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

seems to be some connection, as might have been expected, between the two sets of cases. For the leaves of many plants sleep, whilst their cotyledons do not do so—of which fact Desmodium gyrans offers a good instance, as likewise do three species of Nicotiana observed by us; also Sida rhombifolia, Abutilon Darwinii, and Chenopodium album. On the other

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hand, the cotyledons of some plants sleep and not the leaves, as with the species of Beta, Brassica, Geranium, Apium, Solanum, and Mirabilis, named in our list. Still more striking is the fact that, in the same genus, the leaves of several or of all the species may sleep, but the cotyledons of only some of them, as occurs with Trifolium, Lotus, Gossypium, and partially with Oxalis. Again, when both the cotyledons and the leaves of the same plant sleep, their movements may be of a widely dissimilar nature: thus with Cassia the cotyledons rise vertically up at night, whilst their leaves sink down and twist round so as to turn their lower surfaces outwards. With seedlings of Oxalis Valdiviana, having 2 or 3 well-developed leaves, it was a curious spectacle to behold at night each leaflet folded inwards and hanging perpendicularly downwards, whilst at the same time and on the same plant the cotyledons stood vertically upwards.

 

These several facts, showing the independence of the nocturnal movements of the leaves and cotyledons on the same plant, and on plants belonging to the same genus, lead to the belief that the cotyledons have acquired their power of movement for some special purpose. Other facts lead to the same conclusion, such as the presence of pulvini, by the aid of which the nocturnal movement is continued during some weeks. In Oxalis the cotyledons of some species move vertically upwards, and of others vertically downwards at night; but this great difference within the same natural genus is not so surprising as it may at first appear, seeing that the cotyledons of all the species are continually oscillating up and down during the day, so that a small cause might determine whether they should rise or sink at night.

Again, the peculiar nocturnal movement of the left-hand coty-

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ledon of Trifolium strictum, in combination with that of the first true leaf. Lastly, the wide distribution in the dicotyledonous series of plants with cotyledons which sleep. Reflecting on these several facts, our conclusion seems justified, that the nyctitropic movements of cotyledons, by which the blade is made to stand either vertically or almost vertically upwards or downwards at night, has been acquired, at least in most cases, for some special purpose; nor can we doubt that this purpose is the protection of the upper surface of the blade, and perhaps of the central bud or plumule, from radiation at night.

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CHAPTER VII.

 

MODIFIED CIRCUMNUTATION: NYCTITROPIC OR SLEEP MOVEMENTS OF LEAVES.

 

Conditions necessary for these movements—List of Genera and Families, which include sleeping plants—Description of the movements in the several Genera—Oxalis: leaflets folded at night—Averrhoa: rapid movements of the leaflets—Porlieria: leaflets close when plant kept very dry—Tropaeolum: leaves do not sleep unless well illuminated during day—Lupinus: various modes of sleeping—Melilotus: singular movements of terminal leaflet—

Trifolium—Desmodium: rudimentary lateral leaflets, movements of, not developed on young plants, state of their pulvini—Cassia: complex movements of the leaflets—Bauhinia: leaves folded at night—Mimosa pudica: compounded movements of leaves, effect of darkness—Mimosa albida, reduced leaflets of—Schrankia: downward movement of the pinnae—Marsilea: the only cryptogam known to sleep—Concluding remarks and summary—Nyctitropism consists of modified circumnutation, regulated by the alternations of light and darkness—Shape of first true leaves.

 

WE now come to the nyctitropic or sleep movements of leaves. It should be remembered that we confine this term to leaves which place their blades at night either in a vertical position or not more than 30o from the vertical,—that is, at least 60o above or beneath the horizon. In some few cases this is effected by the rotation of the blade, the petiole not being either raised or lowered to any considerable extent. The limit of 30o from the vertical is obviously an arbitrary one, and has been selected for reasons previously assigned, namely, that when the blade approaches the perpendicular as nearly as this, only half as much of the surface is exposed at night to the

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zenith and to free radiation as when the blade is horizontal. Nevertheless, in a few instances, leaves which seem to be prevented by their structure from moving to so great an extent as 60o above or beneath the horizon, have been included amongst sleeping plants.

