Tendril

“ 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 ”

“ 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 best proof of the intimate connexion between the spiral contraction of a tendril and the previous act of clasping a support, is afforded by those tendrils which, when caught, invariably contract into a spire, whilst as long as they remain unattached they continue straight, though dependent, and thus wither and drop off. ”

“ We shall find later that different plant organs, after moderate stimulation, exhibit acceleration of growth as an after-effect. The phenomenon of responsive reaction of tendril is therefore not unique, but similar to that of other organs under all forms of stimulation. The only speciality in tendril is that owing to anatomical peculiarities, the perceptive power of the organ for mechanical stimulus is highly developed. ”

“ Others, like those of the passion-flower, will occasionally become discouraged and curl up all by themselves, in which case, the other tip being free, the curl is perfect and continuous and without the reverse loop, which is now unnecessary. But the function of the tendril is to clasp and hold. Its growth is not complete until thus quickened by the new responsibility. Tendrils on duty become tough and sinewy in comparison to their idling neighbors. How firm and rigid are these swollen coils upon the grape-vine! ”

“ SKELETON OF AN ICHTHYOSAURUS. How, once more, can we conceive the peculiar actions of the tendrils of some climbing plants to have been produced by minute modifications? These, according to Mr. Darwin, [26] oscillate till they touch an object, and then embrace it. It is stated by that observer, "that a thread weighing no more than the thirty-second of a grain, if placed on the tendril of the Passiflora gracilis, will cause it to bend; and merely to touch the tendril with a twig causes it to bend; but if the twig is at once removed, the tendril soon straightens itself. ”

“ That growth is the chief cause of the spiral contraction of tendrils may be safely admitted, as shown by the recent researches of H. de Vries. I will, however, add one little fact in support of this conclusion.
If the short, nearly straight portion of an attached tendril of Passiflora gracilis, (and, as I believe, of other tendrils,) between the opposed spires, be examined, it will be found to be transversely wrinkled in a conspicuous manner on the outside ”

“ Even the middle part of the tarsus is sensitive to prolonged contact, as soon as the tendril has arrived at maturity. After it has grown old, the sensitiveness is confined to the toes, and these are only able to curl very slowly round a stick. A tendril is perfectly ready to act, as soon as the three toes have diverged, and at this period their outer surfaces first become irritable. The irritability spreads but little from one part when excited to another: thus, when a stick was caught by the part immediately beneath the three toes, these seldom clasped it, but remained sticking straight out. ”

“ The flexibility of the tendrils is of service in allowing them to be blown about by a breath of wind, and they can thus be made to seize hold of objects which are out of reach of the ordinary revolving movements. Many tendrils can only seize a stick by curling round it, and this even in the most sensitive tendril must take a minute or two; but with Cobœa the sharp hooks catch hold of little irregularities on the bark the moment the tendril comes into contact with it, and afterward the tendril can curl round and make the attachment permanent. ”

“ Why a delicate touch should cause one side of a tendril to contract we know as little as why, on the view held by Sachs, it should lead to extraordinarily rapid growth of the opposite side. The chief or sole reason for the belief that the curvature of a tendril when touched is due to rapid growth, seems to be that tendrils lose their sensitiveness and power of movement after they have grown to their full length; but this fact is intelligible, if we bear in mind that all the functions of a tendril are adapted to drag up the terminal growing shoot towards the light. ”

“ As soon as all three branches of the tendril have begun to stiffen themselves in this remarkable manner, as if by a process of turgescence, and to rise from an inclined into a vertical position, the revolving movement becomes more rapid; and as soon as the tendril has succeeded in passing the extremity of the shoot, its revolving motion, coinciding with that from gravity, often causes it to fall into its previously inclined position so quickly, that the end of the tendril could be distinctly seen travelling like the minute hand of a gigantic clock. ”

“ In the foregoing three species, when the foot-like tendril has caught any object, it continues to grow and to thicken, and ultimately it becomes wonderfully strong, in the same manner as we have seen with the petioles of leaf-climbers. If the tendril catches nothing, it first slowly bends downwards, and then its power of clasping is lost. Very soon afterwards it disarticulates itself from the petiole, like a leaf in autumn from the stem, and drops off. I have seen this process of disarticulation in no other tendrils, but when uncaught they soon wither away. ”

“ When the branches of a revolving tendril strike against a stick, they quickly bend round and clasp it; but the little hooks play an important part, especially if only the extremity of the tendril be caught, in preventing its being dragged by the rapid revolving movement away too quickly for its irritability to act. ”

