The Power of Movement in Plants · Charles Darwin
CHAPTER VIII. MODIFIED CIRCUMNUTATION: MOVEMENTS EXCITED BY LIGHT.
Fig. 178. Bignonia capreolata: apheliotropic movement of a tendril, traced on a horizontal glass from 6.45 A.M. July 19th to 10 A.M. 20th. Movements as originally traced, little magnified, here reduced to two-thirds of the original scale. Bignonia capreolata.—No organ of any plant, as far as we have seen, bends away so quickly from the light as do the tendrils of this Bignonia. They are also remarkable from circumnutating much less regularly than most other tendrils, often remaining stationary; they depend on apheliotropism for coming into contact with the trunks of trees.[2] The stem of a young plant was tied to a stick at the base of a pair of fine tendrils, which projected almost vertically upwards; and it was placed in front of a north-east window, being protected on all other sides from the light. The first dot was made at 6.45 A.M., and by 7.35 A.M. both tendrils felt the full influence of the light, for they moved straight away from it until 9.20 A.M., when they circumnutated for a time, still moving, but only a little, from the light (see Fig. 178 of the left-hand tendril). After 3 P.M. they again moved rapidly away from the light in zigzag lines. By a late hour in the evening both had moved so far, that they pointed in a direct line from the light. During the night they returned a little in a nearly opposite direction. On the following morning they again moved from the light and converged, so that by the evening they had become interlocked, still pointing from the light. The right-hand tendril, whilst converging, zigzagged much more than the one figured. Both tracings showed that the apheliotropic movement was a modified form of circumnutation.
Relation between Circumnutation and Heliotropism.—Any one who will look at the foregoing diagrams, showing the movements of the stems of various plants towards a lateral and more or less dimmed light, will be forced to admit that ordinary circumnutation and heliotropism graduate into one another. When a plant is exposed to a dim lateral light and continues during the whole day bending towards it, receding late in the evening, the movement unquestionably is one of heliotropism. Now, in the case of Tropaeolum (Fig. 175) the stem or epicotyl obviously circumnutated during the whole day, and yet it continued at the same time to move heliotropically; this latter movement being effected by the apex of each successive elongated figure or ellipse standing nearer to the light than the previous one. In the case of Cassia (Fig. 177) the comparison of the movement of the hypocotyl, when exposed to a dim lateral light and to darkness, is very instructive; as is that between the ordinary circumnutating movement of a seedling Brassica (Figs. 172, 173), or that of Phalaris (Figs. 49, 174), and their heliotropic movement towards a window protected by blinds. In both these cases, and in many others, it was interesting to notice how gradually the stems began to circumnutate as the light waned in the evening. We have therefore many kinds of gradations from a movement towards the light, which must be considered as one of circumnutation very slightly modified and still consisting of ellipses or circles,—though a movement more or less strongly zigzag, with loops or ellipses occasionally formed,—to a nearly straight, or even quite straight, heliotropic course.
A plant, when exposed to a lateral light, though this may be bright, commonly moves at first in a zigzag line, or even directly from the light; and this no doubt is due to its circumnutating at the time in a direction either opposite to the source of the light, or more or less transversely to it. As soon, however, as the direction of the circumnutating movement nearly coincides with that of the entering light, the plant bends in a straight course towards the light, if this is bright. The course appears to be rendered more and more rapid and rectilinear, in accordance with the degree of brightness of the light—firstly, by the longer axes of the elliptical figures, which the plant continues to describe as long as the light remains very dim, being directed more or less accurately towards its source, and by each successive ellipse being described nearer to the light. Secondly, if the light is only somewhat dimmed, by the acceleration and increase of the movement towards it, and by the retardation or arrestment of that from the light, some lateral movement being still retained, for the light will interfere less with a movement at right angles to its direction, than with one in its own direction.[4] The result is that the course is rendered more or less zigzag and unequal in rate. Lastly, when the light is very bright all lateral movement is lost; and the whole energy of the plant is expended in rendering the circumnutating movement rectilinear and rapid in one direction alone, namely, towards the light.
