The Power of Movement in Plants · Charles Darwin
CHAPTER I. THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
Little squares of card were affixed with shellac on two occasions to the tips of 22 young and short secondary radicles, which had been emitted from the primary radicle whilst growing in water, but were now suspended in damp air. Besides the difficulty of attaching the squares to such finely pointed objects as were these radicles, the temperature was too high,—varying on the first occasion from 72° to 77° F., and on the second being almost steadily 78° F.; and this probably lessened the sensitiveness of the tips. The result was that after an interval of 8 h. 30 m., 6 of the 22 radicles were bowed upwards (one of them greatly) in opposition to gravity, and 2 laterally; the remaining 14 were not affected. Considering the unfavourable circumstances, and bearing in mind the case of the bean, the evidence appears sufficient to show that the tips of the secondary radicles of the pea are sensitive to slight contact.
Phaseolus multiflorus: Sensitiveness of the apex of the Radicle.—Fifty-nine radicles were tried with squares of various sizes of the same card-like paper, also with bits of thin glass and rough cinders, affixed with shellac to one side of the apex. Rather large drops of the dissolved shellac were also placed on them and allowed to set into hard beads. The specimens were subjected to various temperatures between 60° and 72° F., more commonly at about the latter. But out of this considerable number of trials only 5 radicles were plainly bent, and 8 others slightly or even doubtfully, from the attached objects; the remaining 46 not being at all affected. It is therefore clear that the tips of the radicles of this Phaseolus are much less sensitive to contact than are those of the bean or pea. We thought that they might be sensitive to harder pressure, but after several trials we could not devise any method for pressing harder on one side of the apex than on the other, without at the same time offering mechanical resistance to its growth. We therefore tried other irritants.
The tips of 13 radicles, dried with blotting-paper, were thrice touched or just rubbed on one side with dry nitrate of silver. They were rubbed thrice, because we supposed from the foregoing trials, that the tips were not highly sensitive. After 24 h. the tips were found greatly blackened; 6 were blackened equally all round, so that no curvature to any one side could be expected; 6 were much blackened on one side for a length of about 1/10th of an inch, and this length became curved at right angles towards the blackened surface, the curvature afterwards increasing in several instances until little hooks were formed. It was manifest that the blackened side was so much injured that it could not grow, whilst the opposite side continued to grow. One alone out of these 13 radicles became curved from the blackened side, the curvature extending for some little distance above the apex.
After the experience thus gained, the tips of six almost dry radicles were once touched with the dry caustic on one side; and after an interval of 10 m. were allowed to enter water, which was kept at a temperature of 65°–67° F. The result was that after an interval of 8 h. a minute blackish speck could just be distinguished on one side of the apex of five of these radicles, all of which became curved towards the opposite side—in two cases at about an angle of 45°—in two other cases at nearly a rectangle—and in the fifth case at above a rectangle, so that the apex was a little hooked; in this latter case the black mark was rather larger than in the others. After 24 h. from the application of the caustic, the curvature of three of these radicles (including the hooked one) had diminished; in the fourth it remained the same, and in the fifth it had increased, the tip being now hooked. It has been said that after 8 h. black specks could be seen on one side of the apex of five of the six radicles; on the sixth the speck, which was extremely minute, was on the actual apex and therefore central; and this radicle alone did not become curved. It was therefore again touched on one side with caustic, and after 15 h. 30 m. was found curved from the perpendicular and from the blackened side at an angle of 34°, which increased in nine additional hours to 54°.
It is therefore certain that the apex of the radicle of this Phaseolus is extremely sensitive to caustic, more so than that of the bean, though the latter is far more sensitive to pressure. In the experiments just given, the curvature from the slightly cauterised side of the tip, extended along the radicle for a length of nearly 10 mm.; whereas in the first set of experiments, when the tips of several were greatly blackened and injured on one side, so that their growth was arrested, a length of less than 3 mm. became curved towards the much blackened side, owing to the continued growth of the opposite side. This difference in the results is interesting, for it shows that too strong an irritant does not induce any transmitted effect, and does not cause the adjoining, upper and growing part of the radicle to bend. We have analogous cases with Drosera, for a strong solution of carbonate of ammonia when absorbed by the glands, or too great heat suddenly applied to them, or crushing them, does not cause the basal part of the tentacles to bend, whilst a weak solution of the carbonate, or a moderate heat, or slight pressure always induced such bending. Similar results were observed with Dionaea and Pinguicula.
