The Power of Movement in Plants

teft cotyledon. This seems to be the object gained by these singular movements. Altogether 20 seedlings were examined on successive nights, and in 19 of them it was the left cotyledon alone which became twisted, with the true leaf always so twisted tbat its upper surface approached closely and fronted that of the left cotyledon. In only one instance was the right cotyledon twisted, with the true leaf twisted towards it; but this seedling was in an abnormal condition, as the left cotyledon did not rise up properly at night. This whole case is remarkable, as with the cotyledons of no other plant bave we seen any nocturnal movement except vertically upwards or downwards. It is the more remarkable, because we shall meet with an analogous case in the leaves of the allied genus Melilotus, in which the ter- minal leaflet rotates at night so as to present one edge to the zenith and at the same time bends to one side, so that its upper surface comes into contact with that of one of tbe two now ver- tical lateral leaflets.

Concluding Remarks on the Nyetitropic Movements of Cotyledons.—The sleep of cotyledons (though this is a subject which has been little attended to), seems to be a more common phenomenon than that of leaves. We observed the position of the cotyledons during the day and night in 153 genera, widely distributed through- out the dicotyledonous series, but otherwise selected almost by hazard; and one or more species in 26 of these genera placed their cotyledons at night so as to stand vertically or almost vertically, having gene- rally moved through an angle of at least 60°. If we lay on one side the Leguminose, the cotyledons of which are particularly liable to sleep, 140 genera remain ; and out of these, the cotyledons of at least one species in 19 genera slept. Now if we were to select by hazard 140 genera, excluding the Leguminose, and observed their leaves at night, assuredly not nearly so many as 19 would be found to include sleeping species. We here refer exclusively to the plants observed by ourselves.

In our entire list of seedlings, there are 30 genera, belonging to 16 Families, the cotyledons of which in some of the species rise or sink in the evening or early night, so as to stand at least 60° above or be- neath the horizon. In a large majority of the genera, namely, 24, the movement is a rising one; so that the same direction prevails in these nyctitropic move- ments as in the lesser periodic ones described in the second chapter. The cotyledons move downwards during the early part of the night in only 6 of the genera; and in one of them, Cannabis, the curving down of the tip is probably due to epinasty, as Kraus believes to be the case with the leaves. The down- ward movement to the amount of 90° is very decided in Oxalis Valdiviana and sensitiva, and in Geranium rotundifolium. It is a remarkable fact that with Anoda Wrightti, one species of Gossypium and at least 3 species of Ipomeea, the cotyledons whilst young and light sink at night.very little or not at all; although this movement becomes well pronounced as soon as they have grown large and heavy. Although the downward movement cannot be attributed to the weight of the cotyledons in the several cases which were investigated, namely, in those of the Anoda, Ipomeea purpurea and bona-nowx, nor in that of I coc- zinea, yet bearing in mind that cotyledons are con- tinually circumnutating, a slight cause might at first have determined whether the great nocturnal move- ment should be upwards or downwards. We may therefore suspect that in some aboriginal member of the groups in question, the weight of the cotyledons first determined the downward direction. The fact ot the cotyledons of these species not sinking down much whilst they are young and tender, seems opposed to the belief that the greater movement when they are

grown older, has been acquired for the sake of pro- tecting them from radiation at night; but then we should remember that there are many plants, the leaves of which sleep, whilst the cotyledons do not; and if in some cases the leaves are protected from cold at night whilst the cotyledons are not protected, so in other cases it may be of more importance to the species that the nearly full-grown cotyledons should be better protected than the young ones.

In all the species of Oxalis observed by us, the coty- ledons are provided with pulvini; but this organ has become more or less rudimentary in O. corniculata, and the amount of upward movement of its cotyledons at night is very variable, but is never enough to be called sleep. We omitted to ascertain whether the cotyledons of Geranium rotundifolium possess pulvini. In the Leguminose all the cotyledons which sleep, as far as we have seen, are provided with pulvini. But with Lotus Jacobeus, these are not fully developed during the first few days of the life of the seedling, and the cotyledons do not then rise much at night. With Trifolium strictum the blades of the cotyledons rise at night by the aid of their pulvini; whilst the petiole of one cotyledon twists half-round at the same time, independently of its pulvinus.

