a collar of brown paper so that they received light only from above, were open to the extent of 39o; at 10 P.m. they were completely closed; next morning (November 28th) at 6.45 A.m. whilst it was still dark, two of them [page 53] were partially open and all opened in the course of the morning; but at 10.20 P.m. all four (not to mention nine others which had been open in the morning and six others on another occasion) were again completely closed. On the morning of the 29th they were open, but at night only one of the four was closed, and this only partially; the three others had their cotyledons much more raised than during the day. On the night of the 30th the cotyledons of the four were only slightly raised.
Ricinus Borboniensis (Euphorbiaceae).—Seeds were purchased under the above name—probably a variety of the common castor-oil plant. As soon as an arched hypocotyl had risen clear above the ground, a filament was attached to the upper leg bearing the cotyledons which were still buried beneath the surface, and the movement of the bead was traced on a horizontal glass during a period of 34 h. The lines traced were strongly zigzag, and as the bead twice returned nearly parallel to its former course in two different directions, there could be no doubt that the arched hypocotyl circumnutated. At the close of the 34 h. the upper part began to rise and straighten itself, dragging the cotyledons out of the ground, so that the movements of the bead could no longer be traced on the glass.
Quercus (American sp.) (Cupuliferae).—Acorns of an American oak which had germinated at Kew were planted in a pot in the greenhouse. This transplantation