The effect of cutting off with a razor a thin slice from one side of the conical apex of 14 young and short radicles was next tried. Six of them after being operated on were suspended in damp air; the tips of the other eight, similarly suspended, were allowed to enter water at a temperature of about 65° F. It was recorded in each case which side of the apex had been sliced off, and when they were afterwards examined the direction of the curvature was noted, before the record was consulted. Of the six radicles in damp air, three had their tips curved after an interval of 10 h. 15 m. directly away from the sliced surface, whilst the other three were not affected and remained straight; nevertheless, one of them after 13 additional hours became slightly curved from the sliced surface. Of the eight radicles with their tips immersed in water, seven were plainly curved away from the sliced surfaces after 10 h. 15 m.; and with respect to the eighth which remained quite straight, too thick a slice had been accidentally removed, so that it hardly formed a real exception to the general result. When the seven radicles were looked at again, after an interval of 23 h. from the time of slicing, two had become distorted; four were deflected at an angle of about 70° from the perpendicular and from the cut surface; and one was deflected at nearly 90°, so that it projected almost horizontally, but with the extreme tip now beginning to bend downwards through the action of geotropism. It is therefore manifest that a thin slice cut off one side of the conical apex, causes the upper growing part of the radicle of this Phaseolus to bend, through the transmitted effects of the irritation, away from the sliced surface.
Tropaeolum majus: Sensitiveness of the apex of the Radicle to contact.—Little squares of card were attached with shellac to one side of the tips of 19 radicles, some of which were subjected to 78° F., and others to a much lower temperature. Only 3 became plainly curved from the squares, 5 slightly, 4 doubtfully, and 7 not at all. These seeds were, as we believed, old, so we procured a fresh lot, and now the results were widely different. Twenty-three were tried in the same manner; five of the squares produced no effect, but three of these cases were no real exceptions, for in two of them the squares had slipped and were parallel to the apex, and in the third the shellac was in excess and had spread equally all round the apex. One radicle was deflected only slightly from the perpendicular and from the card; whilst seventeen were plainly deflected. The angles in several of these latter cases varied between 40° and 65° from the perpendicular; and in two of them it amounted after 15 h. or 16 h. to about 90°. In one instance a loop was nearly completed in 16 h. There can, therefore, be no doubt that the apex is highly sensitive to slight contact, and that the upper part of the radicle bends away from the touching object.
Gossypium herbaceum: Sensitiveness of the apex of the Radicle.—Radicles were experimented on in the same manner as before, but they proved ill-fitted for our purpose, as they soon became unhealthy when suspended in damp air. Of 38 radicles thus suspended, at temperatures varying from 66° to 69° F., with squares of card attached to their tips, 9 were plainly and 7 slightly or even doubtfully deflected from the squares and from the perpendicular; 22 not being affected. We thought that perhaps the above temperature was not high enough, so 19 radicles with attached squares, likewise suspended in damp air, were subjected to a temperature of from 74° to 79° F., but not one of them was acted on, and they soon became unhealthy. Lastly, 19 radicles were suspended in water at a temperature from 70° to 75° F., with bits of glass or squares of the card attached to their tips by means of Canada-balsam or asphalte, which adhered rather better than shellac beneath the water. The radicles did not keep healthy for long. The result was that 6 were plainly and 2 doubtfully deflected from the attached objects and the perpendicular; 11 not being affected. The evidence consequently is hardly conclusive, though from the two sets of cases tried under a moderate temperature, it is probable that the radicles are sensitive to contact; and would be more so under favourable conditions.
