The Power of Movement in Plants, Charles Darwin [the reading list book TXT] 📗

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. Nevertheless, 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 [page 131]

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 rectangular 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 transmitted 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 experiments which were tried, as the result proved remarkable. The fact of the apex of a radicle being sensitive to contact has never been observed, though, as we shall

 

* ‘Arbeiten Bot. Inst. W�rzburg,’ Heft iii. 1873, p. 398.

 

[page 132]

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 direction, such as the stamens of Berberis, the lobes of Dionaea, etc.; and many organs, such as tendrils, whether 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 24 h., 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 card 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 1/20th of an inch (i.e.

about 1 � mm.), or oblong bits of nearly the same size, were found to [page 133]

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 circumstances, 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. It should be premised that the beans were [page 134]

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

 

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.

 

as about 45o 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

[page 135]

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.

 

[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, 50o 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 20o from the perpendicular and from the card, and in opposition to Sachs’ curvature: after 23 h. no change.

[page 136]

 

(3.) Radicle 11 mm. in length, card fixed behind: after 9 h. deflected in the plane of the bean 40o 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 7o or 8o 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 30o 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, 20o 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 40o 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 40o 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 20o 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 40o and 50o from the vertical and from the card. In the plane of the bean itself the deflection amounted to 8o or 9o 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