The Power of Movement in Plants - Charles Darwin (top reads txt) 📗

same time sink greatly, and sometimes hang almost perpendicularly downwards. The main petiole, on the other hand, rises much: by 8.30 P.M. one stood 34o higher than at noon, and by 6.40 A.M. on the following morning it was still higher by 10o; shortly after this hour the diurnal sinking movement commenced. The course of a nearly full-grown leaf was traced during 14 h.; it was strongly zigzag, and apparently

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represented five ellipses, with their longer axes differently directed.

 

Albizzia lophantha (Tribe 23).—The leaflets at night come into contact with one another, and are directed towards the apex of the pinna. The pinnae approach one another, but remain in the same plane as during the day; and in this respect they differ much from those of the above Schrankia and Acacia. The main petiole rises but little. The first-formed leaf above the cotyledons bore 11 leaflets on each side, and these slept like those on the subsequently formed leaves; but the petiole of this first leaf was curved downwards during the day and at night straightened itself, so that the chord of its arc then stood 16o higher than in the daytime.

 

Melaleuca ericaefolia (Myrtaceae).—According to Bouch� (‘Bot. Zeit.,’

1874, p. 359) the leaves sleep at night, in nearly the same manner as those of certain species of Pimelia.

 

Oenothera mollissima (Onagrarieae).—According to Linnaeus (‘Somnus Plantarum’), the leaves rise up vertically at night.

 

Passiflora gracilis (Passifloracae).—The young leaves sleep by their blades hanging vertically downwards, and the whole length of the petiole then becomes somewhat curved downwards. Externally no trace of a pulvinus can be seen. The petiole of the uppermost leaf on a young shoot stood at 10.45 A.M. at 33o above the horizon; and at 10.30 P.M., when the blade was vertically dependent, at only 15o, so the petiole had fallen 18o. That of the next older leaf fell only 7o. From some unknown cause the leaves do not always sleep properly. The stem of a plant, which had stood for some time before a north-east window, was secured to a stick at the base of a young leaf, the blade of which was inclined at 40o below the horizon. From its position the leaf had to be viewed obliquely, consequently the vertically ascending and descending movements appeared when traced oblique. On the first day (Oct. 12th) the leaf descended in a zigzag line until late in the evening; and by 8.15 A.M. on the 13th had risen to nearly the same level as on the previous morning. A new tracing was now begun (Fig. 161). The leaf continued to rise until 8.50 A.M., then moved a little to the right, and afterwards descended. Between 11 A.M. and 5 P.M. it circumnutated, and after the latter hour the great nocturnal fall commenced. At 7.15 P.M. it depended vertically. The dotted line ought to have been prolonged much lower down in the figure. By 6.50 A.M. on the following morning (14th) the [page 384]

leaf had risen greatly, and continued to rise till 7.50 A.M., after which hour it redescended. It should be observed that the lines traced on this second morning would have coincided with and confused those previously traced, had not the pot been slided a very little to the left. In the evening (14th) a mark was placed behind the filament attached to the apex of the leaf, and its movement was carefully traced from 5 P.M. to 10.15

P.M.

 

Fig. 161. Passiflora gracilis: circumnutation and nyctitropic movement of leaf, traced on vertical glass, from 8.20 A.M. Oct. 13th to 10 A.M. 14th.

Figure reduced to two-thirds of original scale.

 

Between 5 and 7.15 P.M. the leaf descended in a straight line, and at the latter hour it appeared vertically dependent. But between 7.15 and 10.15

P.M. the line consisted of a succession of steps, the cause of which we could not understand; it was, however, manifest that the movement was no longer a simple descending one.

 

Siegesbeckia orientalis (Compositae).—Some seedlings were raised in the middle of winter and kept in the hot-house; they flowered, but did not grow well, and their leaves never showed any signs of sleep. The leaves on other seedlings raised in May were horizontal at noon (June 22nd), and depended at a consi-

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derable angle beneath the horizon at 10 P.M. In the case of four youngish leaves which were from 2 to 2 � inches in length, these angles were found to be 50o, 56o, 60o, and 65o. At the end of August when the plants had grown to a height of 10 to 11 inches, the younger leaves were so much curved downwards at night that they might truly be said to be asleep. This is one Fig. 162. Nicotiana glauca: shoots with leaves expanded during the day, and asleep at night. Figures copied from photographs, and reduced.

 

of the species which must be well illuminated during the day in order to sleep, for on two occasions when plants were kept all day in a room with north-east windows, the leaves did not sleep at night. The same cause probably accounts for the leaves on our seedlings raised in the dead of the winter not sleeping. Professor Pfeffer informs us that the leaves of another species (S. Jorullensis ?) hang vertically down at night.

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Ipomoea caerulea and purpurea (Convolvulaceae).—The leaves on very young plants, a foot or two in height, are depressed at night to between 68o and 80o beneath the horizon; and some hang quite vertically downwards. On the following morning they again rise into a horizontal position. The petioles become at night downwardly curved, either through their entire length or in the upper part alone; and this apparently causes the depression of the blade. It seems necessary that the leaves should be well illuminated during the day in order to sleep, for those which stood on the back of a plant before a north-east window did not sleep.