Fifteen radicles which had germinated in friable peat were suspended vertically over water. Seven of them served as controls, and they remained quite straight during 24 h. The tips of the other eight radicles were just touched with dry caustic on one side. After only 5 h. 10 m. five of them were slightly curved from the perpendicular and from the side bearing the little blackish marks. After 8 h. 40 m., 4 out of these 5 were deflected at angles between 15° and 65° from the perpendicular. On the other hand, one which had been slightly curved after 5 h. 10 m., now became straight. After 24 h. the curvature in two cases had considerably increased; also in four other cases, but these latter radicles had now become so contorted, some being turned upwards, that it could no longer be ascertained whether they were still curved from the cauterised side. The control specimens exhibited no such irregular growth, and the two sets presented a striking contrast. Out of the 8 radicles which had been touched with caustic, two alone were not affected, and the marks left on their tips by the caustic were extremely minute. These marks in all cases were oval or elongated; they were measured in three instances, and found to be of nearly the same size, viz. 2/3 of a mm. in length. Bearing this fact in mind, it should be observed that the length of the curved part of the radicle, which had become deflected from the cauterised side in the course of 8 h. 40 m. was found to be in three cases 6, 7, and 9 mm.
Cucurbita ovifera: Sensitiveness of the apex of the Radicle.—The tips proved ill-fitted for the attachment of cards, as they are extremely fine and flexible. Moreover, owing to the hypocotyls being soon developed and becoming arched, the whole radicle is quickly displaced and confusion is thus caused. A large number of trials were made, but without any definite result, excepting on two occasions, when out of 23 radicles 10 were deflected from the attached squares of card, and 13 were not acted on. Rather large squares, though difficult to affix, seemed more efficient than very small ones. We were much more successful with caustic; but in our first trial, 15 radicles were too much cauterised, and only two became curved from the blackened side; the others being either killed on one side, or blackened equally all round. In our next trial the dried tips of 11 radicles were touched momentarily with dry caustic, and after a few minutes were immersed in water. The elongated marks thus caused were never black, only brown, and about ½ mm. in length, or even less. In 4 h. 30 m. after the cauterisation, 6 of them were plainly curved from the side with the brown mark, 4 slightly, and 1 not at all. The latter proved unhealthy, and never grew; and the marks on 2 of the 4 slightly curved radicles were excessively minute, one being distinguishable only with the aid of a lens. Of 10 control specimens tried in the same jars at the same time, not one was in the least curved. In 8 h. 40 m. after the cauterisation, 5 of the radicles out of the 10 (the one unhealthy one being omitted) were deflected at about 90°, and 3 at about 45° from the perpendicular and from the side bearing the brown mark. After 24 h. all 10 radicles had increased immensely in length; in 5 of them the curvature was nearly the same, in 2 it had increased, and in 3 it had decreased. The contrast presented by the 10 controls, after both the 8 h. 40 m. and the 24 h. intervals, was very great; for they had continued to grow vertically downwards, excepting two which, from some unknown cause, had become somewhat tortuous.
In the chapter on Geotropism we shall see that 10 radicles of this plant were extended horizontally on and beneath damp friable peat, under which conditions they grow better and more naturally than in damp air; and their tips were slightly cauterised on the lower side, brown marks about ½ mm. in length being thus caused. Uncauterised specimens similarly placed became much bent downwards through geotropism in the course of 5 or 6 hours. After 8 h. only 3 of the cauterised ones were bowed downwards, and this in a slight degree; 4 remained horizontal; and 3 were curved upwards in opposition to geotropism and from the side bearing the brown mark. Ten other specimens had their tips cauterised at the same time and in the same degree, on the upper side; and this, if it produced any effect, would tend to increase the power of geotropism; and all these radicles were strongly bowed downwards after 8 h. From the several foregoing facts, there can be no doubt that the cauterisation of the tip of the radicle of this Cucurbita on one side, if done lightly enough, causes the whole growing part to bend to the opposite side. Raphanus sativus: Sensitiveness of the apex of the Radicle.—We here encountered many difficulties in our trials, both with squares of card and with caustic; for when seeds were pinned to a cork-lid, many of the radicles, to which nothing had been done, grew irregularly, often curving upwards, as if attracted by the damp surface above; and when they were immersed in water they likewise often grew irregularly. We did not therefore dare to trust our experiments with attached squares of card; nevertheless some of them seemed to indicate that the tips were sensitive to contact. Our trials with caustic generally failed from the difficulty of not injuring too greatly the extremely fine tips. Out of 7 radicles thus tried, one became bowed after 22 h. at an angle of 60°, a second at 40°, and a third very slightly from the perpendicular and from the cauterised side.