We are therefore fully justified, as it seems to us, in believing that whenever light enters laterally, it is the movement of circumnutation which gives rise to, or is converted into, heliotropism and apheliotropism. On this view we need not assume against all analogy that a lateral light entirely stops circumnutation; it merely excites the plant to modify its movement for a time in a beneficial manner. The existence of every possible gradation, between a straight course towards a lateral light and a course consisting of a series of loops or ellipses, becomes perfectly intelligible. Finally, the conversion of circumnutation into heliotropism or apheliotropism, is closely analogous to what takes place with sleeping plants, which during the daytime describe one or more ellipses, often moving in zigzag lines and making little loops; for when they begin in the evening to go to sleep, they likewise expend all their energy in rendering their course rectilinear and rapid. In the case of sleep-movements, the exciting or regulating cause is a difference in the intensity of the light, coming from above, at different periods of the twenty-four hours; whilst with heliotropic and apheliotropic movements, it is a difference in the intensity of the light on the two sides of the plant.
_Transversal-heliotropismus_ (_of Frank_[5]) _or Diaheliotropism_.—The cause of leaves placing themselves more or less transversely to the light, with their upper surfaces directed towards it, has been of late the subject of much controversy. We do not here refer to the object of the movement, which no doubt is that their upper surfaces may be fully illuminated, but the means by which this position is gained. Hardly a better or more simple instance can be given of diaheliotropism than that offered by many seedlings, the cotyledons of which are extended horizontally. When they first burst from their seed-coats they are in contact and stand in various positions, often vertically upwards; they soon diverge, and this is effected by epinasty, which, as we have seen, is a modified form of circumnutation. After they have diverged to their full extent, they retain nearly the same position, though brightly illuminated all day long from above, with their lower surfaces close to the ground and thus much shaded. There is therefore a great contrast in the degree of illumination of their upper and lower surfaces, and if they were heliotropic they would bend quickly upwards. It must not, however, be supposed that such cotyledons are immovably fixed in a horizontal position.
When seedlings are exposed before a window, their hypocotyls, which are highly heliotropic, bend quickly towards it, and the upper surfaces of their cotyledons still remain exposed at right angles to the light; but if the hypocotyl is secured so that it cannot bend, the cotyledons themselves change their position. If the two are placed in the line of the entering light, the one furthest from it rises up and that nearest to it often sinks down; if placed transversely to the light, they twist a little laterally; so that in every case they endeavour to place their upper surfaces at right angles to the light. So it notoriously is with the leaves on plants nailed against a wall, or grown in front of a window. A moderate amount of light suffices to induce such movements; all that is necessary is that the light should steadily strike the plants in an oblique direction. With respect to the above twisting movement of cotyledons, Frank has given many and much more striking instances in the case of the leaves on branches which had been fastened in various positions or turned upside down. [5] ‘Die natürliche Wagerechte Richtung von Pflanzentheilen,’ 1870. See also some interesting articles by the same author, “Zur Frage über Transversal-Geo-und Heliotropismus,” ‘Bot. Zeitung,’ 1873, p. 17 et seq.
In our observations on the cotyledons of seedling plants, we often felt surprise at their persistent horizontal position during the day, and were convinced before we had read Frank’s essay, that some special explanation was necessary. De Vries has shown[6] that the more or less horizontal position of leaves is in most cases influenced by epinasty, by their own weight, and by apogeotropism. A young cotyledon or leaf after bursting free is brought down into its proper position, as already remarked, by epinasty, which, according to De Vries, long continues to act on the midribs and petioles. Weight can hardly be influential in the case of cotyledons, except in a few cases presently to be mentioned, but must be so with large and thick leaves. With respect to apogeotropism, De Vries maintains that it generally comes into play, and of this fact we shall presently advance some indirect evidence. But over these and other constant forces we believe that there is in many cases, but we do not say in all, a preponderant tendency in leaves and cotyledons to place themselves more or less transversely with respect to the light. [6] ‘Arbeiten des Bot. Instituts in Würzburg,’ Heft. ii. 1872, pp. 223–277.
