The Power of Movement in Plants · Charles Darwin
A SUMMARY OF CHAPTER.
A part or organ may be called sensitive, when its irritation excites movement in an adjoining part. Now it has been shown in this chapter, that the tip of the radicle of the bean is in this sense sensitive to the contact of any small object attached to one side by shellac or gum-water; also to a slight touch with dry caustic, and to a thin slice cut off one side. The radicles of the pea were tried with attached objects and caustic, both of which acted. With Phaseolus multiflorus the tip was hardly sensitive to small squares of attached card, but was sensitive to caustic and to slicing. The radicles of Tropaeolum were highly sensitive to contact; and so, as far as we could judge, were those of Gossypium herbaceum, and they were certainly sensitive to caustic. The tips of the radicles of Cucurbita ovifera were likewise highly sensitive to caustic, though only moderately so to contact. Raphanus sativus offered a somewhat doubtful case. With Æsculus the tips were quite indifferent to bodies attached to them, though sensitive to caustic. Those of Quercus robur and Zea mays were highly sensitive to contact, as were the radicles of the latter to caustic. In several of these cases the difference in sensitiveness of the tip to contact and to caustic was, as we believe, merely apparent; for with Gossypium, Raphanus, and Cucurbita, the tip was so fine and flexible that it was very difficult to attach any object to one of its sides. With the radicles of Æsculus, the tips were not at all sensitive to small bodies attached to them; but it does not follow from this fact that they would not have been sensitive to somewhat greater continued pressure, if this could have been applied.
The peculiar form of sensitiveness which we are here considering, is confined to the tip of the radicle for a length of from 1 mm. to 1.5 mm. When this part is irritated by contact with any object, by caustic, or by a thin slice being cut off, the upper adjoining part of the radicle, for a length of from 6 or 7 to even 12 mm., is excited to bend away from the side which has been irritated. Some influence must therefore be transmitted from the tip along the radicle for this length. The curvature thus caused is generally symmetrical. The part which bends most apparently coincides with that of the most rapid growth. The tip and the basal part grow very slowly and they bend very little. Considering the widely separated position in the vegetable series of the several above-named genera, we may conclude that the tips of the radicles of all, or almost all, plants are similarly sensitive, and transmit an influence causing the upper part to bend. With respect to the tips of the secondary radicles, those of Vicia faba, Pisum sativum, and Zea mays were alone observed, and they were found similarly sensitive. In order that these movements should be properly displayed, it appears necessary that the radicles should grow at their normal rate. If subjected to a high temperature and made to grow rapidly, the tips seem either to lose their sensitiveness, or the upper part to lose the power of bending. So it appears to be if they grow very slowly from not being vigorous, or from being kept at too low a temperature; also when they are forced to germinate in the middle of the winter.
The curvature of the radicle sometimes occurs within from 6 to 8 hours after the tip has been irritated, and almost always within 24 h., excepting in the case of the massive radicles of Æsculus. The curvature often amounts to a rectangle,—that is, the terminal part bends upwards until the tip, which is but little curved, projects almost horizontally. Occasionally the tip, from the continued irritation of the attached object, continues to bend up until it forms a hook with the point directed towards the zenith, or a loop, or even a spire. After a time the radicle apparently becomes accustomed to the irritation, as occurs in the case of tendrils, for it again grows downwards, although the bit of card or other object may remain attached to the tip. It is evident that a small object attached to the free point of a vertically suspended radicle can offer no mechanical resistance to its growth as a whole, for the object is carried downwards as the radicle elongates, or upwards as the radicle curves upwards. Nor can the growth of the tip itself be mechanically checked by an object attached to it by gum-water, which remains all the time perfectly soft. The weight of the object, though quite insignificant, is opposed to the upward curvature. We may therefore conclude that it is the irritation due to contact which excites the movement. The contact, however, must be prolonged, for the tips of 15 radicles were rubbed for a short time, and this did not cause them to bend. Here then we have a case of specialised sensibility, like that of the glands of Drosera; for these are exquisitely sensitive to the slightest pressure if prolonged, but not to two or three rough touches.
