Another analogous fact deserves notice: we observed on several occasions that a greater number of free leaves were injured on the branches which had been kept motionless by some of their leaves having been pinned to the corks, than on the other branches. This was conspicuously the case with those of Melilotus Petitpierreana, but the injured leaves in this instance were not actually counted. With Arachis hypogaea, a young plant with 7 stems bore 22 free leaves, and of these 5 were injured by the frost, all of which were on two stems, bearing four leaves pinned to the cork-supports. With Oxalis carnosa, 7 free leaves were injured, and every one of them belonged to a cluster of leaves, some of which had been pinned to the cork. We could account for these cases only by supposing that the branches which were quite free had been slightly waved about by the wind, and that their leaves had thus been a little warmed by the surrounding warmer air. If we hold our hands motionless before a hot fire, and then wave them about, we immediately feel relief; and this is evidently an analogous, though reversed, case. These several facts—in relation to leaves pinned close to or a little above the cork-supports—to their tips projecting beyond it—and to the leaves on branches kept motionless—seem to us curious, as showing how a difference, apparently trifling, may determine the greater or less injury of the leaves. We may even infer as probable that the less or greater destruction during a frost of the leaves on a plant which does not sleep, may often depend on the greater or less degree of flexibility of their petioles and of the branches which bear them.

NYCTITROPIC OR SLEEP MOVEMENTS OF COTYLEDONS.

We now come to the descriptive part of our work, and will begin with cotyledons, passing on to leaves in the next chapter. We have met with only two brief notices of cotyledons sleeping. Hofmeister,[[13]] after stating that the cotyledons of all the observed seedlings of the Caryophylleae (Alsineae and Sileneae) bend upwards at night (but to what angle he does not state), remarks that those of Stellaria media rise up so as to touch one another; they may therefore safely be said to sleep. Secondly, according to Ramey,[[14]] the cotyledons of Mimosa pudica and of Clianthus Dampieri rise up almost vertically at night and approach each other closely. It has been shown in a previous chapter that the cotyledons of a large number of plants bend a little upwards at night, and we here have to meet the difficult question at what inclination may they be said to sleep? According to the view which we maintain, no movement deserves to be called nyctitropic, unless it has been acquired for the sake of lessening radiation; but this could be discovered only by a long series of experiments, showing that the leaves of each species suffered from this cause, if prevented from sleeping. We must therefore take an arbitrary limit. If a cotyledon or leaf is inclined at 60° above or beneath the horizon, it exposes to the zenith about one-half of its area; consequently the intensity of its radiation will be lessened by about half, compared with what it would have been if the cotyledon or leaf had remained horizontal. This degree of diminution certainly would make a great difference to a plant having a tender constitution. We will therefore speak of a cotyledon and hereafter of a leaf as sleeping, only when it rises at night to an angle of about 60°, or to a still higher angle, above the horizon, or sinks beneath it to the same amount. Not but that a lesser diminution of radiation may be advantageous to a plant, as in the case of Datura stramonium, the cotyledons of which rose from 31° at noon to 55° at night above the horizon. The Swedish turnip may profit by the area of its leaves being reduced at night by about 30 per cent., as estimated by Mr. A. S. Wilson; though in this case the angle through which the leaves rose was not observed. On the other hand, when the angular rise of cotyledons or of leaves is small, such as less than 30°, the diminution of radiation is so slight that it probably is of no significance to the plant in relation to radiation. For instance, the cotyledons of Geranium Ibericum rose at night to 27° above the horizon, and this would lessen radiation by only 11 per cent.: those of Linum Berendieri rose to 33°, and this would lessen radiation by 16 per cent.

[13] ‘Die Lehre von der Pflanzenzelle,’ 1867, p. 327.

[14] ‘Adansonia,’ March 10th, 1869.

There are, however, some other sources of doubt with respect to the sleep of cotyledons. In certain cases, the cotyledons whilst young diverge during the day to only a very moderate extent, so that a small rise at night, which we know occurs with the cotyledons of many plants, would necessarily cause them to assume a vertical or nearly vertical position at night; and in this case it would be rash to infer that the movement was effected for any special purpose. On this account we hesitated long whether we should introduce several Cucurbitaceous plants into the following list; but from reasons, presently to be given, we thought that they had better be at least temporarily included. This same source of doubt applies in some few other cases; for at the commencement of our observations we did not always attend sufficiently to whether the cotyledons stood nearly horizontally in the middle of the day. With several seedlings, the cotyledons assume a highly inclined position at night during so short a period of their life, that a doubt naturally arises whether this can be of any service to the plant. Nevertheless, in most of the cases given in the following list, the cotyledons may be as certainly said to sleep as may the leaves of any plant. In two cases, namely with the cabbage and radish, the cotyledons of which rise almost vertically during the few first nights of their life, it was ascertained by placing young seedlings in the klinostat, that the upward movement was not due to apogeotropism.

