The Power of Movement in Plants. Charles Darwin
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 plant is generally supposed to produce only a single cotyledon, but Dr. H. Gressner* has shown that a second one is developed after a long interval of time. The hypocotyl is converted into a globular corm, even before the first cotyledon has broken through the ground with its blade closely enfolded and with its petiole in the form of an arch, like the arched hypocotyl or epicotyl of any ordinary dicotyledonous plant. A glass filament was affixed to a cotyledon, .55 of an inch in height, the petiole of which had straightened itself and stood nearly vertical, but with the blade not as yet fully expanded. Its movements were traced during 24½ h. on a horizontal glass, magnified 50 times; and in this interval it described two irregular small circles; it therefore circumnutates, though on an extremely small scale.
Fig. 34. Stapelia sarpedon: circumnutation of hypocotyl, illuminated from above, traced on horizontal glass, from 6.45 A.m. June 26th to 8.45 A.m. 28th. Temp. 23–24o C. Movement of bead magnified 21 times.
Stapelia sarpedon (Asclepiadeae).—This plant, when mature, resembles a cactus. The flattened hypocotyl is fleshy, enlarged in the upper part, and bears two rudimentary cotyledons. It breaks through the ground in an arched form, with the rudimentary cotyledons closed or in contact. A filament was affixed almost
* 'Bot. Zeitung,' 1874, p. 837. [page 47]
vertically to the hypocotyl of a seedling half an inch high; and its movements were traced during 50 h. on a horizontal glass (Fig. 34). From some unknown cause it bowed itself to one side, and as this was effected by a zigzag course, it probably circumnutated; but with hardly any other seedling observed by us was this movement so obscurely shown.
Ipomoea caerulea vel Pharbitis nil (Convolvulaceae).—Seedlings of this plant were observed because it is a twiner, the upper internodes of which circumnutate conspicuously; but like other twining plants, the first few internodes which rise above the ground are stiff enough to support themselves, and therefore do not circumnutate in any plainly recognisable manner.* In this particular instance the fifth internode (including the hypocotyl) was the first which plainly circumnutated and twined round a stick. We therefore wished to learn whether circumnutation could be observed in the hypocotyl if carefully observed in our usual manner. Two seedlings were kept in the dark with filaments fixed to the upper part of their hypocotyls; but from circumstances not worth explaining their movements were traced for only a short time. One moved thrice forwards and twice backwards in nearly opposite directions, in the course of 3 h. 15 m.; and the other twice forwards and twice backwards in 2 h. 22 m. The hypocotyl therefore circumnutated at a remarkably rapid rate. It may here be added that a filament was affixed transversely to the summit of the second internode above the cotyledons of a little plant 3½ inches in height; and its movements were traced on a horizontal glass. It circumnutated, and the actual distance travelled from side to side was a quarter of an inch, which was too small an amount to be perceived without the aid of marks.
The movements of the cotyledons are interesting from their complexity and rapidity, and in some other respects. The hypocotyl (2 inches high) of a vigorous seedling was secured to a stick, and a filament with triangles of paper was affixed to one of the cotyledons. The plant was kept all day in the hot-house, and at 4.20 P.m. (June 20th) was placed under a skylight in the house, and observed occasionally during the evening and night. It fell in a slightly zigzag line to a moderate extent from 4.20 P.m. till 10.15 P.m. When looked at shortly after midnight (12.30 P.m.) it had risen a very little, and considerably by
* 'Movements and Habits of Climbing Plants,' p. 33, 1875. [page 48]
3.45 A.m. When again looked at, at 6.10 A.m. (21st), it had fallen largely. A new tracing was now begun (see Fig. 35), and soon afterwards, at 6.42 A.m., the cotyledon had risen a little. During the forenoon it was observed about every hour; but between 12.30 and 6 P.m. every half-hour. If the observations had been made at these short intervals during the whole day, the figure would have been too intricate to have been copied. As it was, the cotyledon moved up and down in the course of 16 h. 20 m. (i.e. between 6.10 A.m. and 10.30 P.m.) thirteen times.
Fig 35. Ipomoea caerulea: circumnutation of cotyledon, traced on vertical glass, from 6.10 A.m. June 21st to 6.45 A.m. 22nd. Cotyledon with petiole 1.6 inch in length, apex of blade 4.1 inch from the vertical glass; so movement not greatly magnified; temp. 20o C.
The cotyledons of this seedling sank downwards during both evenings and the early part of the night, but rose during the latter part. As this is an unusual movement, the cotyledons of twelve other seedlings were observed; they stood almost or quite horizontally at mid-day, and at 10 P.m. were all declined at various angles. The most usual angle was between 30o and 35o; but three stood at about 50o and one at even 70o beneath the horizon. The blades of all these cotyledons had attained almost their full size, viz. from 1 to 1½ inches in length, measured along their midribs. It is a remarkable fact that whilst young—that is, when less than half an inch in length, measured in the same manner—they do not sink [page 49] downwards in the evening. Therefore their weight, which is considerable when almost fully developed, probably came into play in originally determining the downward movement. The periodicity of this movement is much influenced by the degree of light to which the seedlings have been exposed during the day; for three kept