| OCR Text |
Show 334 DARWINIA.NA. of the compass, through an action which circulates around the stem in the direction of the sweep), and of the consequent twm. m. g, 1. . e., " WI't h the sun, " or WI't h the movement of the hands of a watch, in the hop, or in the opposite direction in pole-beans and most twiners. Twining plants, therefore, ascend trees or other stems by an action and a movement of their own, from which they derive advantage. To plants liable to be overshadowed by more robust companions, climbing is an economical method of obtaining a freer exposure to light and air with the smallest possible expenditure of material. But twiners have one disadvantage: to rise ten feet they must produce fifteen feet of stem or thereabouts, according to the diameter of the support, and the openness or closeness of the coil. A rootlet- climber saves much in this respect, but has arestricted range of action, and other disadvantages. There are two other modes, which combine the utmost economy of material with freer range of action. There are, in the first place, leaf-climbers of various sorts, agreeing only in this, that the duty of laying hold is transferred to the leaves, so that the stem may rise in a direct line. Sometimes the blade or leaflets, or some of them, but more commonly their slender stalks, undertake the work, and the plant rises as a boy ascends a tree, gra~ping first with one hand or arm, then with the other. Indeed, the comparison, like the leaf-stalk holds better than would be supposed ; for the gras;ing of the latter is not the result of a blind groping in all directions by a continuous movement, but of a definite sensitiveness which acts only upon the INSECTIVOROUS AND CLIMBING PLANTS. 235 occasion. Most leaves make no regular sweeps ; but when the stalks of a leaf-climbing species come into prolonged contact with any fitting extraneous body, they slowly incurve and make a turn around it and then commonly thicken and harden until they a't tain a strength which may equal that of the stem itself. IIere we have the faculty of movement to a definite end, upon external irritation, of the same nature with that displayed by Dion(Ea and Drosera, although slow~ er for the most part than even in the latter. But the movement of the hour-hand of the clock is not different in nature or cause from that of the second-hand. Finally-distribution of office being, on the whole, most advantageous and economical, and this, in the vegetable kingdom, being led up to by degrees-we reach, through numerous gradations, the highest style of climbing plants in the tendril-climber. A tendril, morphologically, is either a leaf or branch of stem, or a portion of one, specia11y. organized for climbing. Some tendrils simply turn away from light, as do those of grape-vin~s, thus taking the direction in which some supporting object is likely to be encountered; most are indifferent to light ; and many revolve in the manner of the summit of twining stems. As the stems which bear these highly-endowed tendrils in many cases themselves also revolve more or less, though they seldom twine, their reach is the more extensive; and to this endowment of automatic movement most tendrils add the other faculty, th~t of incurving and coiling upon prolonged touch, or even brief contact, in the highest degree. Some long tendrils, when in their best condition, revolve so rapidly that the sweeping |
