The Power of Movement in Plants · Charles Darwin
CHAPTER I. THE CIRCUMNUTATING MOVEMENTS OF SEEDLING PLANTS.
Some preliminary trials were made, presently to be described, by which the proper temperature was determined, and then the following experiments were made. It should be premised that the beans were always fixed to the cork-lids, for the convenience of manipulation, with the edge from which the radicle and plumule protrudes, outwards; and it must be remembered that owing to what we have called Sachs’ curvature, the radicles, instead of growing perpendicularly downwards, often bend somewhat, even as much as about 45° inwards, or under the suspended bean. Therefore when a square of card was fixed to the apex in front, the bowing induced by it coincided with Sachs’ curvature, and could be distinguished from it only by being more strongly pronounced or by occurring more quickly. To avoid this source of doubt, the squares were fixed either behind, causing a curvature in direct opposition to that of Sachs’, or more commonly to the right or left sides. For the sake of brevity, we will speak of the bits of card, etc., as fixed in front, or behind, or laterally. As the chief curvature of the radicle is at a little distance from the apex, and as the extreme terminal and basal portions are nearly straight, it is possible to estimate in a rough manner the amount of curvature by an angle; and when it is said that the radicle became deflected at any angle from the perpendicular, this implies that the apex was turned upwards by so many degrees from the downward direction which it would naturally have followed, and to the side opposite to that to which the card was affixed. That the reader may have a clear idea of the kind of movement excited by the bits of attached card, we append here accurate sketches of three germinating beans thus treated, and selected out of several specimens to show the gradations in the degrees of curvature. We will now give in detail a series of experiments, and afterwards a summary of the results.
Fig. 65. Vicia faba: A, radicle beginning to bend from the attached little square of card; B, bent at a rectangle; C, bent into a circle or loop, with the tip beginning to bend downwards through the action of geotropism. In the first 12 trials, little squares or oblongs of sanded card, 1.8 mm. in length, and 1.5 or only 0.9 mm. in breadth (i.e. .071 of an inch in length and .059 or .035 of an inch in breadth) were fixed with shellac to the tips of the radicles. In the subsequent trials the little squares were only occasionally measured, but were of about the same size. (1.) A young radicle, 4 mm. in length, had a card fixed behind: after 9 h. deflected in the plane in which the bean is flattened, 50° from the perpendicular and from the card, and in opposition to Sachs’ curvature: no change next morning, 23 h. from the time of attachment. (2.) Radicle 5.5 mm. in length, card fixed behind: after 9 h. deflected in the plane of the bean 20° from the perpendicular and from the card, and in opposition to Sachs’ curvature: after 23 h. no change. (3.) Radicle 11 mm. in length, card fixed behind: after 9 h. deflected in the plane of the bean 40° from the perpendicular and from the card, and in opposition to Sachs’ curvature. The tip of the radicle more curved than the upper part, but in the same plane. After 23 h. the extreme tip was slightly bent towards the card; the general curvature of the radicle remaining the same.
(4.) Radicle 9 mm. long, card fixed behind and a little laterally: after 9 h. deflected in the plane of the bean only about 7° or 8° from the perpendicular and from the card, in opposition to Sachs’ curvature. There was in addition a slight lateral curvature directed partly from the card. After 23 h. no change. (5.) Radicle 8 mm. long, card affixed almost laterally: after 9 h. deflected 30° from the perpendicular, in the plane of the bean and in opposition to Sachs’ curvature; also deflected in a plane at right angles to the above one, 20° from the perpendicular: after 23 h. no change. (6.) Radicle 9 mm. long, card affixed in front: after 9 h. deflected in the plane of the bean about 40° from the vertical, away from the card and in the direction of Sachs’ curvature. Here therefore we have no evidence of the card being the cause of the deflection, except that a radicle never moves spontaneously, as far as we have seen, as much as 40° in the course of 9 h. After 23 h. no change. (7.) Radicle 7 mm. long, card affixed to the back: after 9 h. the terminal part of the radicle deflected in the plane of the bean 20° from the vertical, away from the card and in opposition to Sachs’ curvature. After 22 h. 30 m. this part of the radicle had become straight.