As a general rule, cotyledons which are provided with pulvini continue to rise or sink at night during a much longer period than those destitute of this organ. In this latter case the movement no doubt depends on alternately greater growth on the upper and lower side of the petiole, or of the blade, or of both, preceded probably by the increased turgescence of the growing cells. Such movements generally last for a very short period—for instance, with Brassica and Githago for 4 or 5 nights, with Beta for 2 or 3, and with

Raphanus for only a single night. There are, however, some strong exceptions to this rule, as the coty]eduns of Gossypium, Anoda and Ipomeea do not possess pul- vini, yet continue to move and to grow for a long time. We thought at first that when the movement lasted for only 2 or 3 nights, it could hardly be of any servico to the plant, and hardly deserved to be called sleep ; but as many quickly-growing leaves sleep for only a few nights, and as cotyledons are rapidly developed and soon complete their growth, this doubt now seems to us not well-founded, more especially as these move- ments are in many instances so strongly pronounced. We may here mention another point of similarity between sleeping leaves and cotyledons, namely, that some of the latter (for instance, those of Cassia and Githago) are easily affected by the absence of light; and they then either close, or if closed do not open; whereas others (as with the cotyledons of Oxalis) are very little affected by light. In the next chapter it will be shown that the nyctitropic movements both of cotyledons and leaves consist of a modified form of circumnutation.

As in the Leguminose and Oxalide, the leaves and the cotyledons of the same species generally sleep, the idea at first naturally occurred tous, that the sleep of the cotyledons was merely an early development of a habit proper to a more advanced stage of life. But no such explanation can be admitted, although there 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 Nico- tiana observed by us; also Sida rhombifulia, Abutilon Darwinii, and Cienopodium album. On. the other

hand, the cotyledons of some plants slecp 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 Ozalis Valdiviana, having 2 or 3 well-developed leaves, it was a curious spectacle to behold at night each leaflet folded inwards and hanging perpendicularly down- wards, 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-

ledon of Lrifolium 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 move- ments 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 vr plumule, from radiation at night.

Moprrrep CrrctmNuTaT1IoN: Nyortirrovic or SLEEP MoveMEN13 OF Lraves. Conditions necessary for these movements--List of Genera and Families, which include sleeping plants—Description of the movements in the severul Genera—Osalis: leaflets folded at night—Averrhoa : rapid movements of the leaflets—Porliein: leaflets close when plant kept very dry—Tropzolum: leaves do not sleep unless welt Wuminated during day—Lupinus: varous modes of sleeping— Melilotus: singular movements of terminal leaflet-—Trifolium— Desmodium: rudimentary lateral leaflets, movements of, not de- veloped on young plants, state of their pulvini—Cassia : complex movements vf the leaflets—Bauhinia: leaves folded at night— Mimosa pudica: compounded movements of leaves, effect of dark- ness—Mimosa albida, reduced leaflets of—Schrankia: downward movement of the pinne—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 move- ments 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 80° from the vertical,—that is, at least 60° 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 30° 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

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 60° above or beneath the horizon, have been included amongst sleeping plants. It should be premised that the nyctitropic move- ments 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 deter- mines the variable amount of sinking of the leaves of Polygonum convolvulus at night ; and if so, their move- ments are not in our sense strictly nyctitropic. Plants in order to sleep must have been exposed to a proper temperature: Erythrina -crista-galli, oat 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 de- pendent at night. In a kitchen-garden the leaflets of Phaseolus vulgaris did not sleep during the early part of the summer. Ch. Royer says,t referring I suppose to the native plants in France, that they do not sleep when the temperature is below 5° C. or 41° F. In the case of several sleeping plants, viz., species ot

* Dassen, ‘ Tijdschrift vor. Na- turlijke Gesch. en Physiologie,’ 1837, vol. iv. p. 106. See also Ch. Royer on the importance of a proper state of turgescence of the cells, im *Annal. dee Se. Nat. Tropeolum, Lupinus, Ipomcea, 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 Intro- duction. 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 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 Linnzus; 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 Linneus, we are left in doubt.

Cuass I. DICOTYLEDONS. Sub-class I. ANGiospeRss—continucd, Sub-class I. ANGIOSPERMS, ‘ res. ike 5 Tropzolum. Tropzolex (49). Genus. Family. Crotolaria (Thisel-\ Leguminose (75) Githago Caryophyllex (26).| ton Dyer). } Tribe II. Stellaria (Batalin). - Lupinus, 34 4s Portulaca = (Ch. Cytisus. : Pa s Royer). } Honeletcen rn Trigonella. | » Tr. UE Sida. Malvacee (36). Medicago. i a 7 Abutilon. 3s Melilotus. j ao ‘3 Malva (Linnzus Trifolium. : and Pfeffer). } m Securigera. i #5 Tr. IV. Hibiscus my Lotus. | neus), bag Psoralea, ' sy Tr. V. Anoda, #9 Amorpha (Du- Gossypium. si chartre). } ia ” Ayenia (Linnzus). | Sterculacee (37). Delea. si a ‘Lriumfetta (Lin- qe Indigofera, ‘ nei), Tiliacee (38). ATs, : Linum (Batalin). | Liner (39). Wistaria. 5 Oralis. Oxalide (41). Robinia, : i Averrhoa, s Spherophysa. 5 ey Porlieria. Zygophyllex (45). | Colutea, a as Guiacum. % Astragalus. se nn Impatiens (Lin-; Glycyrrhiza, sy e neus, Pfe tn Balsaminex (48). | Coronilla. vw Pav