When the tip of a radicle is lightly touched on one side with dry nitrate of silver, the injury caused is very slight, and the adjoining upper part bends away from the cauterised point, with more certainty in most cases than from an object attached on one side. Here it obviously is not the mere touch, but the effect produced by the caustic, which induces the tip to transmit some influence to the adjoining part, causing it to bend away. If one side of the tip is badly injured or killed by the caustic, it ceases to grow, whilst the opposite side continues growing; and the result is that the tip itself bends towards the injured side and often becomes completely hooked; and it is remarkable that in this case the adjoining upper part does not bend. The stimulus is too powerful or the shock too great for the proper influence to be transmitted from the tip. We have strictly analogous cases with Drosera, Dionaea and Pinguicula, with which plants a too powerful stimulus does not excite the tentacles to become incurved, or the lobes to close, or the margin to be folded inwards.
With respect to the degree of sensitiveness of the apex to contact under favourable conditions, we have seen that with Vicia faba a little square of writing-paper affixed with shellac sufficed to cause movement; as did on one occasion a square of merely damped goldbeaters’ skin, but it acted very slowly. Short bits of moderately thick bristle (of which measurements have been given) affixed with gum-water acted in only three out of eleven trials, and beads of dried shellac under 1/200th of a grain in weight acted only twice in nine cases; so that here we have nearly reached the minimum of necessary irritation. The apex, therefore, is much less sensitive to pressure than the glands of Drosera, for these are affected by far thinner objects than bits of bristle, and by a very much less weight than 1/200th of a grain. But the most interesting evidence of the delicate sensitiveness of the tip of the radicle, was afforded by its power of discriminating between equal-sized squares of card-like and very thin paper, when these were attached on opposite sides, as was observed with the radicles of the bean and oak.
When radicles of the bean are extended horizontally with squares of card attached to the lower sides of their tips, the irritation thus caused was always conquered by geotropism, which then acts under the most favourable conditions at right angles to the radicle. But when objects were attached to the radicles of any of the above-named genera, suspended vertically, the irritation conquered geotropism, which latter power at first acted obliquely on the radicle; so that the immediate irritation from the attached object, aided by its after-effects, prevailed and caused the radicle to bend upwards, until sometimes the point was directed to the zenith. We must, however, assume that the after-effects of the irritation of the tip by an attached object come into play, only after movement has been excited. The tips of the radicles of the pea seem to be more sensitive to contact than those of the bean, for when they were extended horizontally with squares of card adhering to their lower sides, a most curious struggle occasionally arose, sometimes one and sometimes the other force prevailing, but ultimately geotropism was always victorious; nevertheless, in two instances the terminal part became so much curved upwards that loops were subsequently formed. With the pea, therefore, the irritation from an attached object, and from geotropism when acting at right angles to the radicle, are nearly balanced forces. Closely similar results were observed with the horizontally extended radicles of _Cucurbita ovifera_, when their tips were slightly cauterised on the lower side.
Finally, the several co-ordinated movements by which radicles are enabled to perform their proper functions are admirably perfect. In whatever direction the primary radicle first protrudes from the seed, geotropism guides it perpendicularly downwards; and the capacity to be acted on by the attraction of gravity resides in the tip. But Sachs has proved[15] that the secondary radicles, or those emitted by the primary one, are acted on by geotropism in such a manner that they tend to bend only obliquely downwards. If they had been acted on like the primary radicle, all the radicles would have penetrated the ground in a close bundle. We have seen that if the end of the primary radicle is cut off or injured, the adjoining secondary radicles become geotropic and grow vertically downwards. This power must often be of great service to the plant, when the primary radicle has been destroyed by the larvae of insects, burrowing animals, or any other accident. The tertiary radicles, or those emitted by the secondary ones, are not influenced, at least in the case of the bean, by geotropism; so they grow out freely in all directions. From this manner of growth of the various kinds of radicles, they are distributed, together with their absorbent hairs, throughout the surrounding soil, as Sachs has remarked, in the most advantageous manner; for the whole soil is thus closely searched.
