The little fumitory (Fumaria officinalis) is also a humble example of a climber of this kind (fig. 28). Some of the petioles were determined to be sensitive to touching, and responded thereto in about an hour and a quarter. The young internodes forming the terminal shoots of the stem and branches are in constant rotation. The leaves also have their own special spontaneous movement. As this plant is a common weed there need be no difficulty in verifying, and even supplementing, the observations already made. The Corydalis is a closely allied plant, but not so common; it is intermediate between leaf-climbers and tendril-bearers, with some of the habits of both (fig. 29).
Fig. 28.—Common Fumitory (Fumaria officinalis).
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The plants which climb by means of the development of the tips of the leaves into hooks, are so few, and those are exotic, that we may dismiss them with a brief explanation of the process. The end of the leaf (in Gloriosa Plantii) forms a narrow projection, which is thickened and at first nearly straight; subsequently it bends downwards and forms a hook, which becomes strong enough and rigid enough to catch any object and fasten the plant. The inner surface of the hook is somewhat sensitive, and, when a twig is caught by it, the extremity curves a little inwards and permanently seizes it. If nothing is caught the hook remains open and sensitive for some time, but ultimately the extremity slowly curls inwards and forms a coil at the end of the leaf. In one leaf the hook remained open for thirty-three days. When the tip has curled into the form of a ring all sensibility is lost, but as long as it remains open some sensibility is retained.
Fig. 29.—Climbing Corydalis (Corydalis claviculata).
<!-- image -->We now pass on to tendril-bearers, premising that tendrils are in most cases modifications of leaves transformed into filaments, which are used wholly for climbing. In other words, a tendril may be a leaf so modified that it is reduced to the midrib and a few lateral branches, with none of the functions of leaves, but with a new and special function contemporaneous with the modification, viz., that of enabling the plant to climb and maintaining it in that position. But a tendril may also be a modification of the flower-stalk, or of some other organ. It matters not, in so far as the present inquiry is concerned, what organs are so modified; in fact, botanists themselves do not seem to be entirely agreed on this point.
Very few plants with tendrils possess the power of climbing up an erect stick, but most of them exhibit rotation in the growing points, performing revolutions not unlike in character to those of twiners, and in like manner in different directions. This movement, though similar in its action, has a different purpose. In twiners the oscillation is evidently in search of some object around which to entwine; in tendril-bearers in order to bring the tendrils in contact with some support. The tendrils themselves also rotate in many species; in some the tendrils, internodes, and petioles, move in harmony together. In Cobaea scandens, a well-known climber in common cultivation, the tendrils are ten or eleven inches in length, and revolve rapidly and vigorously. Three large circular sweeps were observed within an hour and a quarter, but the growing point does not rotate. In Echinocystis lobata, a plant of the cucumber family, the tendrils, which are from seven to nine inches in length, revolve as well as the internodes, but over a wider surface. The circles swept by the tendrils are from fifteen to sixteen inches in diameter, whilst those of the internodes are not more than about three inches. The quickest rate of motion for the completion of a revolution was about one hour and three-quarters. In a passion-flower the internodes as well as the tendrils rotate, the former very rapidly, performing its revolution in an average period of about an hour. In a species of trumpet-flower (Bignonia littoralis) the mature tendrils rotate much slower than the internodes, the former taking six hours to perform a revolution, and the latter two hours and three-quarters. In the Virginia creeper neither the internodes nor the tendrils possess the power of rotation.
That tendrils are sensitive to a touch, one might expect from the purposes they are called upon to serve, but this faculty varies in different species. In one of the passion-flowers (Passiflora gracilis) where the tendrils are thin, delicate, and straight, except the curved tips, a single delicate touch on the concave surface of the tip caused it to curve immediately, so that in two minutes it formed an open spire. The movement was generally perceptible within half a minute after being touched. A tendril which curls through being touched, but does not embrace anything, straightens itself again, but soon becomes irritated by a second touch. In order to ascertain how often the same tendril may be excited one tendril was selected, and this alternately straightening itself, answered to the stimulus no less than twenty-one times in fifty-four hours.
Professor Asa Gray has observed an equally rapid response to a touch in the tendrils of a plant of the cucumber family, but instances of such rapidity are rare. In some, the movement takes place after a few minutes, in others it is an hour or two, but in all some exhibition of sensibility has been observed. It is noteworthy that drops of water sprinkled with a syringe, so as to resemble falling rain, in no instance appeared to have the least stimulating effect. In most cases a touch from another tendril seemed to have no influence, although, in the bryony and the vine other tendrils have been seen embraced.