In the cases above alluded to of seedlings exposed to a lateral light with their hypocotyls secured, it is impossible that epinasty, weight and apogeotropism, either in opposition or combined, can be the cause of the rising of one cotyledon, and of the sinking of the other, since the forces in question act equally on both; and since epinasty, weight and apogeotropism all act in a vertical plane, they cannot cause the twisting of the petioles, which occurs in seedlings under the above conditions of illumination. All these movements evidently depend in some manner on the obliquity of the light, but cannot be called heliotropic, as this implies bending towards the light; whereas the cotyledon nearest to the light bends in an opposed direction or downwards, and both place themselves as nearly as possible at right angles to the light. The movement, therefore, deserves a distinct name. As cotyledons and leaves are continually oscillating up and down, and yet retain all day long their proper position with their upper surfaces directed transversely to the light, and if displaced reassume this position, diaheliotropism must be considered as a modified form of circumnutation. This was often evident when the movements of cotyledons standing in front of a window were traced. We see something analogous in the case of sleeping leaves or cotyledons, which after oscillating up and down during the whole day, rise into a vertical position late in the evening, and on the following morning sink down again into their horizontal or diaheliotropic position, in direct opposition to heliotropism. This return into their diurnal position, which often requires an angular movement of 90°, is analogous to the movement of leaves on displaced branches, which recover their former positions. It deserves notice that any force such as apogeotropism, will act with different degrees of power[7] in the different positions of those leaves or cotyledons which oscillate largely up and down during the day; and yet they recover their horizontal or diaheliotropic position.
We have seen in the second and fourth chapters that those cotyledons and leaves which do not alter their positions at night sufficiently to be said to sleep, commonly rise a little in the evening and fall again on the next morning, so that they stand during the night at a rather higher inclination than during the middle of the day. It is incredible that a rising movement of 2° or 3°, or even of 10° or 20°, can be of any service to the plant, so as to have been specially acquired. It must be the result of some periodical change in the conditions to which they are subjected, and there can hardly be a doubt that this is the daily alternations of light and darkness. De Vries states in the paper before referred to, that most petioles and midribs are apogeotropic;[8] and apogeotropism would account for the above rising movement, which is common to so many widely distinct species, if we suppose it to be conquered by diaheliotropism during the middle of the day, as long as it is of importance to the plant that its cotyledons and leaves should be fully exposed to the light.
The exact hour in the afternoon at which they begin to bend slightly upwards, and the extent of the movement, will depend on their degree of sensitiveness to gravitation and on their power of resisting its action during the middle of the day, as well as on the amplitude of their ordinary circumnutating movements; and as these qualities differ much in different species, we might expect that the hour in the afternoon at which they begin to rise would differ much in different species, as is the case. Some other agency, however, besides apogeotropism, must come into play, either directly or indirectly, in this upward movement. Thus a young bean (Vicia faba), growing in a small pot, was placed in front of a window in a klinostat; and at night the leaves rose a little, although the action of apogeotropism was quite eliminated. Nevertheless, they did not rise nearly so much at night, as when subjected to apogeotropism. Is it not possible, or even probable, that leaves and cotyledons, which have moved upwards in the evening through the action of apogeotropism during countless generations, may inherit a tendency to this movement? We have seen that the hypocotyls of several Leguminous plants have from a remote period inherited a tendency to arch themselves; and we know that the sleep-movements of leaves are to a certain extent inherited, independently of the alternations of light and darkness.
The so-called Diurnal Sleep of Leaves, Or Paraheliotropism.—This is another class of movements, dependent on the action of light, which supports to some extent the belief that the movements above described are only indirectly due to its action. We refer to the movements of leaves and cotyledons which when moderately illuminated are diaheliotropic; but which change their positions and present their edges to the light, when the sun shines brightly on them. These movements have sometimes been called diurnal sleep, but they differ wholly with respect to the object gained from those properly called nyctitropic; and in some cases the position occupied during the day is the reverse of that during the night. It has long been known[9] that when the sun shines brightly on the leaflets of Robinia, they rise up and present their edges to the light; whilst their position at night is vertically downwards. We have observed the same movement, when the sun shone brightly on the leaflets of an Australian Acacia. Those of Amphicarpæa monoica turned their edges to the sun; and an analogous movement of the little almost rudimentary basal leaflets of Mimosa albida was on one occasion so rapid that it could be distinctly seen through a lens. the elongated, unifoliate, first leaves of Phaseolus Roxburghii stood at 7 A.M. at 20° above the horizon, and no doubt they afterwards sank a little lower. At noon, after having been exposed for about 2 h. to a bright sun, they stood at 56° above the horizon; they were then protected from the rays of the sun, but were left well illuminated from above, and after 30 m. they had fallen 40°, for they now stood at only 16° above the horizon.