This movement, moreover, combined with the sensitiveness of the tip to contact, can hardly fail to be of the highest importance; for as the tip is always endeavouring to bend to all sides it will press on all sides, and will thus be able to discriminate between the harder and softer adjoining surfaces, in the same manner as it discriminated between the attached squares of card-like and thin paper. Consequently it will tend to bend from the harder soil, and will thus follow the lines of least resistance. So it will be if it meets with a stone or the root of another plant in the soil, as must incessantly occur. If the tip were not sensitive, and if it did not excite the upper part of the root to bend away, whenever it encountered at right angles some obstacle in the ground, it would be liable to be doubled up into a contorted mass. But we have seen with radicles growing down inclined plates of glass, that as soon as the tip merely touched a slip of wood cemented across the plate, the whole terminal growing part curved away, so that the tip soon stood at right angles to its former direction; and thus it would be with an obstacle encountered in the ground, as far as the pressure of the surrounding soil would permit. We can also understand why thick and strong radicles, like those of Æsculus, should be endowed with less sensitiveness than more delicate ones; for the former would be able by the force of their growth to overcome any slight obstacle.
After a radicle, which has been deflected by some stone or root from its natural downward course, reaches the edge of the obstacle, geotropism will direct it to grow again straight downward; but we know that geotropism acts with very little force, and here another excellent adaptation, as Sachs has remarked,[16] comes into play. For the upper part of the radicle, a little above the apex, is, as we have seen, likewise sensitive; and this sensitiveness causes the radicle to bend like a tendril towards the touching object, so that as it rubs over the edge of an obstacle, it will bend downwards; and the curvature thus induced is abrupt, in which respect it differs from that caused by the irritation of one side of the tip. This downward bending coincides with that due to geotropism, and both will cause the root to resume its original course. [16] ‘Arbeiten Bot. Inst., Würzburg,’ Heft iii. p. 456. As radicles perceive an excess of moisture in the air on one side and bend towards this side, we may infer that they will act in the same manner with respect to moisture in the earth. The sensitiveness to moisture resides in the tip, which determines the bending of the upper part. This capacity perhaps partly accounts for the extent to which drain-pipes often become choked with roots.
Considering the several facts given in this chapter, we see that the course followed by a root through the soil is governed by extraordinarily complex and diversified agencies,—by geotropism acting in a different manner on the primary, secondary, and tertiary radicles,—by sensitiveness to contact, different in kind in the apex and in the part immediately above the apex, and apparently by sensitiveness to the varying dampness of different parts of the soil. These several stimuli to movement are all more powerful than geotropism, when this acts obliquely on a radicle, which has been deflected from its perpendicular downward course. The roots, moreover, of most plants are excited by light to bend either to or from it; but as roots are not naturally exposed to the light it is doubtful whether this sensitiveness, which is perhaps only the indirect result of the radicles being highly sensitive to other stimuli, is of any service to the plant. The direction which the apex takes at each successive period of the growth of a root, ultimately determines its whole course; it is therefore highly important that the apex should pursue from the first the most advantageous direction; and we can thus understand why sensitiveness to geotropism, to contact and to moisture, all reside in the tip, and why the tip determines the upper growing part to bend either from or to the exciting cause. A radicle may be compared with a burrowing animal such as a mole, which wishes to penetrate perpendicularly down into the ground. By continually moving his head from side to side, or circumnutating, he will feel any stone or other obstacle, as well as any difference in the hardness of the soil, and he will turn from that side; if the earth is damper on one than on the other side he will turn thitherward as a better hunting-ground. Nevertheless, after each interruption, guided by the sense of gravity, he will be able to recover his downward course and to burrow to a greater depth.