 

It should be premised that the nyctitropic movements of leaves are easily affected by the conditions to which the plants have been subjected. If the ground is kept too dry, the movements are much delayed or fail: according to Dassen,* even if the air is very dry the leaves of Impatiens and Malva are rendered motionless. Carl Kraus has also lately insisted** on the great influence which the quantity of water absorbed has on the periodic movements of leaves; and he believes that this cause chiefly determines the variable amount of sinking of the leaves of Polygonum convolvulus at night; and if so, their movements are not in our sense strictly nyctitropic.

Plants in order to sleep must have been exposed to a proper temperature: Erythrina crista-galli, out of doors and nailed against a wall, seemed in fairly good health, but the leaflets did not sleep, whilst those on another plant kept in a warm greenhouse were all vertically dependent at night. In a kitchen-garden the leaflets of Phaseolus vulgaris did not sleep during the early part of the summer. Ch. Royer says,*** referring I suppose to the native plants in France, that they do not sleep when the temperature is below 5o C. or 41o F. In the case of several sleeping plants, viz., species of

 

* Dassen,‘Tijdschrift vor. Naturlijke Gesch. en Physiologie,’ 1837, vol.

iv. p. 106. See also Ch. Royer on the importance of a proper state of turgescence of the cells, in ‘Annal. des Sc. Nat. Bot.’ (5th series), ix.

1868, p. 345.

 

** ‘Beitr�ge zur Kentniss der Bewegungen,’ etc., in ‘Flora,’ 1879, pp. 42, 43, 67, etc.

 

*** ‘Annal. des Sc. Nat. Bot.’ (5th Series), ix. 1868, p. 366.

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Tropaeolum, Lupinus, Ipomoea, Abutilon, Siegesbeckia, and probably other genera, it is indispensable that the leaves should be well illuminated during the day in order that they may assume at night a vertical position; and it was probably owing to this cause that seedlings of Chenopodium album and Siegesbeckia orientalis, raised by us during the middle of the winter, though kept at a proper temperature, did not sleep. Lastly, violent agitation by a strong wind, during a few minutes, of the leaves of Maranta arundinacea (which previously had not been disturbed in the hot-house), prevented their sleeping during the two next nights.

 

We will now give our observations on sleeping plants, made in the manner described in the Introduction. The stem of the plant was always secured (when not stated to the contrary) close to the base of the leaf, the movements of which were being observed, so as to prevent the stem from circumnutating. As the tracings were made on a vertical glass in front of the plant, it was obviously impossible to trace its course as soon as the leaf became in the evening greatly inclined either upwards or downwards; it must therefore be understood that the broken lines in the diagrams, which represent the evening and nocturnal courses, ought always to be prolonged to a much greater distance, either upwards or downwards, than appears in them. The conclusions which may be deduced from our observations will be given near the end of this chapter.

 

In the following list all the genera which include sleeping plants are given, as far as known to us. The same arrangement is followed as in former cases, and the number of the Family is appended. This list possesses some interest, as it shows that the habit of

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sleeping is common to some few plants throughout the whole vascular series.

The greater number of the genera in the list have been observed by ourselves with more or less care; but several are given on the authority of others (whose names are appended in the list), and about these we have nothing more to say. No doubt the list is very imperfect, and several genera might have been added from the ‘Somnus Plantarum’ by Linnaeus; but we could not judge in some of his cases, whether the blades occupied at night a nearly vertical position. He refers to some plants as sleeping, for instance, Lathyrus odoratus and Vicia faba, in which we could observe no movement deserving to be called sleep, and as no one can doubt the accuracy of Linnaeus, we are left in doubt.

 

[List of Genera, including species the leaves of which sleep.

 

CLASS I. DICOTYLEDONS.

 

Sub-class I. ANGIOSPERMS.

 

Genus Family.

 

Githago Caryophylleae (26).

Stellaria (Batalin). “

Portulaca (Ch.Royer). Portulaceae (27).

Sida Malvaceae (36).

Abutilon. “

Malva (Linnaeus and Pfeffer). “

Hibiscus (Linnaeus). “

Anoda. “

Gossypium. “

Ayenia (Linnaeus). Sterculaceae (37).

