Page:Popular Science Monthly Volume 2.djvu/301

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287
SPONTANEOUS MOVEMENTS IN PLANTS.

with some exceedingly sensitive plants, the falling of drops of rain on the tendril will produce no effect whatever. The mode in which a tendril of a Bignonia catches hold of a support is thus described by Darwin: "The main petiole is sensitive to contact with any object; even a small loop of thread after two days caused one to bend upward. The whole tendrils are likewise sensitive to contact. Hence, when a shoot grows through branched twigs, its revolving movement soon brings the tendril into contact with some twig, and then all three

Fig. 4.
PSM V02 D301 Bignonia.jpg

'toes' bend (or sometimes one alone), and, after several hours, seize fast hold of the twig, exactly like a bird when perched." The Virginian creeper has another mode of attaching itself to a wall or other solid support, by the formation, at the extremities of the branches of the tendril, of little disks or cushions, very similar to the disks on the foot of the house-fly, by which it is enabled to attach itself to our windows, and to walk along the ceiling. These disks secrete a glutinous fluid, which attaches the tendril to the support with such strength that it is often impossible to detach it without destroying the tendril, or even removing a portion of the wall itself. As soon as the attachment is accomplished, the tendril gradually thickens, and contracts spirally, as shown in Figs. 3 and 4. This spiral contraction, indeed, is always the result of the tendril meeting with a support; and, if no support is found, the tendril soon shrinks and withers away. Some tendrils exhibit a most remarkable power of selection, which, to use Mr. Darwin's words, "would, in an animal, be called instinct." The tendrils of a species of Bignonia slowly travelled over the surface of a piece of wood, and, when the apex of one of them came to a hole or fissure, it inserted itself; the same tendril would frequently withdraw from one