The Power of Movement in Plants. Charles Darwin

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СКАЧАТЬ in temperature or light. The record commenced at 11 A.m. on February 18th; and from this hour till 3 P.m. the [page 42] cotyledon fell; it then rose rapidly till 9 P.m., then very gradually till 3 A.m. February 19th, after which hour it sank gradually till 4.30 P.m.; but the downward movement was interrupted by one slight rise or oscillation about 1.30 P.m. After 4.30 P.m. (19th) the cotyledon rose till 1 A.m. (in the night of February 20th) and then sank very gradually till 9.30 A.m., when our observations ceased. The amount of movement was greater on the 18th than on the 19th or on the morning of the 20th.

      Cucurbita aurantia.—An arched hypocotyl was found buried a little beneath the surface of the soil; and in order to prevent it straightening itself quickly, when relieved from the surrounding pressure of the soil, the two legs of the arch were tied together. The seed was then lightly covered with loose damp earth. A filament with a bead at the end was affixed to the basal leg, the movements of which were observed during two days in the usual manner. On the first day the arch moved in a zigzag line towards the side of the basal leg. On the next day, by which time the dependent cotyledons had been dragged above the surface of the soil, the tied arch changed its course greatly nine times in the course of 14½ h. It swept a large, extremely irregular, circular figure, returning at night to nearly the same spot whence it had started early in the morning. The line was so strongly zigzag that it apparently represented five ellipses, with their longer axes pointing in various directions. With respect to the periodical movements of the cotyledons, those of several young seedlings formed together at 4 P.m. an angle of about 60o, and at 10 P.m. their lower parts stood vertically and were in contact; their tips, however, as is usual in the genus, were permanently reflexed. These cotyledons, at 7 A.m. on the following morning, were again well expanded.

      Lagenaria vulgaris (var. miniature Bottle-gourd) (Cucurbitaceae).—A seedling opened its cotyledons, the movements of which were alone observed, slightly on June 27th and closed them at night: next day, at noon (28th), they included an angle of 53o, and at 10 P.m. they were in close contact, so that each had risen 26½o. At noon, on the 29th, they included an angle of 118o, and at 10 P.m. an angle of 54o, so each had risen 32o. On the following day they were still more open, and the nocturnal rise was greater, but the angles were not measured. Two other seedlings were observed, and behaved during three days in a closely similar manner. The cotyledons, therefore, [page 43] open more and more on each succeeding day, and rise each night about 30o; consequently during the first two nights of their life they stand vertically and come into contact.

      Fig. 30. Lagenaria vulgaris: circumnutation of a cotyledon, 1½ inch in length, apex only 4¾ inches from the vertical glass, on which its movements were traced from 7.35 A.m. July 11th to 9.5 A.m. on the 14th. Figure here given reduced to one-third of original scale.

      In order to ascertain more accurately the nature of these movements, the hypocotyl of a seedling, with its cotyledons well expanded, was secured to a little stick, and a filament with triangles of paper was affixed to one of the cotyledons. The observations were made under a rather dim skylight, and the temperature during the whole time was between 17½o to 18o C. (63o to 65o F.). Had the temperature been higher and the light brighter, the movements would probably have been greater. On July 11th (see Fig. 30), the cotyledon fell from 7.35 A.m. till 10 A.m.; it then rose (rapidly after 4 P.m.) till it stood quite vertically at 8.40 P.m. During the early morning of the next day (12th) it fell, and continued to fall till 8 A.m., after which hour it rose, then fell, and again rose, so that by 10.35 P.m. it stood much higher than it did in the morning, but was not vertical as on the preceding night. During the following early morning and whole day (13th) it fell and circumnutated, but had not risen when observed late in the evening; and this was probably due to the deficiency of heat or light, or of both. We thus see that the cotyledons became more widely open at noon on each succeeding day; and that they rose considerably each night, though not acquiring a vertical position, except during the first two nights.

