The Power of Movement in Plants, Charles Darwin [the reading list book TXT] 📗

now travelled in the same direction as before, but at the rate of 3 m. 18 s. for five divisions.

 

We shall have to recur to the cotyledons of the cabbage in a future chapter, when we treat of their sleep-movements. The circumnutation, also, of the leaves of fully-developed plants will hereafter be described.

 

Fig. 11. Githago segetum: circumnutation of hypocotyl, traced on a horizontal glass, by means of a filament fixed transversely across its summit, from 8.15 A.M. to 12.15 P.M. on the following day. Movement of bead of filament magnified about 13 times, here reduced to one-half the original scale.

 

Githago segetum (Caryophylleae).—A young seedling was dimly illuminated from above, and the circumnutation of the hypo-

[page 22]

cotyl was observed during 28 h., as shown in Fig. 11. It moved in all directions; the lines from right and to left in the figure being parallel to the blades of the cotyledons. The actual distance travelled from side to side by the summit of the hypocotyl was about .2 of an inch; but it was impossible to be accurate on this head, as the more obliquely the plant was viewed, after it had moved for some time, the more the distances were exaggerated.

 

We endeavoured to observe the circumnutation of the cotyledons, but as they close together unless kept exposed to a moderately bright light, and as the hypocotyl is extremely heliotropic, the necessary arrangements were too troublesome. We shall recur to the nocturnal or sleep-movements of the cotyledons in a future chapter.

 

Fig. 12. Gossypium: circumnutation of hypocotyl, traced on a horizontal glass, from 10.30 A.M. to 9.30 A.M. on following morning, by means of a filament fixed across its summit. Movement of bead of filament magnified about twice; seedling illuminated from above.

 

Gossypium (var. Nankin cotton) (Malvaceae).—The circumnutation of a hypocotyl was observed in the hot-house, but the movement was so much exaggerated that the bead twice passed for a time out of view. It was, however, manifest that two somewhat irregular ellipses were nearly completed in 9 h. Another seedling, 1 � in. in height, was then observed during 23 h.; but the observations were not made at sufficiently short intervals, as shown by the few dots in Fig. 12, and the tracing was not now sufficiently enlarged. Nevertheless there could be no doubt about the circumnutation of the hypocotyl, which described in 12 h. a figure representing three irregular ellipses of unequal sizes.

 

The cotyledons are in constant movement up and down during the whole day, and as they offer the unusual case of moving downwards late in the evening and in the early part of the night, many observations were made on them. A filament was fixed along the middle of one, and its movement traced on a vertical glass; but the tracing is not given, as the hypocotyl was not secured, so that it was impossible to distinguish clearly between its movement and that of the cotyledon. The cotyledons rose from 10.30 A.M. to about 3 P.M.; they then sank till 10 P.M., rising, however, greatly in the latter part of the night.

[page 23]

The angles above the horizon at which the cotyledons of another seedling stood at different hours is recorded in the following short table: —

 

Oct. 20 2.50 P.M…25o above horizon.

Oct. 20 4.20 P.M…22o above horizon.

Oct. 20 5.20 P.M…15o above horizon.

Oct. 20 10.40 P.M…8o above horizon.

Oct. 21 8.40 A.M…28o above horizon.

Oct. 21 11.15 A.M…35o above horizon.

Oct. 21 9.11 P.M…10o below horizon.

 

The position of the two cotyledons was roughly sketched at various hours with the same general result.

 

In the following summer, the hypocotyl of a fourth seedling was secured to a little stick, and a glass filament with triangles of paper having been fixed to one of the cotyledons, its movements were traced on a vertical glass under a double skylight in the house. The first dot was made at 4.20

P.M. June 20th; and the cotyledon fell till 10.15 P.M. in a nearly straight line. Just past midnight it was found a little lower and somewhat to one side. By the early morning, at 3.45 A.M., it had risen greatly, but by 6.20

A.M. had fallen a little. During the whole of this day (21st) it fell in a slightly zigzag line, but its normal course was disturbed by the want of sufficient illumination, for during the night it rose only a little, and travelled irregularly during the whole of the following day and night of June 22nd. The ascending and descending lines traced during the three days did not coincide, so that the movement was one of circumnutation. This seedling was then taken back to the hot-house, and after five days was inspected at 10 P.M., when the cotyledons were found hanging so nearly vertically down, that they might justly be said to have been asleep. On the following morning they had resumed their usual horizontal position.

 

Oxalis rosea (Oxalideae).—The hypocotyl was secured to a little stick, and an extremely thin glass filament, with two triangles of paper, was attached to one of the cotyledons, which was .15 inch in length. In this and the following species the end of the petiole, where united to the blade, is developed into a pulvinus. The apex of the cotyledon stood only 5 inches from the vertical glass, so that its movement was not greatly exaggerated as long as it remained nearly horizontal; but in the course of the day it both rose considerably above and fell beneath a horizontal position, and then of course the movement was much exaggerated.