(1.) The radicle was slightly deflected from the cauterised side in one day (i.e. 24 h.); in three days it stood at 60° from the perpendicular; in four days at 90°; on the fifth day it was curved up about 40° above the horizon; so that it had passed through an angle of 130° in the five days, and this was the greatest amount of curvature observed. (2.) In two days radicle slightly deflected; after seven days deflected 69° from the perpendicular and from the cauterised side; after eight days the angle amounted to nearly 90°. (3.) After one day slight deflection, but the cauterised mark was so faint that the same side was again touched with caustic. In four days from the first touch deflection amounted to 78°, which in an additional day increased to 90°. (4.) After two days slight deflection, which during the next three days certainly increased but never became great; the radicle did not grow well and died on the eighth day. (5.) After two days very slight deflection; but this on the fourth day amounted to 56° from the perpendicular and from the cauterised side. (6.) After three days doubtfully, but after four days certainly deflected from the cauterised side. On the fifth day deflection amounted to 45° from the perpendicular, and this on the seventh day increased to about 90°.
(7.) After two days slightly deflected; on the third day the deflection amounted to 25° from the perpendicular, and this did not afterwards increase. (8.) After one day deflection distinct; on the third day it amounted to 44°, and on the fourth day to 72° from the perpendicular and the cauterised side. (9.) After two days deflection slight, yet distinct; on the third day the tip was again touched on the same side with caustic and thus killed. (10.) After one day slight deflection, which after six days increased to 50° from the perpendicular and the cauterised side. (11.) After one day decided deflection, which after six days increased to 62° from the perpendicular and from the cauterised side. (12.) After one day slight deflection, which on the second day amounted to 35°, on the fourth day to 50°, and the sixth day to 63° from the perpendicular and the cauterised side. (13.) Whole tip blackened, but more on one side than the other; on the fourth day slightly, and on the sixth day greatly deflected from the more blackened side; the deflection on the ninth day amounted to 90° from the perpendicular. (14.) Whole tip blackened in the same manner as in the last case: on the second day decided deflection from the more blackened side, which increased on the seventh day to nearly 90°; on the following day the radicle appeared unhealthy.
(15.) Here we had the anomalous case of a radicle bending slightly _towards_ the cauterised side on the first day, and continuing to do so for the next three days, when the deflection amounted to about 90° from the perpendicular. The cause appeared to lie in the tendril-like sensitiveness of the upper part of the radicle, against which the point of a large triangular flap of the seed-coats pressed with considerable force; and this irritation apparently conquered that from the cauterised apex. These several cases show beyond doubt that the irritation of one side of the apex, excites the upper part of the radicle to bend slowly towards the opposite side. This fact was well exhibited in one lot of five seeds pinned to the cork-lid of a jar; for when after 6 days the lid was turned upside down and viewed from directly above, the little black marks made by the caustic were now all distinctly visible on the upper sides of the tips of the laterally bowed radicles. A thin slice was shaved off with a razor from one side of the tips of 22 radicles, in the manner described under the common bean; but this kind of irritation did not prove very effective. Only 7 out of the 22 radicles became moderately deflected in from 3 to 5 days from the sliced surface, and several of the others grew irregularly. The evidence, therefore, is far from conclusive.