 

Nicotiana tabacum (var. Virginian) and glauca (Solaneae).—The young leaves of both these species sleep by bending vertically upwards. Figures of two shoots of N. glauca, awake and asleep (Fig. 162), are given on p. 385: one of the shoots, from which the photographs were taken, was accidentally bent to one side.

 

Fig. 163. Nicotiana tabacum: circumnutation and nyctitropic movement of a leaf (5 inches in length), traced on a vertical glass, from 3 P.M. July 10th to 8.10 A.M. 13th. Apex of leaf 4 inches from glass. Temp. 17 1/2o -

18 1/2o C. Figure reduced to one-half original scale.

 

At the base of the petiole of N. tabacum, on the outside, there is a mass of cells, which are rather smaller than elsewhere, and [page 387]

have their longer axes differently directed from the cells of the parenchyma, and may therefore be considered as forming a sort of pulvinus.

A young plant of N. tabacum was selected, and the circumnutation of the fifth leaf above the cotyledons was observed during three days. On the first morning (July 10th) the leaf fell from 9 to 10 A.M., which is its normal course, but rose during the remainder of the day; and this no doubt was due to its being illuminated exclusively from above; for properly the evening rise does not commence until 3 or 4 P.M. In the figure as given on p. 386 (Fig. 163) the first dot was made at 3 P.M.; and the tracing was continued for the following 65 h. When the leaf pointed to the dot next above that marked 3 P.M. it stood horizontally. The tracing is remarkable only from its simplicity and the straightness of the lines. The leaf each day described a single great ellipse; for it should be observed that the ascending and descending lines do not coincide. On the evening of the 11th the leaf did not descend quite so low as usual, and it now zigzagged a little. The diurnal sinking movement had already commenced each morning by 7 A.M. The broken lines at the top of the figure, representing the nocturnal vertical position of the leaf, ought to be prolonged much higher up.

 

Mirabilis longiflora and jalapa (Nyctagineae).—The first pair of leaves above the cotyledons, produced by seedlings of both these species, were considerably divergent during the day, and at night stood up vertically in close contact with one another. The two upper leaves on an older seedling were almost horizontal by day, and at night stood up vertically, but were not in close contact, owing to the resistance offered by the central bud.

 

Polygonum aviculare (Polygoneae).—Professor Batalin informs us that the young leaves rise up vertically at night. This is likewise the case, according to Linnaeus, with several species of Amaranthus (Amaranthaceae); and we observed a sleep movement of this kind in one member of the genus.

Again, with Chenopodium album (Chenopodieae), the upper young leaves of some seedlings, about 4 inches in height, were horizontal or sub-horizontal during the day, and at 10 P.M. on March 7th were quite, or almost quite, vertical. Other seedlings raised in the greenhouse during the winter (Jan.

28th) were observed day and night, and no difference could be perceived in the position of their leaves. According to Bouch� (‘Bot. Zeitung,’ 1874, p.

359) the leaves of Pimelia linoides and spectabilis (Thymeleae) sleep at night.

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Euphorbia jacquiniaeflora (Euphorbiaceae).—Mr. Lynch called our attention to the fact that the young leaves of this plant sleep by depending vertically. The third leaf from the summit (March 11th) was inclined during the day 30o beneath the horizon, and at night hung vertically down, as did some of the still younger leaves. It rose up to its former level on the following morning. The fourth and fifth leaves from the summit stood horizontally during the day, and sank down at night only 38o. The sixth leaf did not sensibly alter its position. The sinking movement is due to the downward curvature of the petiole, no part of which exhibits any structure like that of a pulvinus. Early on the morning of June 7th a filament was fixed longitudinally to a young leaf (the third from the summit, and 2 5/8 inches in length), and its movements were traced on a vertical glass during 72 h., the plant being illuminated from above through a skylight. Each day the leaf fell in a nearly straight line from 7 A.M. to 5 P.M., after which hour it was so much inclined downwards that the movement could no longer be traced; and during the latter part of each night, or early in the morning, the leaf rose. It therefore circumnutated in a very simple manner, making a single large ellipse every 24 h., for the ascending and descending lines did not coincide. On each successive morning it stood at a less height than on the previous one, and this was probably due partly to the increasing age of the leaf, and partly to the illumination being insufficient; for although the leaves are very slightly heliotropic, yet, according to Mr. Lynch’s and our own observations, their inclination during the day is determined by the intensity of the light. On the third day, by which time the extent of the descending movement had much decreased, the line traced was plainly much more zigzag than on any previous day, and it appeared as if some of its powers of movement were thus expended. At 10 P.M. on June 7th, when the leaf depended vertically, its movements were observed by a mark being placed behind it, and the end of the attached filament was seen to oscillate slowly and slightly from side to side, as well as upwards and downwards.

 

Phyllanthus Niruri (Euphorbiaceae).—The leaflets of this plant sleep, as described by Pfeffer,* in a remarkable manner, apparently like those of Cassia, for they sink downwards at night and twist