The vine has served rhetoricians ever since the Dark Ages as a type of clinging helplessness and utter dependence. It has symbolized the condition of woman under the old regime, before she entered the learned professions and the business world, donned short skirts, mounted the bicycle, and wrote herself down Woman. Therefore, we learn, with some surprise, that the vine, like many a woman in unreconstructed societies, is only appar-ently relieved of the burdens of existence, and that it works as hard for its living as the "sturdy oak," to which it clings.
The charitable soul is now and then defrauded by a ne'er-do-well, who puts into his schemes for the avoidance of work an amount of astuteness, adroitness, and energy which would win success in some legitimate field of labor.
Vines, when one studies their habits, are somewhat suggestive of such characters, for they expend much vital energy in searching for something to support them, and in holding fast to the support when it is found. The "movements and habits of climbing plants" have been carefully studied by Darwin, whose book on the subject is the source of most of the facts here and now set forth. He divides climbing plants into four classes. Those of the first class twine spirally around a support, and have no other spontaneous movements. To this category beans and hops belong.
Vines of the second class ascend by means of special organs. Sometimes, as is the case with the clematis, the leaf-stalks do double duty, and not only uphold the leaves, but also embrace any slender thing within reach. And sometimes the plant bears tendrils, which reach out like the feelers of an octopus, seeking what they may clasp and hold. By this method sweet-peas get on in the world. But no sharp line of distinction can be drawn between "leaf-climbers' and "tendril-bearers." They are closely connected, and are classified together.
Vines of the third class scramble upward by means of hooks, and this is the way some roses clamber. Many of these hook-climbers are natives of tropical forests, and they are most successful where masses of tangled vegetation uphold and "boost" them.
In the fourth class Darwin places the "rootclimbers." These produce roots both in earth and air. The aerial roots, or "rootlets," are short, woody threads, which half cover the main stem and branches, and cling tightly to bare walls, naked rocks, or the trunks of large trees. The English-ivy, the poison-ivy, and the climbing-fig all clamber by this expedient, and their grip upon their supports is amazingly tenacious.
Fig. 65. - English ivy (Hedera Helix). Climbing by means of aerial rootlets. (From the Vegetable World).
Darwin observed that the rootlets of the climbing-fig, when they were a few days old, began to emit minute drops of a clear, viscid fluid. This fig is a first cousin to the plant which produces the india-rubber of commerce, and, like all members of the family, it abounds in caoutchouc. So the liquid which glues its rootlets to the wall is fluid india-rubber, and with time and exposure to the air this substance becomes converted into a brittle, resinous matter, very similar to shellac. "Whether other plants which climb by their rootlets emit any cement," says Darwin, "I do not know; but the rootlets of the ivy, placed against glass, barely adhered to it, yet secreted a little yellowish matter".
But hook- and root-climbers, however lovely and pleasant to the landscape-gardener, have little interest for the student of plant habits. His attention is given rather to the twiners and tendril-bearers, whose movements seem instinct with life, akin to that of the animal world; for every tender tip of every growing twiner sweeps around and around continuously. As the growth of the plant causes the ends of its main stem and branches to ascend, the motion of each vine is not a series of circles, but one close, continuous spiral. This revolving movement is quicker by day than it is by night. It is accelerated by the warmth of sunshiny summer noons, and retarded by overcast or chilly weather. It is most rapid, generally speaking, in June heats, when all plant-life reaches its uttermost fulness, and it slows down gradually with the waning of the year. But all summer long, in glad or in gloomy weather, this strange movement goes on in growing tips of twining and tendril-bearing vines.
Decrease in temperature always has the effect of retarding the revolution of a vine-tip. When twining plants grow in a window the sprays travel faster when in the sunlight, and their speed slackens as they twine into the shadow. Thus, a morning-glory, living in a sunny window, has been found to make a complete revolution in five hours and thirty minutes, but the half of its orbit which lay in the light was traversed in one hour, and all the rest of the time was spent in getting around the semicircle which lay in shadow.
When a hop begins to grow, the two or three first-formed joints, or "internodes," of the stem are straight, and stand erect and still. "But the next formed," says Darwin, "whilst very young, may be seen to bend to one side, and to travel slowly around toward all points of the compass, moving like the hands of a watch, with the sun." The movement very soon acquires its full ordinary velocity, and it continues as long as the plant continues to grow; but each separate internode, as it becomes old, ceases to move. The internodes travel slowly when they are very young, and accelerate their speed as they approach maturity.
So the tender tip and the lower and older part of the spray are moving in the same direction, but at varying rates; and this difference sometimes gives a serpentine twist to the shoots of vigorous twiners. The ends of many vine-sprays are bent over so as to form hooks, which are of great assistance to the plants in their efforts to rise in the world. For not only does the terminal hook lay hold of any support within reach, but it causes the tip of the shoot to embrace this support much more closely than it could otherwise do, and thus may prevent the stem from being blown aside in windy weather. It is very noticeable in the young sprays of the Virginia creeper (see Fig. 67).