Quercus robur: Sensitiveness of the apex of the Radicle.—The tips of the radicles of the common oak are fully as sensitive to slight contact as are those of any plant examined by us. They remained healthy in damp air for 10 days, but grew slowly. Squares of the card-like paper were fixed with shellac to the tips of 15 radicles, and ten of these became conspicuously bowed from the perpendicular and from the squares; two slightly, and three not at all. But two of the latter were not real exceptions, as they were at first very short, and hardly grew afterwards. Some of the more remarkable cases are worth describing. The radicles were examined on each successive morning, at nearly the same hour, that is, after intervals of 24 h. No. 1. This radicle suffered from a series of accidents, and acted in an anomalous manner, for the apex appeared at first insensible and afterwards sensitive to contact. The first square was attached on Oct 19th; on the 21st the radicle was not at all curved, and the square was accidentally knocked off; it was refixed on the 22nd, and the radicle became slightly curved from the square, but the curvature disappeared on the 23rd, when the square was removed and refixed. No curvature ensued, and the square was again accidentally knocked off, and refixed. On the morning of the 27th it was washed off by having reached the water in the bottom of the jar. The square was refixed, and on the 29th, that is, ten days after the first square had been attached, and two days after the attachment of the last square, the radicle had grown to the great length of 3.2 inches, and now the terminal growing part had become bent away from the square into a hook (see Fig. 68).
Fig. 68. Quercus robur: radicle with square of card attached to one side of apex, causing it to become hooked. Drawing one-half natural scale. No. 2. Square attached on the 19th; on the 20th radicle slightly deflected from it and from the perpendicular; on the 21st deflected at nearly right angles; it remained during the next two days in this position, but on the 25th the upward curvature was lessened through the action of geotropism, and still more so on the 26th. No. 3. Square attached on the 19th; on the 21st a trace of curvature from the square, which amounted on the 22nd to about 40°, and on the 23rd to 53° from the perpendicular. No. 4. Square attached on the 21st; on the 22nd trace of curvature from the square; on the 23rd completely hooked with the point turned up to the zenith. Three days afterwards (i.e. 26th) the curvature had wholly disappeared and the apex pointed perpendicularly downwards. No. 5. Square attached on the 21st; on the 22nd decided though slight curvature from the square; on the 23rd the tip had curved up above the horizon, and on the 24th was hooked with the apex pointing almost to the zenith, as in Fig. 68. No. 6. Square attached on the 21st; on the 22nd slightly curved from the square; 23rd more curved; 25th considerably curved; 27th all curvature lost, and the radicle was now directed perpendicularly downwards.
No. 7. Square attached on the 21st; on the 22nd a trace of curvature from the square, which increased next day, and on the 24th amounted to a right angle. It is, therefore, manifest that the apex of the radicle of the oak is highly sensitive to contact, and retains its sensitiveness during several days. The movement thus induced was, however, slower than in any of the previous cases, with the exception of that of Æsculus. As with the bean, the terminal growing part, after bending, sometimes straightened itself through the action of geotropism, although the object still remained attached to the tip. The same remarkable experiment was next tried, as in the case of the bean; namely, little squares of exactly the same size of the card-like sanded paper and of very thin paper (the thicknesses of which have been given under Vicia faba) were attached with shellac on opposite sides (as accurately as could be done) of the tips of 13 radicles, suspended in damp air, at a temperature of 65°–66° F. The result was striking, for 9 out of these 13 radicles became plainly, and 1 very slightly, curved from the thick paper towards the side bearing the thin paper. In two of these cases the apex became completely hooked after two days; in four cases the deflection from the perpendicular and from the side bearing the thick paper, amounted in from two to four days to angles of 90°, 72°, 60°, and 49°, but in two other cases to only 18° and 15°. It should, however, be stated that in the case in which the deflection was 49°, the two squares had accidentally come into contact on one side of the apex, and thus formed a lateral gable; and the deflection was directed in part from this gable and in part from the thick paper. In three cases alone the radicles were not affected by the difference in thickness of the squares of paper attached to their tips, and consequently did not bend away from the side bearing the stiffer paper.
Text from Project Gutenberg, public domain in the United States.