      Cucumis dudaim (Cucurbitaceae).—Two seedlings had opened [page 44] their cotyledons for the first time during the day—one to the extent of 90o and the other rather more; they remained in nearly the same position until 10.40 P.m.; but by 7 A.m. on the following morning the one which had been previously open to the extent of 90o had its cotyledons vertical and completely shut; the other seedling had them nearly shut. Later in the morning they opened in the ordinary manner. It appears therefore that the cotyledons of this plant close and open at somewhat different periods from those of the foregoing species of the allied genera of Cucurbita and Lagenaria.

      Fig. 31. Opuntia basilaris: conjoint circumnutation of hypocotyl and cotyledon; filament fixed longitudinally to cotyledon, and movement traced during 66 h. on horizontal glass. Movement of the terminal bead magnified about 30 times, here reduced to one-third scale. Seedling kept in hot-house, feebly illuminated from above.

      Opuntia basilaris (Cacteae).—A seedling was carefully observed, because, considering its appearance and the nature of the mature plant, it seemed very unlikely that either the hypocotyl or cotyledons would circumnutate to an appreciable extent. The cotyledons were well developed, being .9 of an inch in length, .22 in breadth, and .15 in thickness. The almost cylindrical hypocotyl, now bearing a minute spinous bud on its summit, was only .45 of an inch in height, and .19 in diameter. The tracing (Fig. 31) shows the combined movement of the hypocotyl and of one of the cotyledons, from 4.45 P.m. on May 28th to 11 A.m. on the 31st. On the 29th a nearly perfect ellipse was completed. On the 30th the hypocotyl moved, from some unknown cause, in the same general direction in a zigzag line; but between 4.30 and 10 P.m. almost completed a second small ellipse. The cotyledons move only a little up and down: thus at 10.15 P.m. they stood only 10o higher than at noon. The chief seat of movement therefore, at least when the cotyledons are rather old as in the present case, lies in the hypocotyl. The ellipse described on the 29th had its longer axis directed at nearly right angles to a line joining the two cotyledons. The actual amount of movement of the bead at the end of the [page 45] filament was, as far as could be ascertained, about .14 of an inch.

      Fig. 32. Helianthus annuus: circumnutation of hypocotyl, with filament fixed across its summit, traced on a horizontal glass in darkness, from 8.45 A.m. to 10.45 P.m., and for an hour on following morning. Movement of bead magnified 21 times, here reduced to one-half of original scale.

      Helianthus annuus (Compositae).—The upper part of the hypocotyl moved during the day-time in the course shown in the annexed figure (Fig. 32). As the line runs in various directions, crossing itself several times, the movement may be considered as one of circumnutation. The extreme actual distance travelled was at least .1 of an inch. The movements of the cotyledons of two seedlings were observed; one facing a north-east window, and the other so feebly illuminated from above us as to be almost in darkness. They continued to sink till about noon, when they began to rise; but between 5 and 7 or 8 P.m. they either sank a little, or moved laterally, and then again began to rise. At 7 A.m. on the following morning those on the plant before the north-east window had opened so little that they stood at an angle of 73o above the horizon, and were not observed any longer. Those on the seedling which had been kept in almost complete darkness, sank during the whole day, without rising about mid-day, but rose during the night. On the third and fourth days they continued sinking without any alternate ascending movement; and this, no doubt, was due to the absence of light.

      Primula Sinensis (Primulaceae).—A seedling was placed with the two cotyledons parallel to a north-east window on a day when the light was nearly uniform, and a filament was affixed to one of them. From observations subsequently made on another seedling with the stem secured to a stick, the greater part of the movement shown in the annexed figure (Fig. 33), must have been that of the hypocotyl, though the cotyledons certainly move up and down to a certain extent both during the day and night. The movements of the same seedling were traced [page 46] on the following day with nearly the same result; and there can be no doubt about the circumnutation of the hypocotyl.

      Fig. 33. Primula Sinensis: conjoint circumnutation of hypocotyl and cotyledon, traced on vertical glass, from 8.40 A.m. to 10.45 P.m. Movements of bead magnified about 26 times.

      Cyclamen Persicum (Primulaceae).—This СКАЧАТЬ