Charles Darwin

“ Many authors speak as if the movement of a plant towards the light was as directly the result of the evaporation or of the oxygenation of the sap in the stem, as the elongation of a bar of iron from an increase in its temperature. But, seeing that tendrils are either attracted to or repelled by the light, it is more probable that their movements are only guided and stimulated by its action, in the same manner as they are guided by the force of attraction from or towards the centre of gravity. ”

“ I have seen no other exceptions to the rule that all tendrils, after clasping by their extremities a support, contract spirally. When, however, the tendril of any plant of which the stem happens to be immoveably fixed, catches some fixed object, it does not contract, simply because it cannot; this, however, rarely occurs. In the common Pea only the lateral branches, and not the central stem of the tendril, contract; and with most plants, such as the Vine, Passiflora, Bryony, the basal portion never contracts into a spire. ”

“ Any one who will look during windy weather at twining plants will see that they are easily blown from their support; not so with tendril-bearers or leaf-climbers, for they quickly and firmly grasp their support by a much more efficient kind of movement. In those plants which still twine, but at the same time possess tendrils or sensitive petioles, as some species of Bignonia, Clematis, and Tropæolum, we can readily observe how incomparably more securely they grasp an upright stick than do simple twiners. From possessing the power of movement on contact, tendrils can be made very long and thin ”

“ It has often been vaguely asserted that plants are distinguished from animals by not having the power of movement. It should rather be said that plants acquire and display this power only when it is of some advantage to them; but that this is of comparatively rare occurrence, as they are affixed to the ground, and food is brought to them by the wind and rain. We see how high in the scale of organization a plant may rise, when we look at one of the more perfect tendril-bearers. ”

“ Does this gradation, it may be asked, indicate that plants belonging to one subdivision have passed, during the lapse of ages, or can pass, from one state to the other; has, for instance, a tendril-bearing plant assumed its present structure without having previously existed as either a leaf-climber or a twiner? If we consider leaf-climbers alone, the idea that they were primordially twiners is forcibly suggested. The internodes of all, without exception, revolve in exactly the same manner as twiners; and some few can still twine well, and many others in a more or less imperfect manner. ”

“ Bignonia venusta ascended a vertical stick by spirally twining and by seizing it alternately with its two tendrils like a sailor pulling himself up a rope hand over hand; our present species pulls itself straight up, like a sailor seizing with both hands together the rope above his head.
The tendrils are almost identical in structure with those of the last species. They continue growing for some time, even after clasping an object, and when fully grown, though borne by a young plant, were 9 inches in length. ”

“ If we further inquire how the stems, petioles, tendrils, and flower-peduncles of climbing plants first acquired their power of spontaneously revolving, or, to speak more accurately, of successively bending to all points of the compass, we are again silenced, or at most can only remark, that the power of movement, both spontaneous and from various stimuli, is far more common with plants, as we shall presently see, than is generally supposed to be the case by those who have not attended to the subject. ”

“ The young leaves change their position in a rather odd manner: when first developed the petioles are upturned, parallel to the stem; they then slowly bend downwards, remaining for a short time at right angles to the stem, and then become so much arched downwards that the blade of the leaf points to the ground with its tip curled inwards, so that the whole petiole and leaf together form a hook. If they come into contact with no object, they retain this position for a considerable time, and then bending upwards they reassume their original upturned position, which is retained ever afterwards. ”

“ In the cases given of certain South African plants belonging to great twining families, which in certain districts of their native country never twine, but reassume this habit when cultivated in England, we have a case in point. In the leaf-climbing Clematis flammula, and in the tendril-bearing Vine, we see no loss in the power of climbing, but only a remnant of that revolving-power which is indispensable to all twiners, and is so common, as well as so advantageous, to most climbers. ”

“ If the tendril be displaced, it is acted on by the force of gravity and rights itself. It is acted on by the light, and bends towards or from it, or disregards it, whichever may be most advantageous. During several days the tendril or internodes, or both, spontaneously revolve with a steady motion. The tendril strikes some object, and quickly curls round and firmly grasps it. In the course of some hours it contracts into a spire, dragging up the stem, and forming an excellent spring. All movements now cease. By growth the tissues soon become wonderfully strong and durable. ”

“ The whole length of a tendril often fits itself closely to the surface of the wood with which it is in contact; and I have seen a tendril bend at right angles and place itself in a wide and deep fissure, with the apex again abruptly bent and inserted into a minute lateral hole. After a tendril has clasped a stick, it contracts spirally; if it catches nothing, it does not contract. When it has adapted itself to the inequalities of a thick post, though it has clasped nothing, or when it has inserted its apex into some little fissure, the stimulus suffices to induce spiral contraction ”

“ The leaves of at least two and probably of most of the species of Clematis, and of Fumaria and Adlumia, spontaneously curve from side to side, like the internodes, and are thus better adapted to seize any distant object. The petioles of the perfect leaves, as well as the rudimentary or tendril-like leaves of Tropæolum tricolorum move spontaneously and slowly towards their own stem or the supporting stick, which they then clasp; these petioles also show some tendency to contract spirally. The tips of the uncaught leaves of the Gloriosa, as they grow old, contract into a flat spire. ”