Cuase 1. DICOTYLEDONS (continued). Sub-class 1. ANUIOSPFtMS (continued), Sub-class I. ANGIOSPERMS, ie Reha. . Family. Genus. Family. Hata : = Onagraries (100). Githago segetwm (Caryophyllee).—The first leaves produced -by young seedlings, rise up and close together at night. Ona rather older seedling, two young leawes stood at noon at 55° . above the horizon, and at night at 86°, so each had risen 31°. 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. Ist, 1873, p. 487) that the young leaves of Stellaria close up so completely at night that they form together great buds.

Sida (Malvacee).—The nyctitropic movements of the leaves in this genus are remarkable in some respects. Batalin informs Sida rhombifolia: circumnutation and nyctitropic (or sleep) movements of a leaf on a young plant, 94 inches high; filament fixed to midrib of nearly full-grown leaf, 23 inches in length ; movement traced under a sky- light. Apex of leaf 53 inches from the vertical glass, so diagram not greatly enlarged. S. napea fall at night, but to what angle he cannot remember. The leaves of S. rhombifolia and retusa, on the other hand, rise up vertically, and are pressed against the stem. We havo therefore here within the same genus, directly op- posite movements. Again, the leaves of S. rhombifolia are furnished with a pul- vinus, formed of a mass of small cells destitute of chlo- rophyll, and with their longer axes perpendicular to the axis of the petiole. As measured along this latter line, these cells are only ith of the length of those of the petiole; but instead of being abruptly separated from them (as is usual with the pulvinus in inost plants), they graduate into the larger cells of the petiole. On the other hand, S. napea, according to Ba- talin, does not possess a pulvinus; and he informs us that a gradation may Le traced in the several species of the genus between these two states of the petiole. Sida rhumbifolia presents another peculiarity, of which we have seen no other in- stance with leaves that sleep: for those on very young plants, though they

rise somewhat in the evening, do not go to sleep, as we observed vn several occasions; whilst those on rather older plants sleep in a conspicuous manner. For instance, a leaf (°85 of an inch in length) on a very young seedling 2 inches high, stuod at noon 9° above the horizon, and at 10 p.m. at 28°, so it had risen only 19°; another leaf (1:4 inch in length) on a seedling of the same height, stood at the same two periods at 7° and 32°, and therefore had risen 25°. These leaves, which moved so little, had a fairly well-developed pulvinus. After an interval of some weeks, when the same seedlings were 24 and 3 inches in height, some of the young leaves stood up at night quite vertically, and others were highly inclined; and so it was with bushes which were fully grown and were flowering.

The movement of a leaf was traced from 9.15 a.m. on May 28th to 8.30 a.m. on the 80th. The temperature was too low (15°—16° C.), and the illumination hardly sufficient; con- sequently the leaves did not become quite so highly inclined at night, as they had done previously and as they did subse- quently in the hot-house; but the movements did not appear otherwise disturbed. On the first day the leaf sank till 5.15 p.m.; it then rose rapidly and greatly till 10.5 pm., and only a little higher during the rest of the night (Fig. 126). Early on the next day (29th) it fell in a slightly zigzag line rapidly until 9 a.m., by which time it had reached nearly the same place as on the previous morning. During the remainder of the day it fell slowly, and zigzagged laterally. The evening rise began after 4 p.m.in the same manner as before, and on the second morning it again fell rapidly. The ascending and descending lines do not coincide, as may be seen in the diagram. On the 30th a new tracing was made (not here given) on a rather enlarged scale, as the apex of the leaf now stood 9 inches from the vertical glass. In order to observe more carefully the course pursued at the time when the diurnal fall changes into the nocturnal rise, dots were made every half-hour between 4 p.m. and 10.30 p.m. This rendered the lateral zigzagging movement during the evening more conspicuous than in the diagram given, but it was of the same nature as there shown. The impression forced on our minds was that the leaf was expending superfluous movement, so that the great nocturnal rise might not occur at too early an hour.

Abutilon Darwinti (Malvaceze).—The leaves on some very young plants stood almost horizontally during the day, and hung down vertically at night. Very fine plants kept in a large hall, lighted only from the roof, did not sleep at night, for in order to do so the leaves must be well illuminated during the day. The cotyledons do not sleep. Linnzus says. that the leaves of his Sid« abutilon sink perpendicularly down at night, though the petioles rise. Prof. Pfeffer informs us that the leaves of a Malva, allied to M. sylvestris, rise greatly at night; and this genus, as well as that of Hibiscus, are included by Linneus in his list of sleeping plants.

