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

brought out of a warm greenhouse and observed under a skylight, where the temperature was between 15o and 16o C. We have seen in Chap. I. that a frond of this Fern, as yet only slightly lobed and with a rachis only .23 inch in height, plainly circumnutated.*

 

* Mr. Loomis and Prof. Asa Gray have described (‘Botanical Gazette,’ 1880, pp. 27, 43), an extremely curious case of movement in the fronds, but only in the fruiting fronds, of Asplenium trichomanes. They move almost as rapidly as the little leaflets of Desmodium gyrans, alternately backwards and forwards through from 20 to 40 degrees, in a plane at right angles to that of the frond. The apex of the frond describes “a long and very narrow ellipse,” so that it circumnutates. But the movement differs from ordinary [[page 258]]

circumnutation as it occurs only when the plant is exposed to the light; even artificial light “is sufficient to excite motion for a few minutes.”

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In the chapter on the Sleep of Plants the conspicuous circumnutation of Marsilea quadrifoliata (Marsileaceae, Fam. 4) will be described.

 

It has also been shown in Chap. I. that a very young Selaginella (Lycopodiaceae, Fam. 6), only .4 inch in height, plainly circumnutated; we may therefore conclude that older plants, whilst growing, would do the same.

 

Fig. 120. Lunularia vulgaris: circumnutation of a frond, traced from 9 A.M.

Oct 25th to 8 A.M. 27th.

 

(35.) Lunularia vulgaris (Hepaticae, Fam. 11, Muscales).—The earth in an old flowerpot was coated with this plant, bearing gemmae. A highly inclined frond, which projected .3 inch above the soil and was .4 inch in breadth, was selected for observation. A glass hair of extreme tenuity, .75

inch in length, with its end whitened, was cemented with shellac to the frond at right angles to its breadth; and a white stick with a minute black spot was driven into the soil close behind the end of the hair. The white end could be accurately brought into a line with the black spot, and dots could thus be successively made on the vertical glass-plate in front. Any movement of the frond would of course be exhibited and increased by the long glass hair; and the black spot was placed so close behind the end of the hair, relatively to the distance of the glass-plate in front, that the movement of the end was magnified about 40 times. Nevertheless, we are convinced that our tracing gives a fairly faithful representation of the movements of the frond. In the intervals between each observation, the plant was covered by a small bell-glass. The frond, as already stated, [page 259]

was highly inclined, and the pot stood in front of a north-east window.

During the five first days the frond moved downwards or became less inclined; and the long line which was traced was strongly zigzag, with loops occasionally formed or nearly formed; and this indicated circumnutation. Whether the sinking was due to epinastic growth, or apheliotropism, we do not know. As the sinking was slight on the fifth day, a new tracing was begun on the sixth day (Oct. 25th), and was continued for 47 h.; it is here given (Fig. 120). Another tracing was made on the next day (27th) and the frond was found to be still circumnutating, for during 14 h. 30 m. it changed its course completely (besides minor changes) 10

times. It was casually observed for two more days, and was seen to be continually moving.

 

The lowest members of the vegetable series, the Thallogens, apparently circumnutate. If an Oscillaria be watched under the microscope, it may be seen to describe circles about every 40 seconds. After it has bent to one side, the tip first begins to bend back to the opposite side and then the whole filament curves over in the same direction. Hofmeister* has given a minute account of the curious, but less regular though constant, movements of Spirogyra: during 2 � h. the filament moved 4 times to the left and 3

times to the right, and he refers to a movement at right angles to the above. The tip moved at the rate of about 0.1 mm. in five minutes. He compares the movement with the nutation of the higher plants.** We shall hereafter see that heliotropic movements result from modified circumnutation, and as unicellular Moulds bend to the light we may infer that they also circumnutate.]

 

CONCLUDING REMARKS ON THE CIRCUMNUTATION OF LEAVES.

 

The circumnutating movements of young leaves in 33 genera, belonging to 25

families, widely distributed

 

* ‘Ueber die Bewegungen der Faden der Spirogyra princeps: Jahreshefte des Vereins f�r vaterl�ndische Naturkunde in W�rttemberg,’ 1874, p. 211.

 

** Zukal also remarks (as quoted in ‘Journal R. Microscop. Soc.,’ 1880, vol. iii. p. 320) that the movements of Spirulina, a member of the Oscillatorieae, are closely analogous “to the well-known rotation of growing shoots and tendrils.”

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amongst ordinary and gymnospermous Dicotyledons and amongst Monocotyledons, together with several Cryptogams, have now been described. It would, therefore, not be rash to assume that the growing leaves of all plants circumnutate, as we have seen reason to conclude is the case with cotyledons. The seat of movement generally lies in the petiole, but sometimes both in the petiole and blade, or in the blade alone. The extent of the movement differed much in different plants; but the distance passed over was never great, except with Pistia, which ought perhaps to have been included amongst sleeping plants. The angular movement of the leaves was only occasionally measured; it commonly varied from only 2o (and probably even less in some instances) to about 10o; but it amounted to 23o in the common bean. The movement is chiefly in a vertical plane, but as the ascending and descending lines never coincided, there was always some lateral movement, and thus irregular ellipses were formed. The movement, therefore, deserves to be called one of circumnutation; for all circumnutating organs tend to describe ellipses,—that is, growth on one side is succeeded by growth on nearly but not quite the opposite side. The ellipses, or the zigzag lines representing drawn-out ellipses, are generally very narrow; yet with the Camellia, their minor axes were half as long, and with the Eucalyptus more than half as long as their major axes.