Some young plants of Phaseolus Hernandesii had been exposed to the same bright sunlight, and their broad, unifoliate, first leaves now stood up almost or quite vertically, as did many of the leaflets on the trifoliate secondary leaves; but some of the leaflets had twisted round on their own axes by as much as 90° without rising, so as to present their edges to the sun. The leaflets on the same leaf sometimes behaved in these two different manners, but always with the result of being less intensely illuminated. These plants were then protected from the sun, and were looked at after 1½ h.; and now all the leaves and leaflets had reassumed their ordinary sub-horizontal positions. The copper-coloured cotyledons of some seedlings of Cassia mimosoides were horizontal in the morning, but after the sun had shone on them, each had risen 45½° above the horizon. the movement in these several cases must not be confounded with the sudden closing of the leaflets of Mimosa pudica, which may sometimes be noticed when a plant which has been kept in an obscure place is suddenly exposed to the sun; for in this case the light seems to act, as if it were a touch. [9] Pfeffer gives the names and dates of several ancient writers in his ‘Die Periodischen Bewegungen,’ 1875, p. 62.
From Prof. Wiesner’s interesting observations, it is probable that the above movements have been acquired for a special purpose. the chlorophyll in leaves is often injured by too intense a light, and Prof. Wiesner[10] believes that it is protected by the most diversified means, such as the presence of hairs, colouring matter, etc., and amongst other means by the leaves presenting their edges to the sun, so that the blades then receive much less light. He experimented on the young leaflets of Robinia, by fixing them in such a position that they could not escape being intensely illuminated, whilst others were allowed to place themselves obliquely; and the former began to suffer from the light in the course of two days. [10] ‘Die Näturlichen Einrichtungen zum Schutze des Chlorophylls,’ etc., 1876. Pringsheim has recently observed under the microscope the destruction of chlorophyll in a few minutes by the action of concentrated light from the sun, in the presence of oxygen. See, also, Stahl on the protection of chlorophyll from intense light, in ‘Bot. Zeitung,’ 1880.
In the cases above given, the leaflets move either upwards or twist laterally, so as to place their edges in the direction of the sun’s light; but Cohn long ago observed that the leaflets of Oxalis bend downwards when fully exposed to the sun. We witnessed a striking instance of this movement in the very large leaflets of O. Ortegesii. A similar movement may frequently be observed with the leaflets of Averrhoa bilimbi (a member of the Oxalidæ); and a leaf is here represented (Fig. 180) on which the sun had shone. A diagram (Fig. 134) was given in the last chapter, representing the oscillations by which a leaflet rapidly descended under these circumstances; and the movement may be seen closely to resemble that (Fig. 133) by which it assumed its nocturnal position. It is an interesting fact in relation to our present subject that, as Prof. Batalin informs us in a letter, dated February, 1879, the leaflets of Oxalis acetosella may be daily exposed to the sun during many weeks, and they do not suffer if they are allowed to depress themselves; but if this be prevented, they lose their colour and wither in two or three days. Yet the duration of a leaf is about two months, when subjected only to diffused light; and in this case the leaflets never sink downwards during the day.
Fig. 180. Averrhoa bilimbi: leaf with leaflets depressed after exposure to sunshine; but the leaflets are sometimes more depressed than is here shown. Figure much reduced. As the upward movements of the leaflets of Robinia, and the downward movements of those of Oxalis, have been proved to be highly beneficial to these plants when subjected to bright sunshine, it seems probable that they have been acquired for the special purpose of avoiding too intense an illumination. As it would have been very troublesome in all the above cases to have watched for a fitting opportunity and to have traced the movement of the leaves whilst they were fully exposed to the sunshine, we did not ascertain whether paraheliotropism always consisted of modified circumnutation; but this certainly was the case with the Averrhoa, and probably with the other species, as their leaves were continually circumnutating.
Ideas discussed in this chapter
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