Triumfetta (Linnaeus). Tiliaceae (38).

Linum (Batalin). Lineae (39).

Oxalis. Oxalidae (41).

Averrhoa. “

Porlieria. Zygophylleae (45).

Guiacum. “

Impatiens (Linnaeus, Pfeffer, Batalin). Balsamineae (48).

Tropaeolum. Tropaeoleae (49).

Crotolaria (Thiselton Dyer). Leguminosae (75) Tribe II.

Lupinus. ” “

Cytisus. ” “

Trigonella. ” Tr. III.

Medicago. “

Melilotus. ” “

Trifolium. ” “

Securigera. ” Tr. IV.

Lotus. ” “

Psoralea. ” Tr. V.

Amorpha (Cuchartre). ” “

Daelea. ” “

Indigofera. ” “

Tephrosia. ” “

Wistaria. ” “

Robinia. ” “

Sphaerophysa. ” “

Colutea. ” “

Astragalus. ” “

Glycyrrhiza. ” “

Coronilla. ” Tr. VI.

Hedysarum. ” “

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List of Genera (continued).

 

CLASS I. DICOTYLEDONS.

 

Sub-class I. ANGIOSPERMS.

 

Genus Family.

Onobrychis. Leguminosae (75) Tr. VI.

Smithia. ” “

Arachis. ” “

Desmodium. ” “

Urania. ” “

Vicia. ” Tr. VII.

Centrosema. ” Tr. VIII.

Amphicarpaea. ” “

Glycine. ” “

Erythrina. ” “

Apios. ” “

Phaseolus. ” “

Sophora. ” Tr. X.

Caesalpinia. ” Tr. XIII.

Haematoxylon. ” “

Gleditschia (Duchartre). ” “

Poinciana. ” “

Cassia. ” Tr. XIV.

Bauhinia. ” Tr. XV.

Tamarindus. ” Tr. XVI.

Adenanthera. ” Tr. XX.

Prosopis. ” “

Neptunia. ” “

Mimosa. ” “

Schrankia. ” “

Acacia. ” Tr. XXII.

Albizzia. ” Tr. XXIII.

Melaleuca (Bouch�). Myrtaceae (94).

 

Sub-class I. ANGIOSPERMS (continued).

 

Genus Family.

Aenothera (Linnaeus). Omagrarieae (100).

Passiflora. Passifloracea (105).

Siegesbeckia. Compositae (122).

Ipomoea. Convolvulacea (151).

Nicotiana. Solaneae (157).

Mirabilis. Nyctagineae (177).

Polygonum (Batalin). Polygoneae (179).

Amaranthus. Amaranthaceae (180).

Chenopodium. Chenopodieae (181).

Pimelia (Bouch�). Thymeteae (188).

Euphorbia. Euphorbiaceae (202) Phyllanthus (Pfeffer). “

 

Sub-class II. GYMNOSPERMS.

Aies (Chatin).

 

CLASS II. MONOCOTYLEDONS.

 

Thalia. Cannaceae (21).

Maranta. “

Colocasia. Aroideae (30).

Strephium. Gramineae (55).

 

CLASS III. ACOTYLEDONS.

 

Marsilea. Marsileaceae (4).

 

Githago segetum (Caryophylleae).—The first leaves produced by young seedlings, rise up and close together at night. On a rather older seedling, two young leaves stood at noon at 55o above the horizon, and at night at 86o, so each had risen 31o. The angle, however, was less in some cases.

Similar observations were occasionally made on young leaves (for the older ones moved very little) produced by nearly full-grown plants. Batalin says (‘Flora,’ Oct. 1st, 1873, p. 437) that the young leaves of Stellaria close up so completely at night that they form together great buds.

 

Sida (Malvaceae).—the nyctitropic movements of the leaves in this genus are remarkable in some respects. Batalin informs [page 322]

us (see also ‘Flora,’ Oct. 1st, 1873, p. 437) that those of S. napaea fall at night, but to what angle he cannot remember. The leaves of S.

rhombifolia and retusa, on the other hand,