Page:Popular Science Monthly Volume 2.djvu/300

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286
THE POPULAR SCIENCE MONTHLY.

when climbing. When a climbing plant first springs from the ground, the extremity of the shoot performs slow gyrations in the air, as if, as Darwin expresses it, it were searching for a support. I do not here discuss the question whether this habit may be the result of a tendency transmitted and enhanced through thousands of generations; the movement itself is, in the individual plant, entirely "spontaneous," in every sense of the term; that is, is not the necessary result of known physical laws acting upon the individual. Darwin's paper, "On the Movements and Habits of Climbing Plants," published in the Journal of the Linnæan Society, contains a number of the most interesting observations on this class of plants; and the language employed is everywhere suggestive of some hidden, sentient controlling power in the plant itself.

Fig. 3.
PSM V02 D300 Climbing plant.jpg

The same purpose as that served by a climbing stem is answered in other plants, as the vine, Virginian creeper, and passion-flower, by tendrils; and the phenomena of spontaneous motion in tendrils are, if possible, still more curious. Some tendrils display the same power of rotatory motion possessed by the extremities of the shoots of climbing plants; others do not revolve, but are sensitive, bending to the touch. The curling movement, consequent on a single touch, continues to increase for a considerable time, then ceases; after a few hours the tendril uncurls itself, and is again ready for action. A tendril will thus show a tendency to curl round any object with which it comes into contact, with the singular exception that it will seldom twine itself round another tendril of the same plant. It is also very curious that,