The Power of Movement in Plants by Charles Darwin (book club books TXT) 📖

some special purpose, which may be to protect the young plumule from radiation, by the close contact of the whole basal portion of the two cotyledons.

 

Geranium rotundifolium (Geraniaceae).—A single seedling came up accidentally in a pot, and its cotyledons were observed to bend perpendicularly downwards during several successive nights, having been horizontal at noon. It grew into a fine plant but died before flowering: it was sent to Kew and pronounced to be certainly a Geranium, and in all probability the above-named species. This case is remarkable because the cotyledons of G. cinereum, Endressii, Ibericum, Richardsoni, and subcaulescens were observed during some weeks in the winter, and they did not sink, whilst those of G. Ibericum rose 27o at night.

 

Apium petroselinum (Umbelliferae).—A seedling had its cotyledons (Nov.

22nd) almost fully expanded during the day; by 8.30 P.M. they had risen considerably, and at 10.30 P.M. were almost closed, their tips being only 8/100 of an inch apart. On the following morning (23rd) the tips were 58/100 of an inch apart,

 

* This plant, from Dammara Land in S. Africa, is remarkable from being the one known member of the Family which is not a climber; it has been described in ‘Transact. Linn. Soc.,’ xxvii. p. 30.

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or more than seven times as much. On the next night the cotyledons occupied nearly the same position as before. On the morning of the 24th they stood horizontally, and at night were 60o above the horizon; and so it was on the night of the 25th. But four days afterwards (on the 29th), when the seedlings were a week old, the cotyledons had ceased to rise at night to any plain degree.

 

Apium graveolens.—The cotyledons at noon were horizontal, and at 10 P.M.

stood at an angle of 61o above the horizon.

 

Lactuca scariola (Compositae).—The cotyledons whilst young stood sub-horizontally during the day, and at night rose so as to be almost vertical, and some were quite vertical and closed; but this movement ceased when they had grown old and large, after an interval of 11 days.

 

Helianthus annuus (Compositae).—This case is rather doubtful; the cotyledons rise at night, and on one occasion they stood at 73o above the horizon, so that they might then be said to have been asleep.

 

Ipomoea caerulea vel Pharbitis nil (Convolvulaceae).—The cotyledons behave in nearly the same manner as those of the Anoda and Nankin cotton, and like them grow to a large size. Whilst young and small, so that their blades were from .5 to .6 of an inch in length, measured along the middle to the base of the central notch, they remained horizontal both during the middle of the day and at night. As they increased in size they began to sink more and more in the evening and early night; and when they had grown to a length (measured in the above manner) of from 1 to 1.25 inch, they sank between 55o and 70o beneath the horizon. They acted, however, in this manner only when they had been well illuminated during the day.

Nevertheless, the cotyledons have little or no power of bending towards a lateral light, although the hypocotyl is strongly heliotropic. They are not provided with a pulvinus, but continue to grow for a long time.

 

Ipomoea purpurea (vel Pharbitis hispida).—The cotyledons behave in all respects like those of I. caerulea. A seedling with cotyledons .75 inch in length (measured as before) and 1.65 inch in breadth, having a small true leaf developed, was placed at 5.30 P.M. on a klinostat in a darkened box, so that neither weight nor geotropism could act on them. At 10 P.M. one cotyledon stood at 77o and the other at 82o beneath the horizon. Before being placed in the klinostat they stood at 15o and 29o [page 306]

beneath the horizon. The nocturnal position depends chiefly on the curvature of the petiole close to the blade, but the whole petiole becomes slightly curved downwards. It deserves notice that seedlings of this and the last-named species were raised at the end of February and another lot in the middle of March, and the cotyledons in neither case exhibited any nyctitropic movement.

 

Ipomoea bona-nox.—The cotyledons after a few days grow to an enormous size, those on a young seedling being 3 1/4 inches in breadth. They were extended horizontally at noon, and at 10 P.M. stood at 63o beneath the horizon. five days afterwards they were 4 � inches in breadth, and at night one stood at 64o and the other 48o beneath the horizon. Though the blades are thin, yet from their great size and from the petioles being long, we imagined that their depression at night might be determined by their weight; but when the pot was laid horizontally, they became curved towards the hypocotyl, which movement could not have been in the least aided by their weight, at the same time they were somewhat twisted upwards through apogeotropism. Nevertheless, the weight of the cotyledons is so far influential, that when on another night the pot was turned upside down, they were unable to rise and thus to assume their proper nocturnal position.

 

Ipomoea coccinea.—The cotyledons whilst young do not sink at night, but when grown a little older, but still only .4 inch in length (measured as before) and .82 in breadth, they became greatly depressed. In one case they were horizontal at noon, and at 10 P.M. one of them stood at 64o and the other at 47o beneath the horizon. The blades are thin, and the petioles, which become much curved down at night, are short, so that here weight can hardly have produced any effect. With all the above species of Ipomoea, when the two cotyledons on the same seedling were unequally depressed at night, this seemed to depend on the position which they had held during the day with reference to the light.

