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

facts before us, bearing also in mind that the tendrils of not a few plants consist of modified peduncles, we may admit without much doubt that all growing flower-stems circumnutate.

 

CIRCUMNUTATION OF LEAVES: DICOTYLEDONS.

 

Several distinguished botanists, Hofmeister, Sachs, Pfeffer, De Vries, Batalin, Millardet, etc., have ob-

 

* ‘Jenaische Zeitsch.,’ B. v. p. 133.

 

** ‘N. Mem. de l’Acad. R. de Bruxelles,’ tom. xiv. 1841, p. 3.

 

*** ‘Sitzungbericht des bot. Vereins der P. Brandenburg,’ xxi. p. 84.

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served, and some of them with the greatest care, the periodical movements of leaves; but their attention has been chiefly, though not exclusively, directed to those which move largely and are commonly said to sleep at night. From considerations hereafter to be given, plants of this nature are here excluded, and will be treated of separately. As we wished to ascertain whether all young and growing leaves circumnutated, we thought that it would be sufficient if we observed between 30 and 40 genera, widely distributed throughout the vegetable series, selecting some unusual forms and others on woody plants. All the plants were healthy and grew in pots.

They were illuminated from above, but the light perhaps was not always sufficiently bright, as many of them were observed under a skylight of ground-glass. Except in a few specified cases, a fine glass filament with two minute triangles of paper was fixed to the leaves, and their movements were traced on a vertical glass (when not stated to the contrary) in the manner already described. I may repeat that the broken lines represent the nocturnal course. The stem was always secured to a stick, close to the base of the leaf under observation. The arrangement of the species, with the number of the Family appended, is the same as in the case of stems.

 

Fig. 93. Sarracenia purpurea: circumnutation of young pitcher, traced from 8 A.M. July 3rd to 10.15 A.M. 4th. Temp. 17o - 18o C. Apex of pitcher 20

inches from glass, so movement greatly magnified.

 

(1.) Sarracenia purpurea (Sarraceneae, Fam. 11).—A young leaf, or pitcher, 8 � inches in height, with the bladder swollen but with the hood not as yet open, had a filament fixed transversely

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across its apex; it was observed for 48 h., and during the whole of this time it circumnutated in a nearly similar manner, but to a very small extent. The tracing given (Fig. 93) relates only to the movement during the first 26 h.

 

(2) Glaucium luteum (Papaveraceae, Fam. 12).—A young plant, bearing only 8

leaves, had a filament attached to the youngest leaf but one, which was 3

inches in length, including the petiole. The circumnutating movement was traced during 47 h. On both days the leaf descended from before 7 A.M.

until about 11 A.M., and then ascended slightly during the rest of the day and the early part of the night. During the latter part of the night it fell greatly. It did not ascend so much during the second as during the first day, and it descended considerably lower on the second night than on the first. This difference was probably due to the illumination from above having been insufficient during the two days of observation. Its course during the two days is shown in Fig. 94.

 

Fig. 94. Glaucium luteum: circumnutation of young leaf, traced from 9.30

A.M. June 14th to 8.30 A.M. 16th. Tracing not much magnified, as apex of leaf stood only 5 � inches from the glass.

 

(3.) Crambe maritima (Cruciferae, Fam. 14).—A leaf 9 � inches in length on a plant not growing vigorously was first observed. Its apex was in constant movement, but this could hardly be traced, from being so small in extent.

The apex, however, certainly changed its course at least 6 times in the course of 14 h. A more vigorous young plant, bearing only 4 leaves, was then selected, and a filament was affixed to the midrib of the third leaf from the base, which, with the petiole, was 5 inches in length. The leaf stood up almost vertically, but the tip

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was deflected, so that the filament projected almost horizontally, and its movements were traced during 48 h. on a vertical glass as shown in the accompanying figure (Fig. 95). We here plainly see that the leaf was continually circumnutating; but the proper periodicity of its movements was disturbed by its being only dimly illuminated from above through a double skylight. We infer that this was the case, because two leaves on plants growing out of doors, had their angles above the horizon measured in the middle of the day and at 9 to about 10 P.M. on successive nights, and they were found at this latter hour to have risen by an average angle of 9o above their mid-day position: on the following morning they fell to their former position. Now it may be observed in the diagram that the leaf rose during the second night, so that it stood at 6.40 A.M. higher than at 10.20

P.M. on the preceding night; and this may be attributed to the leaf adjusting itself to the dim light, coming exclusively from above.

 

Fig. 95. Crambe maritima: circumnutation of leaf, disturbed by being insufficiently illuminated from above, traced from 7.50 A.M. June 23rd to 8

A.M. 25th. Apex of leaf 15 1/4 inches from the vertical glass, so that the tracing was much magnified, but is here reduced to one-fourth of original scale.

 

(4.) Brassica oleracea (Cruciferae).—Hofmeister and Batalin* state that the leaves of the cabbage rise at night, and fall by day. We covered a young plant, bearing 8 leaves, under a large bell-glass, placing it in the same position with respect to the

 

* ‘Flora,’ 1873, p. 437.