(8.) Radicle 12 mm. long, card affixed almost laterally: after 9 h. deflected laterally in a plane at right angles to that of the bean between 40° and 50° from the vertical and from the card. In the plane of the bean itself the deflection amounted to 8° or 9° from the vertical and from the card, in opposition to Sachs’ curvature. After 22 h. 30 m. the extreme tip had become slightly curved towards the card. (9.) Card fixed laterally: after 11 h. 30 m. no effect, the radicle being still almost vertical. (10.) Card fixed almost laterally: after 11 h. 30 m. deflected 90° from the vertical and from the card, in a plane intermediate between that of the bean itself and one at right angles to it. Radicle consequently partially deflected from Sachs’ curvature. (11.) Tip of radicle protected with goldbeaters’ skin, with a square of card of the usual dimensions affixed with shellac: after 11 h. greatly deflected in the plane of the bean, in the direction of Sachs’ curvature, but to a much greater degree and in less time than ever occurs spontaneously. (12.) Tip of radicle protected as in last case: after 11 h. no effect, but after 24 h. 40 m. radicle clearly deflected from the card. This slow action was probably due to a portion of the goldbeaters’ skin having curled round and lightly touched the opposite side of the tip and thus irritated it.
(13.) A radicle of considerable length had a small square of card fixed with shellac to its apex laterally: after only 7 h. 15 m. a length of .4 of an inch from the apex, measured along the middle, was considerably curved from the side bearing the card. (14.) Case like the last in all respects, except that a length of only .25 of an inch of the radicle was thus deflected. (15.) A small square of card fixed with shellac to the apex of a young radicle; after 9 h. 15 m. deflected through 90° from the perpendicular and from the card. After 24 h. deflection much decreased, and after an additional day, reduced to 23° from the perpendicular. (16.) Square of card fixed with shellac behind the apex of a radicle, which from its position having been changed during growth had become very crooked; but the terminal portion was straight, and this became deflected to about 45° from the perpendicular and from the card, in opposition to Sachs’ curvature. (17.) Square of card affixed with shellac: after 8 h. radicle curved at right angles from the perpendicular and from the card. After 15 additional hours curvature much decreased. (18.) Square of card affixed with shellac: after 8 h. no effect; after 23 h. 3 m. from time of affixing, radicle much curved from the square. (19.) Square of card affixed with shellac: after 24 h. no effect, but the radicle had not grown well and seemed sickly.
(20.) Square of card affixed with shellac: after 24 h. no effect. (21, 22.) Squares of card affixed with shellac: after 24 h. radicles of both curved at about 45° from the perpendicular and from the cards. (23.) Square of card fixed with shellac to young radicle: after 9 h. very slightly curved from the card; after 24 h. tip curved towards card. Refixed new square laterally, after 9 h. distinctly curved from the card, and after 24 h. curved at right angles from the perpendicular and from the card. (24.) A rather large oblong piece of card fixed with shellac to apex: after 24 h. no effect, but the card was found not to be touching the apex. A small square was now refixed with shellac; after 16 h. slight deflection from the perpendicular and from the card. After an additional day the radicle became almost straight. (25.) Square of card fixed laterally to apex of young radicle; after 9 h. deflection from the perpendicular considerable; after 24 h. deflection reduced. Refixed a fresh square with shellac: after 24 h. deflection about 40° from the perpendicular and from the card. (26.) A very small square of card fixed with shellac to apex of young radicle: after 9 h. the deflection from the perpendicular and from the card amounted to nearly a right angle; after 24 h. deflection much reduced; after an additional 24 h. radicle almost straight.