Anoda Wrightii (Malvacese).—The leaves, produced by very young plants, when grown to a moderate size, sink at night either almost vertically down or to an angle of about 45° beneath the horizon; for there is a considerable degree of variability in the amount of sinking at night, which depends in part on the degree to which they have been illuminated during the day. But the leaves, whilst quite young, do not sink down at night, and this is a very unusual circumstance. The summit of the petiole, where it joins the blade, is developed into a pulvinus, and this is present in very young leaves which do not sleep; though it is not so well defined as in older leaves.

Gossypium (var. Nankin cotton, Malvacere).—Some young leaves, between 1 and 2 inches in length, borne by two seedlings 6 and 7% inches in height, stood horizontally, or were raised a little above the horizon at noon on July 8th and 9th; but by 10 p.m. they had sunk down to between 68° and 90° beneath the horizon. When the same plants had grown to double the above height, their leaves stocd at night almost or quite vertically dependent. The leaves on some large plants of G. maritimum and Bruzilense, which were kept in a very badly lighted hot-house, only occasionally sank much downwards at night, and hardly enough to be called sleep.

Oualis (Oxalide).— In most of the species in this large genus the three leaflets sink vertically down at night; but as their sub-petioles are short the blades could not assume this position from the want of space, unless they were in some manner yen- dered narrower; and this is effected by their becoming more or less folded (Fig. 127). The angle formed by the two halves of the same leaflet was found to vary in different individuals of several species between 92° and 160°; in three of the best folded leaflets of O. fragrans it was 76°, 74°, and 54° The angle is often different in the three leaflets of the same leaf. As the leaflets sink down at night and become folded, their lower surfaces are brought near together (see B), or even into

close contact; and from this circumstance it might be thought that the object of the folding was the protection of their lower surfaces. If this had been the case, it would have formed a strongly marked exception to the rule, that when there is any difference in the degree of protection from radiation of the two surfaces of the leaves, it is always the upper surface which is the best protected. But that the folding of the leaflets, and consequent mutual approximation of their lower surfaces, serves merely to allow them to sink down vertically, may be

O.xalis acetosclla: A, leaf seen from vertically above; B, diagram of leaf asleep, also seen trom vertically above. inferred from the fact that when the leaflets do not radiate from the summit of a common petiole, or, again, when there is plenty of room, from the sub-petioles not being very short, the leaflets sink down without becoming folded. This occurs with the leaflets of O. sensitiva, Plumierit, and bupleurifolia, There is no use in giving a long list of the many species which sleep in the above described manner. This holds good with species having rather fleshy leaves, like those of O. carnosa, or large leaves like those of O. Ort-gesti, or four leaflets like those of O. variabilis. There are, however, some species which show no signs of sleep, viz., O. pentuphylla, enneaphylla, hirta, and rubella. We will now describe the nature of the movements in some of the species.

Oxalis acetosella.—The movement of a leaflet, together with that of the main petiole, are shown in the following dia- gram (Fig. 128), traced between 11 a.m. on October 4th and 7.45 am. on the 5th. After 5.30 p.m. on the 4th the leaflet sank rapidly, and at 7 p.m. depended vertically. For some time before it assumed this latter position, its movemerts could, of course, no longer be traced on the vertival glass, and the broken line in the diagram ought to be extended much further

9xalis acetosella: circumnutation and nyctitropic movements of a nearly full-grown leaf, with filament at- tached to the midrib of one of the leaflets; traced on vertical glass dur- ing 20 h., 45 m. down in this and all other cases. By 6.45 am. on the following morning it had risen considerably, and con- tinued to rise for the next hour; but, judging from other observations, it would soon have begun to fall again. Between 11 4.m. and 5.30 P.M. the leaflet moved at least four times up and four times down before the great noc- turnal fall commenced; it reached its highest point at noon. Similar observations were made on two other leaflets, with nearly the same results, Sachs and Pfeffer have also described briefly * the autonomous movements of the leaves of this plant. On another occasion the petiole of a leaf was secured to a little stick close beneath the leaflets, and a filament tipped with a bead of sealing- Wax was affixed to the mid- rib of one of them, and a mark was placed close behind. At 7PM., when the leaficts were asleep, the filament de- pended vertically down, and the movements of the bead were then traced till 10.40 P.M., aS shown in the fol- lowing diagram (Fig. 129), We here see that the leaflet moved a little from side to side, as well as a little up and down, whilst asleep.