CHAPTER IV. THE CIRCUMNUTATING MOVEMENTS OF THE SEVERAL PARTS OF MATURE PLANTS. Circumnutation of stems: concluding remarks on—Circumnutation of stolons: aid thus afforded in winding amongst the stems of surrounding plants—Circumnutation of flower-stems—Circumnutation of Dicotyledonous leaves—Singular oscillatory movement of leaves of Dionaea—Leaves of Cannabis sink at night—Leaves of Gymnosperms—Of Monocotyledons—Cryptogams—Concluding remarks on the circumnutation of leaves; generally rise in the evening and sink in the morning. We have seen in the first chapter that the stems of all seedlings, whether hypocotyls or epicotyls, as well as the cotyledons and the radicles, are continually circumnutating—that is they grow first on one side and then on another, such growth being probably preceded by increased turgescence of the cells. As it was unlikely that plants should change their manner of growth with advancing age, it seemed probable that the various organs of all plants at all ages, as long as they continued to grow, would be found to circumnutate, though perhaps to an extremely small extent. As it was important for us to discover whether this was the case, we determined to observe carefully a certain number of plants which were growing vigorously, and which were not known to move in any manner. We commenced with stems. Observations of this kind are tedious, and it appeared to us that it would be sufficient to observe the stems in about a score of genera, belonging to widely distinct families and inhabitants of various countries. Several plants were selected which, from being woody, or for other reasons, seemed the least likely to circumnutate. The observations and the diagrams were made in the manner described in the Introduction. Plants in pots were subjected to a proper temperature, and whilst being observed, were kept either in darkness or were feebly illuminated from above. They are arranged in the order adopted by Hooker in Le Maout and Decaisne’s ‘System of Botany.’ The number of the family to which each genus belongs is appended, as this serves to show the place of each in the series.
(1.) Iberis umbellata (Cruciferae, Fam. 14).—The movement of the stem of a young plant, 4 inches in height, consisting of four internodes (the hypocotyl included) besides a large bud on the summit, was traced, as here shown, during 24 h. (Fig. 70). As far as we could judge the uppermost inch alone of the stem circumnutated, and this in a simple manner. The movement was slow, and the rate very unequal at different times. In part of its course an irregular ellipse, or rather triangle, was completed in 6 h. 30 m. Fig. 70. Iberis umbellata: circumnutation of stem of young plant, traced from 8.30 A.M. Sept. 13th to same hour on following morning. Distance of summit of stem beneath the horizontal glass 7.6 inches. Diagram reduced to half of original size. Movement as here shown magnified between 4 and 5 times. (2.) Brassica oleracea (Cruciferae).—A very young plant, bearing three leaves, of which the longest was only three-quarters of an inch in length, was placed under a microscope, furnished with an eye-piece micrometer, and the tip of the largest leaf was found to be in constant movement. It crossed five divisions of the micrometer, that is, 1/100th of an inch, in 6 m. 20 s. There could hardly be a doubt that it was the stem which chiefly moved, for the tip did not get quickly out of focus; and this would have occurred had the movement been confined to the leaf, which moves up or down in nearly the same vertical plane.
(3.) Linum usitatissimum (Lineae, Fam. 39).—The stems of this plant, shortly before the flowering period, are stated by Fritz Müller (‘Jenaische Zeitschrift,’ B. v. p. 137) to revolve, or circumnutate. (4.) Pelargonium zonale (Geraniaceae, Fam. 47).—A young plant, 7½ inches in height, was observed in the usual manner; but, in order to see the bead at the end of the glass filament and at the same time the mark beneath, it was necessary to cut off three leaves on one side. We do not know whether it was owing to this cause, or to the plant having previously become bent to one side through heliotropism, but from the morning of the 7th of March to 10.30 P.M. on the 8th, the stem moved a considerable distance in a zigzag line in the same general direction. During the night of the 8th it moved to some distance at right angles to its former course, and next morning (9th) stood for a time almost still. At noon on the 9th a new tracing was begun (see Fig. 71), which was continued till 8 A.M. on the 11th. Between noon on the 9th and 5 P.M. on the 10th (i.e. in the course of 29 h.), the stem described a circle. This plant therefore circumnutates, but at a very slow rate, and to a small extent.