The names of the plants, the cotyledons of which stand at night at an angle of at least 60° with the horizon, are arranged in the appended list on the same system as previously followed. The numbers of the Families, and with the Leguminosae the numbers of the Tribes, have been added to show how widely the plants in question are distributed throughout the dicotyledonous series. A few remarks will have to be made about many of the plants in the list. In doing so, it will be convenient not to follow strictly any systematic order, but to treat of the Oxalidæ and the Leguminosae at the close; for in these two Families the cotyledons are generally provided with a pulvinus, and their movements endure for a much longer time than those of the other plants in the list.

List of Seedling Plants, the cotyledons of which rise or sink at night to an angle of at least 60° above or beneath the horizon.

Brassica oleracea. Cruciferae (Fam. 14). — napus (as we are informed by Prof. Pfeffer). Raphanus sativus. Cruciferae. Githago segetum. Caryophylleae (Fam. 26). Stellaria media (according to Hofmeister, as quoted). Caryophylleae. Anoda Wrightii. Malvaceae (Fam. 36). Gossypium (var. Nankin cotton). Malvaceae. Oxalis rosea. Oxalidæ (Fam. 41). — floribunda. — articulata. — Valdiviana. — sensitiva. Geranium rotundifolium. Geraniaceae (Fam. 47). Trifolium subterraneum. Leguminosae (Fam. 75, Tribe 3). — strictum. — leucanthemum. Lotus ornithopopoides. Leguminosae (Tribe 4). — peregrinus. — Jacobæus. Clianthus Dampieri. Leguminosae (Tribe 5)—according to M. Ramey. Smithia sensitiva. Leguminosae (Tribe 6). Haematoxylon Campechianum. Leguminosae (Tribe 13)—according to Mr. R. I. Lynch. Cassia mimosoides. Leguminosae (Tribe 14). — glauca. — florida. — corymbosa. — pubescens. — tora. — neglecta. — 3 other Brazilian unnamed species. Bauhinia (sp.?. Leguminosae (Tribe 15). Neptunia oleracea. Leguminosae (Tribe 20). Mimosa pudica. Leguminosae (Tribe 21). — albida. Cucurbita ovifera. Cucurbitaceæ (Fam. 106). — aurantia. Lagenaria vulgaris. Cucurbitaceæ. Cucumis dudaim. Cucurbitaceæ. Apium petroselinum. Umbelliferae (Fam. 113). — graveolens. Lactuca scariola. Compositæ (Fam. 122). Helianthus annuus (?). Compositæ. Ipomœa caerulea. Convolvulaceae (Fam. 151). — purpurea. — bona-nox. — coccinea. Solanum lycopersicum. Solaneae (Fam. 157.) Mimulus, (sp. ?) Scrophularineae (Fam. 159)—from information given us by Prof. Pfeffer. Mirabilis jalapa. Nyctagineae (Fam. 177). Mirabilis longiflora. Beta vulgaris. Polygoneae (Fam. 179). Amaranthus caudatus. Amaranthaceae (Fam. 180). Cannabis sativa (?). Cannabineae (Fam. 195).

Brassica oleracea (Cruciferae).—It was shown in the first chapter that the cotyledons of the common cabbage rise in the evening and stand vertically up at night with their petioles in contact. But as the two cotyledons are of unequal height, they frequently interfere a little with each other’s movements, the shorter one often not standing quite vertically. They awake early in the morning; thus at 6.45 A.M. on Nov. 27th, whilst if was still dark, the cotyledons, which had been vertical and in contact on the previous evening, were reflexed, and thus presented a very different appearance. It should be borne in mind that seedlings in germinating at the proper season, would not be subjected to darkness at this hour in the morning. The above amount of movement of the cotyledons is only temporary, lasting with plants kept in a warm greenhouse from four to six days; how long it would last with seedlings growing out of doors we do not know.