The Power of Movement in Plants - Charles Darwin (top reads txt) 📗

On the causes leading to reversion see chap. xii. vol. ii. and p. 59, chap.

xiv. On peloric flowers, chap. xiii. p. 32; and see p. 337 on their position on the plant. With respect to seeds, p. 340. On reversion by means of buds, p. 438, chap. xi. vol. i.

[page 191]

sible, or even probable, that this tendency to reversion may have been increased, as it is manifestly of service to the plant.

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 Aesculus 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 Aesculus, the tips were not at all sensitive to small bodies attached to them; but it does not follow from this

[page 192]

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.

[page 193]

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 Aesculus. 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

[page 194]

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.

[page 195]

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, [page 196]

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* 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.

Geotropism, as was shown in the last chapter, excites the primary radicle to bend downwards with very little force, quite insufficient to penetrate the ground. Such penetration is effected by the pointed * ‘Arbeiten Bot. Institut, W�rzburg,’ Heft iv. 1874, pp. 605-631.

[page 197]

apex (protected by the root-cap) being pressed down by the longitudinal expansion or growth of the terminal rigid portion, aided by its transverse expansion, both of which forces act powerfully. It is, however, indispensable that the seeds should be at first held down in some manner.

When they lie on the bare surface they are held down by the attachment of the root-hairs to any adjoining objects; and this apparently is effected by the conversion of their outer surfaces into a cement. But many seeds get covered up by various accidents, or they fall into crevices or holes. With some seeds their own weight suffices. The