“ Nevertheless, as the contraction travels from the extremity of a tendril, after it has been stimulated by contact with a support, down to the base, I cannot avoid doubting, from reasons presently to be given, whether the whole effect ought to be attributed to growth. An unattached tendril rolls itself up into a flat helix, as in the case of Cardiospermum, if the contraction commences at the extremity and is quite regular ”

“ 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. ”

“ It is a rare circumstance thus to detect any superfluity or imperfection in the action of tendrils—organs which are so excellently adapted for the functions which they have to perform; but we see that they are not always perfect, and it would be rash to assume that any existing tendril has reached the utmost limit of perfection. ”

“ They readily catch soft wood, or gloves, or the skin of the hands. Excepting these hardened hooks, and excepting the basal part of the central stem of the tendril, every part of every branch is highly sensitive on all sides to a slight touch, and bends in a few minutes towards the touched side. By lightly rubbing several branches on different and opposite sides, the whole tendril rapidly assumes an extraordinarily crooked shape: these movements from contact do not interfere with the ordinary revolving movement. ”

“ Very few tendril-bearing plants can spirally twine up an upright stick. Although the power of twining has generally been lost, either from the stiffness or shortness of the internodes, from the size of the leaves, or from some other unknown cause, the revolving movement of the stem serves to bring the tendrils into contact with surrounding objects.
The tendrils themselves also spontaneously revolve. The movement begins whilst the tendril is young, and is at first slow. The mature tendrils of Bignonia littoralis move much slower than the internodes. ”

“ A tendril which has not become attached to any body, does not contract spirally; and in course of a week or two shrinks into the finest thread, withers and drops off. An attached tendril, on the other hand, contracts spirally, and thus becomes highly elastic, so that when the main foot-stalk is pulled the strain is distributed equally between all the attached discs. For a few days after the attachment of the discs, the tendril remains weak and brittle, but it rapidly increases in thickness and acquires great strength. ”

“ When an unattached tendril contracts spirally, the spire always runs in the same direction from tip to base. A tendril, on the other hand, which has caught a support by its extremity, although the same side is concave from end to end, invariably becomes twisted in one part in one direction, and in another part in the opposite direction ”

“ On the other hand, the effect of stimulation by contact in this case is to transmit the stimulus along the tendril, resulting in the formation of a spiral, and in most cases, if not all, the plant energy induced by the stimulus is directed towards the formation and modification of mechanical tissue, to render the union of the plant with the support more firm.
The formation of mechanical tissue in a tendril is well illustrated m the tendril of the common grapevine, and in various hook climbers. ”

“ The layer is white, whilst the tendril is green, and near the tip it could sometimes be seen to be thicker than the tendril itself; it generally spreads a little beyond the sides of the tendril, and its edge is fringed with free elongated cells, which have enlarged globular or retort-shaped heads. ”

“ And in the tendril the coiling below is just a continuation of the same movement or same change as that which incurved the tip in clasping, that is, a relative shortening of concave or lengthening of the convex side of the tendril. Would you not infer that the action was propagated downward?
So you were astonished at Mrs. Gray’s audacity. Well, “toujours l’audace;” she is all the better for it. ”

“ The speed with which a tendril will take its first turns about a support is so comparatively rapid that, once the support is caught there is scarcely a chance of the vine being torn away by the wind or other agency as would surely happen if tendril movements were the leisurely things that some folks think they are. In the case of one Passion-flower vine, which are gorgeous climbers mostly from the tropics, the tendril made a complete turn in two minutes after it first touched a possible support. ”

“ Unilateral application of direct stimulus induces a positive curvature, but the same stimulus applied at a distance from the responding region induces a negative curvature.
The tendril of Passiflora is excitable both on the upper and under sides: the excitability of the under side is about seven times greater than that of the upper side.
Stimulation of one side of the tendril induces an expansion of the opposite side, even in cases where the contractility of the stimulated side is feeble. ”

“ 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. ”

“ A tendril commonly grows straight and outstretched until it reaches some neighboring support, such as a stem, when its apex hooks around it to secure a hold; then the whole tendril shortens itself by coiling up spirally, and so draws the shoot of the growing plant nearer to the supporting object. ”

“ The plant so advantageously equipped transmits its [188] tendency to its offspring, and has therefore survived in place of its ancient fellows, and is the type perpetuated or "selected" by nature. Such a tendril, then, is a modified leaf. How is it in the pea? Here we find four leaflets in two opposite pairs, but no odd leaflet at the end of the main stalk, such as we see in almost all other plants of its family. But in place of this leaflet we find a branching tendril reaching out on all sides for conquest. ”