The Power of Movement in Plants

Manner in which radicles bend when they encounter an obstacle in the soil—Vicia faba, tips of radicles highly sensitive to contact and other irritants—Effects of too high a temperature—Power of discriminating between objects attached on opposite sides—Tips of secondary radicles sensitive—Pisum, tips of radicles sensitive— Effects of such sensitiveness in overcoming geotropism—Secondary radicles—Phaseolus, tips of radicles hardly sensitive to contact but highly sensitive to caustic and to the removal of a slice—Tro- peolum— Gossy pium—Cucurbita — Raphanus—Aisculus, tip not sensitive to slight contact, highly sensitive to caustic—Quercus, tip highly sensitive to contact—Power of discrimination—Zea tip highly sensitive, secondary radicles—Sensitiveness of radicles to moist air—Summary of chapter.

In order to see how the radicles of seedlings would pass over stones, roots, and other obstacles, which they must incessantly encounter in the soil, germinating beans (Vicia faba) were so placed that the tips of the radicles came into contact, almost rectangularly or at a high angle, with underlying plates of glass. In other cases the beans were turned about whilst their radicles were growing, so that they descended nearly vertically on their own smooth, almost flat, broad upper surfaces. The delicate root-cap, when it first touched any directly opposing surface, was a little flattened transversely; the flattening soon became oblique, and in a few hours quite disappeared, the apex now point- ing at right angles, or at nearly right angles, to its former course. The radicle then seemed to glide in its new direction over the surface which had opposed

it, pressing on it with very little foree. How far such abrupt changes in its former course are aided by the circumnutation of the tip must be left doubtful. Thin slips of wood were cemented on more or less steeply inclined glass-plates, at right angles to the radicles which were gliding down them. Straight lines had been painted along the growing terminal part of some of these radicles, before they met the opposing slip of wood; and the lines became sensibly curved in 2 h. after the apex had come into contact with the slips. In one case of a radicle, which was growing rather slowly, the root-cap, after encountering a rough slip of wood at right angles, was at first slightly flat- tened transversely: .after an interval of 2 h. 30 m. the flattening became oblique; and after an addi- tional 3 hours the flattening had wholly disappeared, and the apex now pointed at right angles to its former course. It then continued to grow in its new direc- tion alongside the slip of wood, until it came to the end of it, round which it bent rectangularly. Soon afterwards when coming to the edge of the plate of glass, it was again bent at a large angle, and de- scended perpendicularly into the damp sand.

When, as in the above cases, radicles encountered an obstacle at right angles to their course, the terminal growing part became curved for a length of between -3 and °4 of an inch (8-10 mm.), measured from the apex. ‘This was well shown by the black lines which had been previously painted on them. The first and most obvious explanation of the curvature is, that it results merely from the mechanical resistance to the growth of the radicle in its original direction. Never- theless, this explanation did not seem to us satisfactory. The radicles did not present the appearance of having been subjected to a sufficient pressure to account for

their curvature; and Sachs has shown* that the growing part is more rigid than the part immediately above which has ceased to grow, so that the latter might have been expected to yield and become curved as soon as the apex encountered an unyielding object ; whereas it was the stiff growing part which became curved. Moreover, an object which yields with the greatest ease will deflect a radicle: thus, as we have seen, when the apex of the radicle of the bean encountered the polished surface of extremely thin tin-foil laid on soft sand, no impression was left on it yet the radicle became deflected at right angles. A second explanation occurred to us, namely, that even the gentlest pressure might check the growth of the apex, and in this case growth could continue only on one side, and thus the radicle would assume a rectan- gular form; but this view leaves wholly unexplained the curvature of the upper part, extending for a length of 8-10 mm.