The sensibility of tendrils to light may also be illustrated by the trumpet-flower (Bignonia capreolata). In his experiments on these plants, Mr. Darwin observes, "In two instances, a pair of leaves stood so that one of the two tendrils was directed towards the light, and the other to the darkest side of the house; the latter did not move, but the opposite one bent itself first upwards and then right over its fellow, so that the two became parallel, one above the other, both pointing to the dark. I then turned the plant half-round, and the tendril which had turned recovered its original position, and the opposite one which had not before moved, now turned over to the dark side. On another plant, three pairs of tendrils were produced at the same time by three shoots, and all happened to be differently directed. I placed the pot in a box open only on one side, and obliquely facing the light. In two days all six tendrils pointed with unerring truth to the darkest corner of the box, though to do this each had to bend in a different manner. Six wind-vanes could not have more truly shown the direction of the wind than did these branched tendrils the course of the stream of light which entered the box. I left these tendrils undisturbed for about twenty-four hours, and then turned the pot half-round; but they had now lost their power of movement, and could not any longer avoid the light." The rotation in the tendrils of some plants is retarded and in others accelerated by the action of the light. Those of the pea, and some others, seem to be insensible to its influence.
The mode by which tendrils clasp and attach themselves to their supports is variable, even in the same genus. In some, they twine spirally, like a corkscrew; in others they grasp a projection in a manner resembling the foot of a bird; in others, again, they attach themselves by hooks or grapnels; and in others, the sharp points are inserted in cracks and fissures, or minute holes, although this latter in some cases, seems to be only a temporary expedient. The most elaborate mode of attachment is one in which the tips of the tendrils undergo special modification, and to this kind we must advert more in detail.
This curious but interesting adaptation of the tendrils of a plant, in order the better to fulfil its function of climbing, is related of an exotic trumpet-flower (Bignonia capreolata). The tendrils are branched, having about five branches, each of which is divided again at the apex, with each point blunt but distinctly hooked. Having placed a piece of wood containing numerous cracks within reach of the plant, it was observed that the tips of the immature tendrils crawled like roots into the minutest crevices. In two or three days after the tips had thus crawled into the crevices, or after the hooked ends had seized on projecting points, another process commenced. The tips of the inner surfaces of the hooks begin to swell, and in two or three days are visibly enlarged. After a few more days the hooks are converted into whitish balls, rather more than the one-twentieth of an inch in diameter, and composed of coarse cellular tissue, sometimes enveloping and concealing the hooks themselves. The surface of the balls secrete a viscid matter, to which small objects adhere. When slender fibres become attached to the balls the tissue grows round and over them, and fresh fibres continuing to adhere, as many as fifty or sixty fibres of flax have been counted imbedded in one of these balls. The fibres are clasped so tightly that they cannot be withdrawn. When two balls from adjacent extremities come into contact they will sometimes coalesce. If the hooked extremities of the tendrils do not touch anything the discs are not formed in this species, although, in an allied plant, Fritz Müller has remarked that smooth shining discs terminate the tendrils without their having come into contact with any object.
fig. 30.—Hooked tendril, like foot of a bird, from Bignonia Tweediana. Tip of hook magnified (a).
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The Virginia creeper (Ampelopsis hederacea) has also branched tendrils five or six inches in length. The tips of the branches are at first curved, and when they come in contact with a wall, or other flat surface, the hooks are brought into apposition to it. In the course of two days after a tendril has arranged its tips so that they touch and press on the surface, the curvatures swell, become bright red, and form little discs or cushions on the under side. In one case the tips were swollen in 38 hours, and in another 48 hours, and in an additional 24 hours were firmly attached to a smooth board. The discs are generally formed on one side of the curved tip, and never, as far as yet observed, without coming in contact with some object. Dr. McNab has observed in another species that small globose discs are formed before the tips come into contact. This also corresponds with the observations on Bignonia.
It seems evident that these discs possess the power of secreting some resinous cement, by means of which they adhere to the support to which they attach themselves. When a tendril does not become attached, its primary object being frustrated, in the course of a few weeks it shrinks and withers, and finally drops off. When the discs have become attached, then the tendril contracts spirally, so as to become very elastic, and at the same time thickens so as to attain increased strength. Even after the tendrils are dead they still continue to adhere, and retain strength. One single branch of a tendril, which had been dead at least for ten years, still remained elastic, and capable of supporting a weight of two pounds, so that assuming all the branches of the same tendril to have been equally attached, and equally strong, the entire tendril would be capable of enduring a strain of ten pounds. Sachs remarks, that the tendrils of different species are adapted to clasp supports of different thicknesses, and that when a tendril has clasped its support it afterwards tightens its hold.