In the case of Cissus, parts of the figure more nearly represented circles than ellipses. The amount of lateral movement is therefore sometimes considerable. Moreover, the longer axes of the successively formed ellipses (as with the Bean, Cissus, and Sea-kale), and in several instances the zigzag lines representing ellipses, were extended in very different directions during the same day or on

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the next day. The course followed was curvilinear or straight, or slightly or strongly zigzag, and little loops or triangles were often formed. A single large irregular ellipse may be described on one day, and two smaller ones by the same plant on the next day. With Drosera two, and with Lupinus, Eucalyptus and Pancratium, several were formed each day.

 

The oscillatory and jerking movements of the leaves of Dionaea, which resemble those of the hypocotyl of the cabbage, are highly remarkable, as seen under the microscope. They continue night and day for some months, and are displayed by young unexpanded leaves, and by old ones which have lost their sensibility to a touch, but which, after absorbing animal matter, close their lobes. We shall hereafter meet with the same kind of movement in the joints of certain Gramineae, and it is probably common to many plants while circumnutating. It is, therefore, a strange fact that no such movement could be detected in the tentacles of Drosera rotundifolia, though a member of the same family with Dionaea; yet the tentacle which was observed was so sensitive, that it began to curl inwards in 23 seconds after being touched by a bit of raw meat.

 

One of the most interesting facts with respect to the circumnutation of leaves is the periodicity of their movements; for they often, or even generally, rise a little in the evening and early part of the night, and sink again on the following morning. Exactly the same phenomenon was observed in the case of cotyledons. The leaves in 16 genera out of the 33

which were observed behaved in this manner, as did probably 2 others. Nor must it be supposed that in the remaining 15 genera there was no periodicity in their movements; for 6 of them were observed during too short a period for any judgment to be formed on this head, [page 262]

and 3 were so young that their epinastic growth, which serves to bring them down into a horizontal position, overpowered every other kind of movement.

In only one genus, Cannabis, did the leaves sink in the evening, and Kraus attributes this movement to the prepotency of their epinastic growth. That the periodicity is determined by the daily alternations of light and darkness there can hardly be a doubt, as will hereafter be shown.

Insectivorous plants are very little affected, as far as their movements are concerned, by light; and hence probably it is that their leaves, at least in the cases of Sarracenia, Drosera, and Dionaea, do not move periodically. The upward movement in the evening is at first slow, and with different plants begins at very different hours;—with Glaucium as early as 11 A.M., commonly between 3 and 5 P.M., but sometimes as late as 7 P.M. It should be observed that none of the leaves described in this chapter (except, as we believe, those of Lupinus speciosus) possess a pulvinus; for the periodical movements of leaves thus provided have generally been amplified into so-called sleep-movements, with which we are not here concerned. The fact of leaves and cotyledons frequently, or even generally, rising a little in the evening and sinking in the morning, is of interest as giving the foundation from which the specialised sleep-movements of many leaves and cotyledons, not provided with a pulvinus, have been developed.

the above periodicity should be kept in mind, by any one considering the problem of the horizontal position of leaves and cotyledons during the day, whilst illuminated from above.

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CHAPTER V.

 

MODIFIED CIRCUMNUTATION: CLIMBING PLANTS; EPINASTIC AND HYPONASTIC

MOVEMENTS.

 

Circumnutation modified through innate causes or through the action of external conditions—Innate causes—Climbing plants; similarity of their movements with those of ordinary plants; increased amplitude; occasional points of difference—Epinastic growth of young leaves—Hyponastic growth of the hypocotyls and epicotyls of seedlings—Hooked tips of climbing and other plants due to modified circumnutation—Ampelopsis tricuspidata—

Smithia Pfundii—Straightening of the tip due to hyponasty—Epinastic growth and circumnutation of the flower-peduncles of Trifolium repens and Oxalis carnosa.

 

THE radicles, hypocotyls and epicotyls of seedling plants, even before they emerge from the ground, and afterwards the cotyledons, are all continually circumnutating. So it is with the stems, stolons, flower-peduncles, and leaves of older plants. We may, therefore, infer with a considerable degree of safety that all the growing parts of all plants circumnutate. Although this movement, in its ordinary or unmodified state, appears in some cases to be of service to plants, either directly or indirectly—for instance, the circumnutation of the radicle in penetrating the ground, or that of the arched hypocotyl and epicotyl in breaking through the surface—yet circumnutation is so general, or rather so universal a phenomenon, that we cannot suppose it to have been gained for any special purpose. We must believe that it follows in some unknown way from the manner in which vegetable tissues grow.

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We shall now consider the many cases in which circumnutation has been modified for various special purposes; that is, a movement already in progress is temporarily increased in some one direction, and temporarily diminished or quite arrested in other directions. These cases may be divided