We were therefore led to suspect that the apex was sensitive to contact, and that an effect was trans- mitted from it to the upper part of the radicle, which was thus excited to bend away from the touching object. As a little loop of fine thread hung on a tendril or on the petiole of a leaf-climbing plant, causes it to bend, we thought that any small hard object affixed to the tip of a radicle, freely suspended and growing in damp air, might cause it to bend, if it were sensitive, and yet would not offer any mechanical resistance to its growth. Full details will be given of the experi: ments which were tried, as the result proved remark- able. The fact of the apex of a radicle being sensitive to contact has never been observed, though, as we shall

hereafter see, Sachs discovered that the radicle a little above the apex is sensitive, and bends like a tendril towards the touching object. But when one side of the apex is pressed by any object, the growing part bends away from the object; and this seems a beautiful adaptation for avoiding obstacles in the soil, and, as we shall see, for following the lines of least resistance. Many organs, when touched, bend in one fixed direc- tion, such as the stamens of Berberis, the lobes of Dionwa, &c.; and many organs, such as tendrils, whe- ther modified leaves or flower-peduncles, and some few stems, bend towards a touching object; but no case, we believe, is known of an organ bending away from a touching object.

Sensitiveness of the Apex of the Radicle of Vicia faba. —Common beans, after being soaked in water for 24h., were pinned with the hilum downwards (in the manner followed by Sachs), inside the cork lids of glass-vessels, which were half filled with water; the sides and the cork were well moistened, and light was excluded. As soon as the beans had protruded radicles, some to a length of less than a tenth of an inch, and others to a length of several tenths, little squares or oblongs of ecard were affixed to the short sloping sides of their conical tips. The squares therefore adhered obliquely with reference to the longitudinal axis of the radicle; and this is a very necessary precaution, for if the bits of card accidentally became displaced, or were drawn by the viscid matter employed, so as to adhere parallel to the side of the radicle, although only a little way above the conical apex, the radicle did not bend in the peculiar manner which we are here considering. Squares of about the syth of an inch (i.e. about 14 mm.), or oblong bits of nearly the same size, were found to

be the most convenient and effective. We employed at first ordinary thin card, such as visiting cards, or bits of very thin glass, and various other objects; but afterwards sand-paper was chiefly employed, for it was almost as stiff as thin card, and the roughened surface favoured its adhesion. At first we generally used very thick gum-water; and this of course, under the cir- cumstances, never dried in the least; on the contrary, it sometimes seemed to absorb vapour, so that the bits of card became separated by a layer of fluid from the tip. When there was no such absorption and the card was not displaced, it acted well and caused the radicle to bend to the opposite side. I should state that thick gum-water by itself induces no action. In most cases the bits of card were touched with an extremely small quantity of a solution of shellac in spirits of wine, which had been left to evaporate until it was thick; it then set hard in a few seconds, and fixed the bits of card well. When small drops of the shellac were placed on the tips without any card, they set into hard little beads, and these acted like any other hard object, causing the radicles to bend to the opposite side. Even extremely minute beads of the shellac occasionally acted in a slight degree, as will hereafter be described. But that it was the cards which chiefly acted in our many trials, was proved by coating one side of the tip with a little bit of goldbeaters’ skin (which by itself hardly acts), and then fixing a bit of card to the skin with shellac which never came into contact with the radicle: nevertheless the radicle bent away from the attached card in the ordinary manner. Some preliminary trials were made, presently to be described, by which the proper temperature was determined, and then the following experiments were made. Jt 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 perpendi- cularly downwards, often bend somewhat, even as much 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.

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, &e., 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 down- ward 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 ger- minating 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.

In the first 12 trials, little squares or oblongs of sanded card, 18 mm. in length, gnd 15 or only 0°9 mm. in breadth (i.e. 071 ot 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 Jength, had a card fixed be- hind: 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, 23h. from the time of attachment.

(2.) Radicle 55 mm. in length, card fixed behind: after 9h. deflected in the plane of the bean 20° from the perpendicular and from the card, and in opposition to Sachs’ curvature: after 23h. no change. (3.) Radicle 11 mm. in length, card fixed behind: after 9h, 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 game plane. After 23h. the extreme tip was slightly bent to- wards the card; the general curvature of the radicle remaining the same.