 

Solanum lycopersicum (Solaneae).—The cotyledons rise so much at night as to come nearly in contact. Those of ‘S. palinacanthum’ were horizontal at noon, and by 10 P.M. had risen only 27o 30 minutes; but on the following morning before it was light they stood at 59o above the horizon, and in the afternoon of the same day were again horizontal. The behaviour of the cotyledons of this latter species seems, therefore, to be anomalous.

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Mirabilis jalapa and longiflora (Nyctagineae).—The cotyledons, which are of unequal size, stand horizontally during the middle of the day, and at night rise up vertically and come into close contact with one another. But this movement with M. longiflora lasted for only the three first nights.

 

Beta vulgaris (Polygoneae).—A large number of seedlings were observed on three occasions. During the day the cotyledons sometimes stood sub-horizontally, but more commonly at an angle of about 50o above the horizon, and for the first two or three nights they rose up vertically so as to be completely closed. During the succeeding one or two nights they rose only a little, and afterwards hardly at all.

 

Amaranthus caudatus (Amaranthaceae).—At noon the cotyledons of many seedlings, which had just germinated, stood at about 45o above the horizon, and at 10.15 P.M. some were nearly and the others quite closed. On the following morning they were again well expanded or open.

 

Cannabis sativa (Cannabineae).—We are very doubtful whether this plant ought to be here included. The cotyledons of a large number of seedlings, after being well illuminated during the day, were curved downwards at night, so that the tips of some pointed directly to the ground, but the basal part did not appear to be at all depressed. On the following morning they were again flat and horizontal. the cotyledons of many other seedlings were at the same time not in any way affected. Therefore this case seems very different from that of ordinary sleep, and probably comes under the head of epinasty, as is the case with the leaves of this plant according to Kraus. The cotyledons are heliotropic, and so is the hypocotyl in a still stronger degree.

 

Oxalis.—We now come to cotyledons provided with a pulvinus, all of which are remarkable from the continuance of the nocturnal movements during several days or even weeks, and apparently after growth has ceased. The cotyledons of O. rosea, floribunda and articulata sink vertically down at night and clasp the upper part of the hypocotyl. Those of O. Valdiviana and sensitiva, on the contrary, rise vertically up, so that their upper surfaces come into close contact; and after the young leaves are developed these are clasped by the cotyledons. As in the daytime they stand horizontally, or are even a little deflected beneath the horizon, they move in the evening through an angle of at least 90o. Their complicated circumnutating movements during the day have [page 308]

been described in the first chapter. The experiment was a superfluous one, but pots with seedlings of O. rosea and floribunda were turned upside down, as soon as the cotyledons began to show any signs of sleep, and this made no difference in their movements.

 

Leguminosae.—It may be seen in our list that the cotyledons of several species in nine genera, widely distributed throughout the Family, sleep at night; and this probably is the case with many others. The cotyledons of all these species are provided with a pulvinus; and the movement in all is continued during many days or weeks. In Cassia the cotyledons of the ten species in the list rise up vertically at night and come into close contact with one another. We observed that those of C. florida opened in the morning rather later than those of C. glauca and pubescens. The movement is exactly the same in C. mimosoides as in the other species, though its subsequently developed leaves sleep in a different manner. The cotyledons of an eleventh species, namely, C. nodosa, are thick and fleshy, and do not rise up at night. The circumnutation of the cotyledons during the day of C.

tora has been described in the first chapter. Although the cotyledons of Smithia sensitiva rose from a horizontal position in the middle of the day to a vertical one at night, those of S. Pfundii, which are thick and fleshy, did not sleep. When Mimosa pudica and albida have been kept at a sufficiently high temperature during the day, the cotyledons come into close contact at night; otherwise they merely rise up almost vertically.

The circumnutation of those of M. pudica has been described. The cotyledons of a Bauhinia from St. Catharina in Brazil stood during the day at an angle of about 50o above the horizon, and at night rose to 77o; but it is probable that they would have closed completely, if the seedlings had been kept in a warmer place.

 

Lotus.—In three species of Lotus the cotyledons were observed to sleep.

Those of L. Jacoboeus present the singular case of not rising at night in any conspicuous manner for the first 5 or 6 days of their life, and the pulvinus is not well developed at this period. Afterwards the sleeping movement is well displayed, though to a variable degree, and is long continued. We shall hereafter meet with a nearly parallel case with the leaves of Sida rhombifolia. The cotyledons of L. Gebelii are only slightly raised at night, and differ much in this respect from the three species in our list.

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Trifolium.—The germination of 21 species was observed. In most of them the cotyledons rise hardly at all, or only slightly, at night; but those of T.

glomeratum, striatum and incarnactum rose from 45o to 55o above the horizon. With T. subterraneum, leucanthemum and strictum, they stood up vertically; and with T. strictum the rising movement is accompanied, as we shall see, by another movement, which makes us