Fig. 71. Pelargonium zonale: circumnutation of stem of young plant, feebly illuminated from above. Movement of bead magnified about 11 times; traced on a horizontal glass from noon on March 9th to 8 A.M. on the 11th. (5.) Tropaeolum majus (?) (dwarfed var. called Tom Thumb); (Geraniaceae, Fam. 47).—The species of this genus climb by the aid of their sensitive petioles, but some of them also twine round supports; but even these latter species do not begin to circumnutate in a conspicuous manner whilst young. The variety here treated of has a rather thick stem, and is so dwarf that apparently it does not climb in any manner. We therefore wished to ascertain whether the stem of a young plant, consisting of two internodes, together 3.2 inches in height, circumnutated. It was observed during 25 h., and we see in Fig. 72 that the stem moved in a zigzag course, indicating circumnutation. Fig. 72. Tropaeolum majus (?): circumnutation of stem of young plant, traced on a horizontal glass from 9 A.M. Dec. 26th to 10 A.M. on 27th. Movement of bead magnified about 5 times, and here reduced to half of original scale. Fig. 73. Trifolium resupinatum: circumnutation of stem, traced on vertical glass from 9.30 A.M. to 4.30 P.M. Nov. 3rd. Tracing not greatly magnified, reduced to half of original size. Plant feebly illuminated from above.
(6.) Trifolium resupinatum (Leguminosae, Fam. 75).—When we treat of the sleep of plants, we shall see that the stems in several Leguminous genera, for instance, those of Hedysarum, Mimosa, Melilotus, etc., which are not climbers, circumnutate in a conspicuous manner. We will here give only a single instance (Fig. 73), showing the circumnutation of the stem of a large plant of a clover, Trifolium resupinatum. In the course of 7 h. the stem changed its course greatly eight times and completed three irregular circles or ellipses. It therefore circumnutated rapidly. Some of the lines run at right angles to one another. Fig. 74. Rubus (hybrid): circumnutation of stem, traced on horizontal glass, from 4 P.M. March 14th to 8.30 A.M. 16th. Tracing much magnified, reduced to half of original size. Plant illuminated feebly from above. (7.) Rubus idæus (hybrid) (Rosaceae, Fam. 76).—As we happened to have a young plant, 11 inches in height and growing vigorously, which had been raised from a cross between the raspberry (Rubus idæus) and a North American Rubus, it was observed in the usual manner. During the morning of March 14th the stem almost completed a circle, and then moved far to the right. At 4 P.M. it reversed its course, and now a fresh tracing was begun, which was continued during 40½ h., and is given in Fig. 74. We here have well-marked circumnutation.
(8.) Deutzia gracilis (Saxifrageae, Fam. 77).—A shoot on a bush about 18 inches in height was observed. The bead changed its course greatly eleven times in the course of 10 h. 30 m. (Fig. 75), and there could be no doubt about the circumnutation of the stem. Fig. 75. Deutzia gracilis: circumnutation of stem, kept in darkness, traced on horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of bead originally magnified about 20 times, here reduced to half scale. (9.) Fuchsia (greenhouse var., with large flowers, probably a hybrid) (Onagrarieae, Fam. 100).—A young plant, 15 inches in height, was observed during nearly 48 h. The accompanying figure (Fig. 76) gives the necessary particulars, and shows that the stem circumnutated, though rather slowly. Fig. 76. Fuchsia (garden var.): circumnutation of stem, kept in darkness, traced on horizontal glass, from 8.30 A.M. to 7 P.M. March 20th. Movement of bead originally magnified about 40 times, here reduced to half scale.
(10.) Cereus speciocissimus (garden var., sometimes called Phyllocactus multiflorus) (Cacteæ, Fam. 109).—This plant, which was growing vigorously from having been removed a few days before from the greenhouse to the hot-house, was observed with especial interest, as it seemed so little probable that the stem would circumnutate. The branches are flat, or flabelliform; but some of them are triangular in section, with the three sides hollowed out. A branch of this latter shape, 9 inches in length and 1½ in diameter, was chosen for observation, as less likely to circumnutate than a flabelliform branch. The movement of the bead at the end of the glass filament, affixed to the summit of the branch, was traced (A, Fig. 77) from 9.23 A.M. to 4.30 P.M. on Nov. 23rd, during which time it changed its course greatly six times. On the 24th another tracing was made (see B), and the bead on this day changed its course oftener, making in 8 h. what may be considered as four ellipses, with their longer axes differently directed. The position of the stem and its commencing course on the following morning are likewise shown. There can be no doubt that this branch, though appearing quite rigid, circumnutated; but the extreme amount of movement during the time was very small, probably rather less than the 1/20th of an inch.
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