The Power of Movement in Plants - Charles Darwin (top reads txt) 📗

Eucalyptus resinifera (Myrtaceae, Fam. 94).—A young leaf, two inches in length together with the petiole, produced by a lateral shoot from a cut-down tree, was observed in the usual manner. The blade had not as yet assumed its vertical position. On June 7th only a few observations were made, and the tracing merely showed that the leaf had moved three times upwards and three downwards. On the following day it was observed more frequently; and two tracings were made (see A and B, Fig. 108), as a single one would have been too complicated. The apex changed its course 13 times in the course of 16 h., chiefly up and down, but with some lateral movement. The actual amount of movement in any one direction was small.

 

Fig. 108. Eucalyptus resinifera: circumnutation of a leaf, traced, A, from 6.40 A.M. to 1 P.M. June 8th; B, from 1 P.M. 8th to 8.30 A.M. 9th. Apex of leaf 14 � inches from the horizontal glass, so figures considerably magnified.

 

(17.) Dahlia (garden var.) (Compositae, Fam. 122).—A fine young [page 245]

leaf 5 3/4 inches in length, produced by a young plant 2 feet high, growing vigorously in a large pot, was directed at an angle of about 45o beneath the horizon. On June 18th the leaf descended from 10 A.M. till 11.35 A.M.

(see Fig. 109); it then ascended greatly till 6 P.M., this ascent being probably due to the light

 

Fig. 109. Dahlia: circumnutation of leaf, traced from 10 A.M. June 18th to 8.10 A.M. 20th, but with a break of 1 h. 40 m. on the morning of the 19th, as, owing to the glass filament pointing too much to one side, the pot had to be slightly moved; therefore the relative position of the two tracings is somewhat arbitrary. The figure here given is reduced to one-fifth of the original scale. Apex of leaf 9 inches from the glass in the line of its inclination, and 4 3/4 in a horizontal line.

coming only from above. It zigzagged between 6 P.M. and 10.35 P.M., and ascended a little during the night. It should be remarked that the vertical distances in the lower part of the diagram are much exaggerated, as the leaf was at first deflected beneath the horizon, and after it had sunk downwards, the filament pointed in a very oblique line towards the glass.

Next

[page 246]

day the leaf descended from 8.20 A.M. till 7.15 P.M., then zigzagged and ascended greatly during the night. On the morning of the 20th the leaf was probably beginning to descend, though the short line in the diagram is horizontal. The actual distances travelled by the apex of the leaf were considerable, but could not be calculated with safety. From the course pursued on the second day, when the plant had accommodated itself to the light from above, there cannot be much doubt that the leaves undergo a daily periodic movement, sinking during the day and rising at night.

 

(18.) Mutisia clematis (Compositae).—The leaves terminate in tendrils and circumnutate like those of other tendril-bearers; but this plant is here mentioned, on account of an erroneous statement* which has been published, namely, that the leaves sink at night and rise during the day. The leaves which behaved in this manner had been kept for some days in a northern room and had not been sufficiently illuminated. A plant therefore was left undisturbed in the hot-house, and three leaves had their angles measured at noon and at 10 P.M. All three were inclined a little beneath the horizon at noon, but one stood at night 2o, the second 21o, and the third 10o higher than in the middle of the day; so that instead of sinking they rise a little at night.

 

(19.) Cyclamen Persicum (Primulaceae, Fam. 135).—A young leaf, 1.8 of an inch in length, petiole included, produced by an old root-stock, was observed during three days in the usual manner (Fig. 110). On the first day the leaf fell more than afterwards, apparently from adjusting itself to the light from above. On all three days it fell from the early morning to about 7 P.M., and from that hour rose during the night, the course being slightly zigzag. The movement therefore is strictly periodic. It should be noted that the leaf would have sunk each evening a little lower down than it did, had not the glass filament rested between 5 and 6 P.M. on the rim of the pot. The amount of movement was considerable; for if we assume that the whole leaf to the base of the petiole became bent, the tracing would be magnified rather less than five times, and this would give to the apex a rise and fall of half an inch, with some lateral movement. This amount, however, would not attract attention without the aid of a tracing or measurement of some kind.

 

* ‘The Movements and Habits of Climbing Plants,’ 1875, p. 118.

[page 247]

 

(20.) Allamanda Schottii (Apocyneae, Fam. 144).—The young leaves of this shrub are elongated, with the blade bowed so much Fig. 110. Cyclamen Persicum: circumnutation of leaf, traced from 6.45 A.M.

June 2nd to 6.40 A.M. 5th. Apex of leaf 7 inches from the vertical glass.