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light in which it had long remained, and a filament was fixed at the distance of .4 of an inch from the apex of a young leaf nearly 4 inches in length. Its movements were then traced during three days, but the tracing is not worth giving. The leaf fell during the whole morning, and rose in the evening and during the early part of the night. The ascending and descending lines did not coincide, so that an irregular ellipse was formed each 24 h. The basal part of the midrib did not move, as was ascertained by measuring at successive periods the angle which it formed with the horizon, so that the movement was confined to the terminal portion of the leaf, which moved through an angle of 11o in the course of 24 h., and the distance travelled by the apex, up and down, was between .8 and .9 of an inch.

 

In order to ascertain the effect of darkness, a filament was fixed to a leaf 5 � inches in length, borne by a plant which after forming a head had produced a stem. The leaf was inclined 44o above the horizon, and its movements were traced on a vertical glass every hour by the aid of a taper.

During the first day the leaf rose from 8 A.M. to 10.40 P.M. in a slightly zigzag course, the actual distance travelled by the apex being .67 of an inch. During the night the leaf fell, whereas it ought to have risen; and by 7 A.M. on the following morning it had fallen .23 of an inch, and it continued falling until 9.40 A.M. It then rose until 10.50 P.M., but the rise was interrupted by one considerable oscillation, that is, by a fall and reascent. During the second night it again fell, but only to a very short distance, and on the following morning reascended to a very short distance. Thus the normal course of the leaf was greatly disturbed, or rather completely inverted, by the absence of light; and the movements were likewise greatly diminished in amplitude.

 

We may add that, according to Mr. A. Stephen Wilson,* the young leaves of the Swedish turnip, which is a hybrid between B. oleracea and rapa, draw together in the evening so much “that the horizontal breadth diminishes about 30 per cent. of the daylight breadth.” Therefore the leaves must rise considerably at night.

 

(5.) Dianthus caryophyllus (Caryophylleae, Fam. 26).—The * ‘Trans. Bot. Soc. Edinburgh,’ vol. xiii. p. 32. With respect to the origin of the Swedish turnip, see Darwin, ‘Animals and Plants under Domestication,’ 2nd edit. vol. i. p. 344.

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terminal shoot of a young plant, growing very vigorously, was selected for observation. The young leaves at first stand up vertically and close together, but they soon bend outwards and downwards, so as to become horizontal, and often at the same time a little to one side. A filament was fixed to the tip of a young leaf whilst still highly inclined, and the first dot was made on the vertical glass at 8.30 A.M. June 13th, but it curved downwards so quickly that by 6.40 A.M. on the following morning it stood only a little above the horizon. In Fig. 96

 

Fig. 96. Dianthus caryophyllus: circumnutation of young leaf, traced from 10.15 P.M. June 13th to 10.35 P.M. 16th. Apex of leaf stood, at the close of our observations, 8 3/4 inches from the vertical glass, so tracing not greatly magnified. The leaf was 5 1/4 inches long. Temp. 15 1/2o - 17 1/2o C.

 

the long, slightly zigzag line representing this rapid downward course, which was somewhat inclined to the left, is not given; but the figure shows the highly tortuous and zigzag course, together with some loops, pursued during the next 2 � days. As the leaf continued to move all the time to the left, it is evident that the zigzag line represents many circumnutations.

 

(6.) Camellia Japonica (Camelliaceae, Fam. 32).—A youngish leaf, which together with its petiole was 2 3/4 inches in length and which arose from a side branch on a tall bush, had a filament attached to its apex. This leaf sloped downwards at an angle of 40o beneath the horizon. As it was thick and rigid, and its

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petiole very short, much movement could not be expected. Nevertheless, the apex changed its course completely seven times in the course of 11 � h., but moved to only a very small distance. On the next day the movement of the apex was traced during 26 h. 20 m. (as shown in Fig. 97), and was nearly of the same nature, but rather less complex. The movement seems to be periodical, for on both days the leaf circumnutated in the forenoon, fell in the afternoon (on the first day until between 3 and 4 P.M., and on the second day until 6 P.M.), and then rose, falling again during the night or early morning.

 

Fig. 97. Camellia Japonica: circumnutation of leaf, traced from 6.40 A.M.

June 14th to 6.50 A.M. 15th. Apex of leaf 12 inches from the vertical glass, so figure considerably magnified. Temp. 16o - 16 1/2o C.

 

In the chapter on the Sleep of Plants we shall see that the leaves in several Malvaceous genera sink

 

Fig. 98. Pelargonium zonale: circumnutation and downward movement of young leaf, traced from 9.30 A.M. June 14th to 6.30 P.M. 16th. Apex of leaf 9 1.4

inches from the vertical glass, so figure moderately magnified. Temp. 15o -

16 1/2o C.

 

at night; and as they often do not then occupy a vertical position, especially if they have not been well illuminated during [page 233]

the day, it is doubtful whether some of these cases ought not to have been included in the present chapter.

 

(7.) Pelargonium zonale (Geraniaceae, Fam. 47).—A young leaf, 1 1/4 inch in breadth, with its petiole 1 inch long, borne on a young plant, was observed in the usual manner during