(27.) Square of card fixed with shellac to apex of young radicle: after 9 h. deflection from the card and from the perpendicular a right angle; next morning quite straight. Refixed a square laterally with shellac; after 9 h. a little deflection, which after 24 h. increased to nearly 20° from the perpendicular and from the card. (28.) Square of card fixed with shellac; after 9 h. some deflection; next morning the card dropped off; refixed it with shellac; it again became loose and was refixed; and now on the third trial the radicle was deflected after 14 h. at right angles from the card. (29.) A small square of card was first fixed with thick gum-water to the apex. It produced a slight effect but soon fell off. A similar square was now affixed laterally with shellac: after 9 h. the radicle was deflected nearly 45° from the perpendicular and from the card. After 36 additional hours angle of deflection reduced to about 30°. (30.) A very small piece, less than 1/20th of an inch square, of thin tin-foil fixed with shellac to the apex of a young radicle; after 24 h. no effect. Tin-foil removed, and a small square of sanded card fixed with shellac; after 9 h. deflection at nearly right angles from the perpendicular and from the card. Next morning deflection reduced to about 40° from the perpendicular.
(31.) A splinter of thin glass gummed to apex, after 9 h. no effect, but it was then found not to be touching the apex of the radicle. Next morning a square of card was fixed with shellac to it, and after 9 h. radicle greatly deflected from the card. After two additional days the deflection had decreased and was only 35° from the perpendicular. (32.) Small square of sanded card, attached with thick gum-water laterally to the apex of a long straight radicle: after 9 h. greatly deflected from the perpendicular and from the card. Curvature extended for a length of .22 of an inch from the apex. After 3 additional hours terminal portion deflected at right angles from the perpendicular. Next morning the curved portion was .36 in length. (33.) Square of card gummed to apex: after 15 h. deflected at nearly 90° from the perpendicular and from the card. (34.) Small oblong of sanded card gummed to apex: after 15 h. deflected 90° from the perpendicular and from the card: in the course of the three following days the terminal portion became much contorted and ultimately coiled into a helix. (35.) Square of card gummed to apex: after 9 h. deflected from card: after 24 h. from time of attachment greatly deflected obliquely and partly in opposition to Sachs’ curvature.
(36.) Small piece of card, rather less than 1/20th of an inch square, gummed to apex: in 9 h. considerably deflected from card and in opposition to Sachs’ curvature; after 24 h. greatly deflected in the same direction. After an additional day the extreme tip was curved towards the card. (37.) Square of card, gummed to apex in front, caused after 8 h. 30 m. hardly any effect; refixed fresh square laterally, after 15 h. deflected almost 90° from the perpendicular and from the card. After 2 additional days deflection much reduced. (38.) Square of card gummed to apex: after 9 h. much deflection, which after 24 h. from time of fixing increased to nearly 90°. After an additional day terminal portion was curled into a loop, and on the following day into a helix. (39.) Small oblong piece of card gummed to apex, nearly in front, but a little to one side; in 9 h. slightly deflected in the direction of Sachs’ curvature, but rather obliquely, and to side opposite to card. Next day more curved in the same direction, and after 2 additional days coiled into a ring. (40.) Square of card gummed to apex: after 9 h. slightly curved from card; next morning radicle straight, and apex had grown beyond the card. Refixed another square laterally with shellac; in 9 h. deflected laterally, but also in the direction of Sachs’ curvature. After 2 additional days’ curvature considerably increased in the same direction.
(41.) Little square of tin-foil fixed with gum to one side of apex of a young and short radicle: after 15 h. no effect, but tin-foil had become displaced. A little square of card was now gummed to one side of apex, which after 8 h. 40 m. was slightly deflected; in 24 h. from the time of attachment deflected at 90° from the perpendicular and from the card; after 9 additional hours became hooked, with the apex pointing to the zenith. In 3 days from the time of attachment the terminal portion of the radicle formed a ring or circle. (42.) A little square of thick letter-paper gummed to the apex of a radicle, which after 9 h. was deflected from it. In 24 h. from time when the paper was affixed the deflection much increased, and after 2 additional days it amounted to 50° from the perpendicular and from the paper. (43.) A narrow chip of a quill was fixed with shellac to the apex of a radicle. After 9 h. no effect; after 24 h. moderate deflection, but now the quill had ceased to touch the apex. Removed quill and gummed a little square of card to apex, which after 8 h. caused slight deflection. On the fourth day from the first attachment of any object, the extreme tip was curved towards the card.
