The Power of Movement in Plants by Charles Darwin (best fiction books to read TXT) 📕

in two sub-classes; in one of which the modification depends on innate or constitutional causes, and is independent of external conditions, excepting in so far that the proper ones for growth must be present. In the second sub-class the modification depends to a large extent on external agencies, such as the daily alternations of light and darkness, or light alone, temperature, or the attraction of gravity. The first small sub-class will be considered in the present chapter, and the second sub-class in the remainder of this volume.

 

THE CIRCUMNUTATION OF CLIMBING PLANTS.

 

The simplest case of modified circumnutation is that offered by climbing plants, with the exception of those which climb by the aid of motionless hooks or of rootlets: for the modification consists chiefly in the greatly increased amplitude of the movement. This would follow either from greatly increased growth over a small length, or more probably from moderately increased growth spread over a considerable length of the moving organ, preceded by turgescence, and acting successively on all sides. The circumnutation of climbers is more regular than that of ordinary plants; but in almost every other respect there is a close similarity between their movements, namely, in their tendency to describe ellipses directed successively to all points of the compass—in their courses being often interrupted by zigzag lines, triangles, loops, or small [page 265]

ellipses—in the rate of movement, and in different species revolving once or several times within the same length of time. In the same internode, the movements cease first in the lower part and then slowly upwards. In both sets of cases the movement may be modified in a closely analogous manner by geotropism and by heliotropism; though few climbing plants are heliotropic.

Other points of similarity might be pointed out.

 

That the movements of climbing plants consist of ordinary circumnutation, modified by being increased in amplitude, is well exhibited whilst the plants are very young; for at this early age they move like other seedlings, but as they grow older their movements gradually increase without undergoing any other change. That this power is innate, and is not excited by any external agencies, beyond those necessary for growth and vigour, is obvious. No one doubts that this power has been gained for the sake of enabling climbing plants to ascend to a height, and thus to reach the light. This is effected by two very different methods; first, by twining spirally round a support, but to do so their stems must be long and flexible; and, secondly, in the case of leaf-climbers and tendril-bearers, by bringing these organs into contact with a support, which is then seized by the aid of their sensitiveness. It may be here remarked that these latter movements have no relation, as far as we can judge, with circumnutation. In other cases the tips of tendrils, after having been brought into contact with a support, become developed into little discs which adhere firmly to it.

 

We have said that the circumnutation of climbing plants differs from that of ordinary plants chiefly by its greater amplitude. But most leaves circumnutate

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in an almost vertical plane, and therefore describe very narrow ellipses, whereas the many kinds of tendrils which consist of metamorphosed leaves, make much broader ellipses or nearly circular figures; and thus they have a far better chance of catching hold of a support on any side. The movements of climbing plants have also been modified in some few other special ways.

Thus the circumnutating stems of Solnanum dulcamara can twine round a support only when this is as thin and flexible as a string or thread. The twining stems of several British plants cannot twine round a support when it is more than a few inches in thickness; whilst in tropical forests some can embrace thick trunks;* and this great difference in power depends on some unknown difference in their manner of circumnutation. The most remarkable special modification of this movement which we have observed is in the tendrils of Echinocystis lobata; these are usually inclined at about 45o above the horizon, but they stiffen and straighten themselves so as to stand upright in a part of their circular course, namely, when they approach and have to pass over the summit or the shoot from which they arise. If they had not possessed and exercised this curious power, they would infallibly have struck against the summit of the shoot and been arrested in their course. As soon as one of these tendrils with its three branches begins to stiffen itself and rise up vertically, the revolving motion becomes more rapid; and as soon as it has passed over the point of difficulty, its motion coinciding with that from its own weight, causes it to fall into its previously inclined position so quickly, that the apex can be seen travelling like the hand of a gigantic clock.

 

* ‘The Movements and Habits of Climbing Plants,’ p. 36.

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A large number of ordinary leaves and leaflets and a few flower-peduncles are provided with pulvini; but this is not the case with a single tendril at present known. The cause of this difference probably lies in the fact, that the chief service of a pulvinus is to prolong the movement of the part thus provided after growth has ceased; and as tendrils or other climbing-organs are of use only whilst the plant is increasing in height or growing, a pulvinus which served to prolong their movements would be useless.

 

It was shown in the last chapter that the stolons or runners of certain plants circumnutate largely, and that this movement apparently aids them in finding a passage between the crowded stems of adjoining plants. If it could be proved that their movements had been modified and increased for this special purpose, they ought to have been included in the present chapter; but as the amplitude of their revolutions is not so conspicuously different from that of ordinary plants, as in the case of climbers, we have no evidence on this head. We encounter the same doubt in the case of some plants which bury their pods in the ground. This burying process is certainly favoured by the circumnutation of the flower-peduncle; but we do not know whether it has been increased for this special purpose.