 

downwards as almost to form a semicircle. The chord—that is, a line drawn from the apex of the blade to the base of the petiole—of a young leaf, 4

3/4 inches in length, stood at 2.50 P.M. on [page 248]

Dec. 5th at an angle of 13o beneath the horizon, but by 9.30 P.M. the blade had straightened itself so much, which implies the raising of the apex, that the chord now stood at 37o above the horizon, and had therefore risen 50o. On the next day similar angular measurements of the same leaf were made; and at noon the chord stood 36o beneath the horizon, and 9.30 P.M. 3

1/2o above it, so had risen 39 1/2o. The chief cause of the rising movement lies in the straightening of the blade, but the short petiole rises between 4o and 5o. On the third night the chord stood at 35o above the horizon, and if the leaf occupied the same position at noon, as on the previous day, it had risen 71o. With older leaves no such change of curvature could be detected. The plant was then brought into the house and kept in a north-east room, but at night there was no change in the curvature of the young leaves; so that previous exposure to a strong light is apparently requisite for the periodical change of curvature in the blade, and for the slight rising of the petiole.

 

(21.) Wigandia (Hydroleaceae, Fam. 149).—Professor Pfeffer informs us that the leaves of this plant rise in the evening; but as we do not know whether or not the rising is great, this species ought perhaps to be classed amongst sleeping plants.

 

Fig. 111. Petunia violacea: downward movement and circumnutation of a very young leaf, traced from 10 A.M. June 2nd to 9.20 A.M. June 6th. N.B.—At 6.40 A.M. on the 5th it was necessary to move the pot a little, and a new tracing was begun at the point where two dots are not joined in the diagram. Apex of leaf 7 inches from the vertical glass. Temp. generally 17

1/2o C.

[page 249]

 

(22.) Petunia violacea (Solaneae, Fam. 157).—A very young leaf, only 3/4

inch in length, highly inclined upwards, was observed for four days. During the whole of this time it bent outwards and downwards, so as to become more and more nearly horizontal. The strongly marked zigzag line in the figure on p. 248 (Fig. 111), shows that this was effected by modified circumnutation; and during the latter part of the time there was much ordinary circumnutation on a small scale. The movement in the diagram is magnified between 10 and 11 times. It exhibits a clear trace of periodicity, as the leaf rose a little each evening; but this upward tendency appeared to be almost conquered by the leaf striving to become more and more horizontal as it grew older. The angles which two older leaves formed together, were measured in the evening and about noon on 3

successive days, and each night the angle decreased a little, though irregularly.

 

Fig. 112. Acanthus mollis: circumnutation of young leaf, traced from 9.20

A.M. June 14th to 8.30 A.M. 16th. Apex of leaf 11 inches from the vertical glass, so movement considerably magnified. Figure here reduced to one-half of original scale. Temp. 15o - 16 1/2o C.

 

(23.) Acanthus mollis (Acanthaceae, Fam. 168).—The younger of two leaves, 2 1/4 inches in length, petiole included, produced by a seedling plant, was observed during 47 h. Early on each of the three mornings, the apex of the leaf fell; and it continued to fall till 3 P.M., on the two afternoons when observed. After 3 P.M. it rose considerably, and continued to rise on the second night until the early morning. But on the first night it fell instead of rising, and we have little doubt that this was owing to the leaf being very young and becoming through epinastic growth more and more horizontal; for it may be seen in the diagram (Fig. 112), that the leaf stood on a higher level on the first than on the second day. The leaves of an allied species (‘A. spinosus’) certainly rose every night; and the rise between noon and 10.15 P.M., when measured on one occasion, was 10o. This rise was chiefly

[page 250]

or exclusively due to the straightening of the blade, and not to the movement of the petiole. We may therefore conclude that the leaves of Acanthus circumnutate periodically, falling in the morning and rising in the afternoon and night.

 

(24.) Cannabis sativa (Cannabineae, Fam. 195).—We have here the rare case of leaves moving downwards in the evening, but not to a sufficient degree to be called sleep.* In the early morning, or in the latter part of the night, they move upwards. For instance, all the young leaves near the summits of several stems stood almost horizontally at 8 A.M. May 29th and at 10.30 P.M. were considerably declined. On a subsequent day two leaves stood at 2 P.M. at 21o and 12o beneath the horizon, and at 10 P.M. at 38o beneath it. Two other leaves on a younger plant were horizontal at 2 P.M., and at 10 P.M. had sunk to 36o beneath the horizon. With respect to this downward movement of the leaves, Kraus believes that it is due to their epinastic growth. He adds, that the leaves are relaxed during the day, and tense at night, both in sunny and rainy weather.

 

(25.) Pinus pinaster (Coniferae, Fam. 223).—The leaves on the summits of the terminal shoots stand at first in a bundle almost upright, but they soon diverge and ultimately become almost horizontal. The movements of a young leaf, nearly one inch in length, on the summit of a seedling plant only 3 inches high, were traced from the early morning of June 2nd to the evening of the 7th. During these five days the leaf diverged, and its apex descended at first in an almost straight line; but during the two latter days it zigzagged so much that it was evidently circumnutating. The same little plant, when grown to a height of 5 inches, was again observed during four days. A filament was fixed transversely to the apex of a leaf, one inch in length, and which had already diverged considerably from its originally upright position. It continued to diverge (see A, Fig. 113), and to descend from 11.45 A.M. July 31st to 6.40 A.M. Aug. 1st. On August 1st it circumnutated about the same small space, and again descended at night.

Next morning the pot was moved