(44.) A rather long and narrow splinter of extremely thin glass, fixed with shellac to apex, it caused in 9 h. slight deflection, which disappeared in 24 h.; the splinter was then found not touching the apex. It was twice refixed, with nearly similar results, that is, it caused slight deflection, which soon disappeared. On the fourth day from the time of first attachment the tip was bent towards the splinter. From these experiments it is clear that the apex of the radicle of the bean is sensitive to contact, and that it causes the upper part to bend away from the touching object. But before giving a summary of the results, it will be convenient briefly to give a few other observations. Bits of very thin glass and little squares of common card were affixed with thick gum-water to the tips of the radicles of seven beans, as a preliminary trial. Six of these were plainly acted on, and in two cases the radicles became coiled up into complete loops. One radicle was curved into a semi-circle in so short a period as 6 h. 10 m. The seventh radicle which was not affected was apparently sickly, as it became brown on the following day; so that it formed no real exception. Some of these trials were made in the early spring during cold weather in a sitting-room, and others in a greenhouse, but the temperature was not recorded. These six striking cases almost convinced us that the apex was sensitive, but of course we determined to make many more trials.
As we had noticed that the radicles grew much more quickly when subjected to considerable heat, and as we imagined that heat would increase their sensitiveness, vessels with germinating beans suspended in damp air were placed on a chimney-piece, where they were subjected during the greater part of the day to a temperature of between 69° and 72° F.; some, however, were placed in the hot-house where the temperature was rather higher. Above two dozen beans were thus tried; and when a square of glass or card did not act, it was removed, and a fresh one affixed, this being often done thrice to the same radicle. Therefore between five and six dozen trials were altogether made. But there was moderately distinct deflection from the perpendicular and from the attached object in only one radicle out of this large number of cases. In five other cases there was very slight and doubtful deflection. We were astonished at this result, and concluded that we had made some inexplicable mistake in the first six experiments.
But before finally relinquishing the subject, we resolved to make one other trial for it occurred to us that sensitiveness is easily affected by external conditions, and that radicles growing naturally in the earth in the early spring would not be subjected to a temperature nearly so high as 70° F. We therefore allowed the radicles of 12 beans to grow at a temperature of between 55° and 60° F. The result was that in every one of these cases (included in the above-described experiments) the radicle was deflected in the course of a few hours from the attached object. All the above recorded successful trials, and some others presently to be given, were made in a sitting-room at the temperatures just specified. It therefore appears that a temperature of about, or rather above, 70° F. destroys the sensitiveness of the radicles, either directly, or indirectly through abnormally accelerated growth; and this curious fact probably explains why Sachs, who expressly states that his beans were kept at a high temperature, failed to detect the sensitiveness of the apex of the radicle.
But other causes interfere with this sensibility. Eighteen radicles were tried with little squares of sanded card, some affixed with shellac and some with gum-water, during the few last days of 1878, and few first days of the next year. They were kept in a room at the proper temperature during the day, but were probably too cold at night, as there was a hard frost at the time. The radicles looked healthy but grew very slowly. The result was that only 6 out of the 18 were deflected from the attached cards, and this only to a slight degree and at a very slow rate. These radicles therefore presented a striking contrast with the 44 above described. On March 6th and 7th, when the temperature of the room varied between 53° and 59° F., eleven germinating beans were tried in the same manner, and now every one of the radicles became curved away from the cards, though one was only slightly deflected. Some horticulturists believe that certain kinds of seeds will not germinate properly in the middle of the winter, although kept at a right temperature.