 

EPINASTY—HYPONASTY.

 

The term epinasty is used by De Vries* to express greater longitudinal growth along the upper than

 

* ‘Arbeiten des Bot. Inst., in W�rzburg,’ Heft ii. 1872, p. 223. De Vries has slightly modified (p. 252) the meaning of the above two terms as first used by Schimper, and they have been adopted in this sense by Sachs.

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along the lower side of a part, which is thus caused to bend downwards; and hyponasty is used for the reversed process, by which the part is made to bend upwards. These actions come into play so frequently that the use of the above two terms is highly convenient. The movements thus induced result from a modified form of circumnutation; for, as we shall immediately see, an organ under the influence of epinasty does not generally move in a straight line downwards, or under that of hyponasty upwards, but oscillates up and down with some lateral movement: it moves, however, in a preponderant manner in one direction. This shows that there is some growth on all sides of the part, but more on the upper side in the case of epinasty, and more on the lower side in that of hyponasty, than on the other sides. At the same time there may be in addition, as De Vries insists, increased growth on one side due to geotropism, and on another side due to heliotropism; and thus the effects of epinasty or of hyponasty may be either increased or lessened.

 

He who likes, may speak of ordinary circumnutation as being combined with epinasty, hyponasty, the effects of gravitation, light, etc.; but it seems to us, from reasons hereafter to be given, to be more correct to say that circumnutation is modified by these several agencies. We will therefore speak of circumnutation, which is always in progress, as modified by epinasty, hyponasty, geotropism, or other agencies, whether internal or external.

 

[One of the commonest and simplest cases of epinasty is that offered by leaves, which at an early age are crowded together round the buds, and diverge as they grow older. Sachs first remarked that this was due to increased growth along the upper side of the petiole and blade; and De Vries has now shown in more detail that the movement is thus caused, aided slightly by

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the weight of the leaf, and resisted as he believes by apogeotropism, at least after the leaf has somewhat diverged. In our observations on the circumnutation of leaves, some were selected which were rather too young, so that they continued to diverge or sink downwards whilst their movements were being traced. This may be seen in the diagrams (Figs. 98 and 112, pp.

232 and 249) representing the circumnutation of the young leaves of Acanthus mollis and Pelargonium zonale. Similar cases were observed with Drosera. The movements of a young leaf, only 3/4 inch in length, of Petunia violacea were traced during four days, and offers a better instance (Fig.

111, p. 248) as it diverged during the whole of this time in a curiously zigzag line with some of the angles sharply acute, and during the latter days plainly circumnutated. Some young leaves of about the same age on a plant of this Petunia, which had been laid horizontally, and on another plant which was left upright, both being kept in complete darkness, diverged in the same manner for 48 h., and apparently were not affected by apogeotropism; though their stems were in a state of high tension, for when freed from the sticks to which they had been tied, they instantly curled upwards.

 

The leaves, whilst very young, on the leading shoots of the Carnation (Dianthus caryophyllus) are highly inclined or vertical; and if the plant is growing vigorously they diverge so quickly that they become almost horizontal in a day. But they move downwards in a rather oblique line and continue for some time afterwards to move in the same direction, in connection, we presume, with their spiral arrangement on the stem. The course pursued by a young leaf whilst thus obliquely descending was traced, and the line was distinctly yet not strongly zigzag; the larger angles formed by the successive lines amounting only to 135o, 154o, and 163o. The subsequent lateral movement (shown in Fig. 96, p. 231) was strongly zigzag with occasional circumnutations. The divergence and sinking of the young leaves of this plant seem to be very little affected by geotropism or heliotropism; for a plant, the leaves of which were growing rather slowly (as ascertained by measurement) was laid horizontally, and the opposite young leaves diverged from one another symmetrically in the usual manner, without any upturning in the direction of gravitation or towards the light.

 

The needle-like leaves of Pinus pinaster form a bundle whilst young; afterwards they slowly diverge, so that those on the upright shoots become horizontal. The movements of one such

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young leaf was traced during 4 � days, and the tracing here given (Fig.

121) shows that it descended at first in a nearly straight line, but afterwards zigzagged, making one or two little loops. The diverging and descending movements of a rather older leaf were also traced (see former Fig. 113, p. 251): it descended during the first day and night in a somewhat zigzag line; it then circumnutated round a small space and again descended. By this time the leaf had nearly assumed its final position, and now plainly circumnutated. As in the case of the Carnation, the leaves, whilst very young, do not seem to