Page:Darwin - On the movements and habits of climbing plants.djvu/79

 way from it. Hence the sweep of the lower part of the tendril of the Hanburya is much restricted. Here a nice case of coadaptation comes into play: in all the other tendrils observed by me the several branches become sensitive at the same period; had this been the case with the Hanburya, the rectangular spur-like branch being pressed, during the revolving movement, against the projecting end of the shoot, would infallibly have seized it in a highly injurious manner. But the main tendril, after revolving for a time in a vertical position, spontaneously bends downwards; and this, of course, raises the rectangular branch, which itself also curves upwards; so that by these combined movements the spur-like branch rises above the projecting end of the shoot, and can now move freely without touching it; then, and not until then, it first becomes sensitive.

The tips of both branches, when they come into contact with a stick, grasp it like any ordinary tendril. In a few days afterwards the inferior surface swells and becomes developed into a cellular layer, which adapts itself closely to the wood, and firmly adheres to it. This layer is analogous to the adhesive disks formed by the tips of the tendrils in some species of Bignonia, but in the Hanburya the layer is developed along the terminal portion of the tendril, sometimes for a length of 1¾ inch, but not at the extreme tip. The layer is white, whilst the tendril is green, and near the tip it could sometimes be seen to be thicker than the tendril itself; it generally spreads a little beyond the sides of the tendril, and its edge is fringed with free elongated cells, which have enlarged globular or retort-shaped heads. This cellular layer apparently secretes some resinous cement; for its adhesion to the wood was not lessened by immersion for 24 h. in alcohol or water, but was quite loosened by the action during the same period of ether and turpentine. After the tendril has once firmly coiled itself round a stick, it is difficult to imagine of what use the formation of the adhesive cellular layer can be. Owing to the spiral contraction, which ensues after a time, whether or not the tendril has clasped any object, it was never able to remain, excepting in one instance, in contact with a thick post or a nearly flat surface; if it could have become attached to such objects by means of the adhesive cellular layer, this layer would evidently have been of service to the plant. I hear from Dr. Hooker that several other Cucurbitaceous plants have adherent tendrils.

Of other Cucurbitaceæ, I observed in Bryonia dioica, Cucurbita ovifera, and Cucumis sativa, that the tendrils were sensitive and