If there really is any proper period for the germination of the bean, the feeble degree of sensibility of the above radicles may have resulted from the trial having been made in the middle of the winter, and not simply from the nights being too cold. Lastly, the radicles of four beans, which from some innate cause germinated later than all the others of the same lot, and which grew slowly though appearing healthy, were similarly tried, and even after 24 h. they were hardly at all deflected from the attached cards. We may therefore infer that any cause which renders the growth of the radicles either slower or more rapid than the normal rate, lessens or annuls the sensibility of their tips to contact. It deserves particular attention that when the attached objects failed to act, there was no bending of any kind, excepting Sachs’ curvature. The force of our evidence would have been greatly weakened if occasionally, though rarely, the radicles had become curved in any direction independently of the attached objects. In the foregoing numbered paragraphs, however, it may be observed that the extreme tip sometimes becomes, after a considerable interval of time, abruptly curved towards the bit of card; but this is a totally distinct phenomenon, as will presently be explained.
A Summary of the Results of the foregoing Experiments on the Radicles of Vicia faba.—Altogether little squares (about 1/20th of an inch), generally of sanded paper as stiff as thin card (between .15 and .20 mm. in thickness), sometimes of ordinary card, or little fragments of very thin glass etc., were affixed at different times to one side of the conical tips of 55 radicles. The 11 last-mentioned cases, but not the preliminary ones, are here included. The squares, etc., were most commonly affixed with shellac, but in 19 cases with thick gum-water. When the latter was used, the squares were sometimes found, as previously stated, to be separated from the apex by a layer of thick fluid, so that there was no contact, and consequently no bending of the radicle; and such few cases were not recorded. But in every instance in which shellac was employed, unless the square fell off very soon, the result was recorded. In several instances when the squares became displaced, so as to stand parallel to the radicle, or were separated by fluid from the apex, or soon fell off, fresh squares were attached, and these cases (described under the numbered paragraphs) are here included.
Out of 55 radicles experimented on under the proper temperature, 52 became bent, generally to a considerable extent from the perpendicular, and away from the side to which the object was attached. Of the three failures, one can be accounted for, as the radicle became sickly on the following day; and a second was observed only during 11 h. 30 m. As in several cases the terminal growing part of the radicle continued for some time to bend from the attached object, it formed itself into a hook, with the apex pointing to the zenith, or even into a ring, and occasionally into a spire or helix. It is remarkable that these latter cases occurred more frequently when objects were attached with thick gum-water, which never became dry, than when shellac was employed. The curvature was often well-marked in from 7 h. to 11 h.; and in one instance a semicircle was formed in 6 h. 10 m, from the time of attachment. But in order to see the phenomenon as well displayed as in the above described cases, it is indispensable that the bits of card, etc., should be made to adhere closely to one side of the conical apex; that healthy radicles should be selected and kept at not too high or too low a temperature, and apparently that the trials should not be made in the middle of the winter.
In ten instances, radicles which had curved away from a square of card or other object attached to their tips, straightened themselves to a certain extent, or even completely, in the course of from one to two days from the time of attachment. This was more especially apt to occur when the curvature was slight. But in one instance (No. 27) a radicle which in 9 h. had been deflected about 90° from the perpendicular, became quite straight in 24 h. from the period of attachment. With No. 26, the radicle was almost straight in 48 h. We at first attributed the straightening process to the radicles becoming accustomed to a slight stimulus, in the same manner as a tendril or sensitive petiole becomes accustomed to a very light loop of thread, and unbends itself though the loop remains still suspended; but Sachs states[2] that radicles of the bean placed horizontally in damp air after curving downwards through geotropism, straighten themselves a little by growth along their lower or concave sides. Why this should occur is not clear: but perhaps it likewise occurred in the above ten cases. There is another occasional movement which must not be passed over: the tip of the radicle, for a length of from 2 to 3 mm., was found in six instances, after an interval of about 24 or more hours, bent towards the bit of still attached card,—that is, in a direction exactly opposite to the previously induced curvature of the whole growing part for a length of from 7 to 8 mm. This occurred chiefly when the first curvature was small, and when an object had been affixed more than once to the apex of the same radicle. The attachment of a bit of card by shellac to one side of the tender apex may sometimes mechanically prevent its growth; or the application of thick gum-water more than once to the same side may injure it; and then checked growth on this side with continued growth on the opposite and unaffected side would account for the reversed curvature of the apex.
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