(4.) Radicle 9 mm. long, card fixed behind and a little laterally: after 9h. 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 23h. no change. (5.) Radicle 8 mm. long, card affixed almost laterally: after 9h. 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 perpen- dicular: after 23h. no change. '

(6.) Radicle 9 mm. long, card affixed in front: after 9h. de- flected 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 4U° in the course of 9h. After 23h. no change. (7.) Radicle 7 mm. long, card affixed to the back: after 9h. 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 22h. 30 m. this pat of the radicle had become straight.

(8.) Radicle 12 mm. long, card affixed almost laterally: after 9h. deflected latera!ly 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 &° or 9° from the vertical aud from the card, in opposition to Sachs’ curvature. After 22h. 30m. the extreme tip had become slightly curved towards the card. (9.) Card fixed laterally: after 11h. 80m. no effect, the raclicle being still almost vertical.

(10.) Card fixed almost laterally: after 11h. 30m. deflected 90° from the vertical and from the card, in a plane inter- mediate 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 11h. greatly deflected in the plane of the bean, in the direction of Sachs’ curvature, but to a much greater degree and in Jess time than ever occurs spontaneously.

(12.) Tip of radicle protected as in last case: after 11h. no effect, but after 24h. 40m. 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. (18.) A radicle of considerable length had a small square of card fixed with shellac to its apex laterally: after only 7h.15m. 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 9h. 15 m. deflected through 90° from the perpendicular and from the card. After 24h. 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 heen 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 8h. no effect; after 23h. 3m. from time of affixing, radicle much curved from the square. (19.) Square of card affixed with shellac: after 24h. no effect, but the radicle had not grown well and seemed sickly: (20.) Square of card affixed with shellac: after 24h. no effect.

(21, 22.) Squares of card affixed with shellac: after 24h. 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 9h. very slightly curved from the card; after 24h. tip curved towards card. Refixed new square laterally, atter 9h. distinctly curved from the card, and after 24h. curved at right angles frcm the perpendicular and from the card. (24.) A rather large oblong piece of card fixed with shellac te apex: after ¥4h. 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 9h. deflection from the perpendicular considerable; after 24h. deflection reduced. Refixed a fresh square with shellac: after 24h. 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 9h. the deflection from the perpendicular and from the card amounted to nearly a right angle; after 24h. 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 perpen- dicular a right angle; next morning quite straight. Refixed a square laterally with shellac; after 9h. a little deflection, which after 24h. increased to nearly 20° from the perpendicular and from the card. (23.) 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 14h. 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 9h. the radicle was deflected nearly 45° from the perpen- dicular and from the card. Alter 36 additional hours angle of deflection reduced to about 30°. (30.) A very small piece, less than goth of an inch square, of thin tin-foil fixed with shellac to the apex of a young radicle; after 24h. no effect. Tin-foil removed, and a small square of sanded card fixed with shellac; after 9h. deflection at nearly vight angles from the perpendicular and from the card. Next

morning deflection reduced to about 40° from the perpen- dicular. (81.) 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 9h. radicle greatly deflected from the card. After two additional days the deflection had decreased and was only 85° 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 15h. deflected at. nearly 90° from the perpendicular and from the card. (84.) Small oblong of sanded card gummed to apex: after 15h. 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.) Squave of card gummed to apex: after 9 h. deflected from card: after 24h. from time of attachment greatly deflected obliquely and partly in opposition to Sachs’ curvature.

(36.) Small piece of card, rather less than j5th 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. (87.) Square of card, gummed to apex in front, caused after 8h. 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.

(88.) Square of card gummed to apex: after 9 h. much deflec- tion, 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 cciled 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 consider- ably 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 aftcr 8 h. 40 m. was slightly dleflected ; 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 Jetter-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. (48.) 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, tle extreme tip was curved towards the card.