[page 24]

In Fig. 13 its course is shown from 6.45 A.M. on June 17th, to 7.40 A.M. on the following morning; and we see that during the daytime, in the course of 11 h. 15 m., it travelled thrice down and twice up. After 5.45 P.M. it moved rapidly downwards, and in an hour or two depended vertically; it thus remained all night asleep. This position could not be represented on the vertical glass nor in the figure here given. By 6.40 A.M. on the following morning (18th) both cotyledons had risen greatly, and they continued to rise until 8 A.M., when they stood almost horizontally. Their movement was traced during the whole of this day and until the next morning; but a tracing is not given, as it was closely similar to Fig. 13, excepting that the lines were more zigzag. The cotyledons moved 7 times, either upwards or downwards; and at about 4 P.M. the great nocturnal sinking movement commenced.

 

Fig. 13. Oxalis rosea: circumnutation of cotyledons, the hypocotyl being secured to a stick; illuminated from above. Figure here given one-half of original scale.

 

Another seedling was observed in a similar manner during nearly 24 h., but with the difference that the hypocotyl was left free. The movement also was less magnified. Between 8.12 A.M. and 5 P.M. on the 18th, the apex of the cotyledon moved 7 times upwards or downwards (Fig. 14). The nocturnal sinking movement, which is merely a great increase of one of the diurnal oscillations, commenced about 4 P.M.

 

Oxalis Valdiviana.—This species is interesting, as the coty-

[page 25]

ledons rise perpendicularly upwards at night so as to come into close contact, instead of sinking vertically downwards, as in the case of O.

rosea. A glass filament was fixed to a cotyledon, .17 of an inch in length, and the hypocotyl was left free. On

 

Fig. 14. Oxalis rosea: conjoint circumnutation of the cotyledons and hypocotyl, traced from 8.12 A.M. on June 18th to 7.30 A.M. 19th. The apex of the cotyledon stood only 3 3/4 inches from the vertical glass. Figure here given one-half of original scale.

 

Fig. 15. Oxalis Valdiviana: conjoint circumnutation of a cotyledon and the hypocotyl, traced on vertical glass, during 24 hours. Figure here given one-half of original scale; seedling illuminated from above.

 

the first day the seedling was placed too far from the vertical glass; so that the tracing was enormously exaggerated and the movement could not be traced when the cotyledon either rose or sank much; but it was clearly seen that the cotyledons rose thrice and fell twice between 8.15 A.M. and 4.15

P.M. Early on the following morning (June 19th) the apex of a cotyledon was [page 26]

placed only 1 7/8 inch from the vertical glass. At 6.40 A.M. it stood horizontally; it then fell till 8.35, and then rose. Altogether in the course of 12 h. it rose thrice and fell thrice, as may be seen in Fig. 15.

The great nocturnal rise of the cotyledons usually commences about 4 or 5

P.M., and on the following morning they are expanded or stand horizontally at about 6.30 A.M. In the present instance, however, the great nocturnal rise did not commence till 7 P.M.; but this was due to the hypocotyl having from some unknown cause temporarily bent to the left side, as is shown in the tracing. To ascertain positively that the hypocotyl circumnutated, a mark was placed at 8.15 P.M. behind the two now closed and vertical cotyledons; and the movement of a glass filament fixed upright to the top of the hypocotyl was traced until 10.40 P.M. During this time it moved from side to side, as well as backwards and forwards, plainly showing circumnutation; but the movement was small in extent. Therefore Fig. 15

represents fairly well the movements of the cotyledons alone, with the exception of the one great afternoon curvature to the left.

 

Oxalis corniculata (var. cuprea).—The cotyledons rise at night to a variable degree above the horizon, generally about 45o: those on some seedlings between 2 and 5 days old were found to be in continued movement all day long; but the movements were more simple than in the last two species. This may have partly resulted from their not being sufficiently illuminated whilst being observed, as was shown by their not beginning to rise until very late in the evening.

 

Oxalis (Biophytum) sensitiva.—The cotyledons are highly remarkable from the amplitude and rapidity of their movements during the day. The angles at which they stood above or beneath the horizon were measured at short intervals of time; and we regret that their course was not traced during the whole day. We will give only a few of the measurements, which were made whilst the seedlings were exposed to a temperature of 22 1/2o to 24 �

decrees C. One cotyledon rose 70o in 11 m.; another, on a distinct seedling, fell 80o in 12 m. Immediately before this latter fall the same cotyledon had risen from a vertically downward to a vertically upward position in 1 h. 48 m., and had therefore passed through 180o in under 2 h.

We have met with no other instance of a circumnutating movement of such great amplitude as 180o; nor of such rapidity of movement as the passage through 80o in 12 m. The cotyledons of this plant sleep at night by rising [page 27]

vertically and coming into close contact. This upward movement differs from one of the great diurnal oscillations above described only by the position being permanent during the night and by its periodicity, as it always commences late in the evening.

 

Tropaeolum minus (?) (var. Tom Thumb) (Tropaeoleae).—The cotyledons are hypogean, or never rise above the ground. By removing the soil a buried epicotyl or plumule was found, with its summit arched abruptly downwards, like the arched hypocotyl of the cabbage previously described. A glass filament with a bead at its end was affixed to the basal half or leg, just above the hypogean cotyledons, which were again almost surrounded by loose earth. The tracing (Fig. 16) shows the course of the bead during 11 h.

After the last dot given in the figure, the bead