“ The main petiole (but not the two sub-petioles of the lateral leaflets) is sensitive to contact with any object: even a small loop of thread after two days caused one to bend upwards. The whole tendrils, namely the tarsus and three toes, especially their under surfaces, are likewise sensitive to contact. Hence, when a shoot grows through branched twigs, its revolving movement soon brings the tendril into contact with some twig, and then all three toes bend (or sometimes one alone) , and, after several hours, seize fast hold of the twig, exactly like a bird when perched. ”

“ If we hold in our left hand a bundle of parallel strings, we can with our right hand turn these round and round, and imitate the revolving movement of a twining plant, and the strings do not become twisted. But if we now at the same time hold a stick in our left hand, in such a position that the strings become spirally turned round it, they will inevitably become twisted. Hence a straight coloured line, painted along the internodes of a twining plant before it has wound round a support, becomes twisted or spiral after it has so wound round. ”

“ In the course of about two days after a tendril has arranged its branches so as to press on any surface, the curved tips swell, become bright red, and form on their under-sides the well-known little disks or cushions, which adhere firmly to the surface. ”

“ The free internodes of vigorously growing plants, when they cease to revolve, become straight, and show no tendency to be spiral; but when any shoot has nearly ceased to grow, or when the plant is unhealthy, the extremity does occasionally become spiral. I have seen this in a remarkable degree with the ends of the shoots of the Stauntonia and of the allied Akebia, which became closely wound up spirally, just like a tendril, especially after the small, ill-formed leaves had perished. ”

“ Each leaf consists of a petiole bearing a pair of leaflets, and terminating in a tendril, which is exactly like that above figured, but a little larger. The whole tendril in a young plant was only about half an inch in length, and is very unlike most tendrils in shape. It curiously resembles the leg and foot of a small bird with the hind toe cut off. The straight leg or tarsus is longer than the three toes, which latter are of equal length, and, diverging, lie in the same plane; the toes terminate in sharp and hard claws, much curved downwards, exactly like the claws on a bird's foot. ”

“ The rate of revolution in different plants varies from one to five hours, and consequently in some cases is more rapid than with any twining plant, and is never so slow as with those many twiners, which take more than five hours for each revolution. The direction is variable even in the same individual plant. In Passiflora, the internodes of only one of the species have the power of revolving. The Vine is the weakest revolver observed by me, apparently exhibiting only a trace of a former power. In the Eccremocarpus the movement is interrupted by many long pauses. ”

“ All tendrils are sensitive, but in very various degrees, to contact with any object, and curve towards the touched side. With several plants a single touch, so slight as only just to move the highly flexible tendril, is enough to induce curvature. Passiflora gracilis has the most sensitive tendrils which I have seen: a hit of platina wire 1⁄50th of a grain in weight, gently placed on the concave point, caused two tendrils to become hooked, as did (and this perhaps is a better proof of sensitiveness) a loop of soft, thin cotton thread weighing 1⁄32nd of a grain, or about two milligrammes. ”

“ Differently from most twiners, there is a strong tendency in the same shoot to revolve first in one and then in the opposite direction. The object gained by the revolving movement, as could be plainly seen, was to bring the petioles or the tips of the leaves into contact with surrounding objects; without this aid there would be a poor chance of success. With rare exceptions, the petioles are sensitive only whilst young; they are sensitive on all sides, but in different degrees in different plants, and in some species of Clematis in very different degrees in different parts of the same petiole. ”

“ But the best evidence that the twisting does not cause the revolving movement is afforded by many leaf-climbing and tendril-bearing plants (as Pisum sativum, Echinocystis lobata, Bignonia capreolata, Eccremocarpus scaber, and with the leaf-climbers, Solanum jasminoides and various species of Clematis) , of which the internodes are not regularly twisted, but which regularly perform, as we shall hereafter see, revolving movements like those of true twining-plants. ”

“ But he who believes in the slow modification of species will not be content simply to ascertain the homological nature of different tendrils; he will wish to learn, as far as possible, by what steps parts acting as leaves or as flower-peduncles can have wholly changed their function, and have come to serve as prehensile organs.
In the whole group of leaf-climbers abundant evidence has been given that an organ, still subserving its proper function as a leaf, may become sensitive to a touch, and thus grasp an adjoining object. ”

“ Climbing plants may be conveniently divided into those which spirally twine round a support, those which ascend by the movement of the foot-stalks or tips of their leaves, and those which ascend by true tendrils,—these tendrils being either modified leaves or flower-peduncles, or perhaps branches. But these subdivisions, as we shall see, nearly all graduate into each other. There are two other distinct classes of climbing-plants, namely those furnished with hooks and those with rootlets; but, as such plants exhibit no special movements, we are but little concerned with them ”