Page:Darwin - On the movements and habits of climbing plants.djvu/88

 accompanying diagram we may compare the differences of a tendril (B) some weeks after attachment to a wall, with one (A) from the same plant, fully grown but unattached. That the change in the nature of the tissues of the tendril, as well as the act of spiral contraction, is consequent on the formation of the disks, is well shown by any lateral branches which have not become attached; for these in a week or two wither and drop off, in the same manner as does a whole tendril when unattached. The gain in strength and durability in a tendril after its attachment is something wonderful. There are tendrils now adhering to my house which are still strong and have been exposed to the weather in a dead state for fourteen or fifteen years. One single lateral branchlet of a tendril, estimated to be at least ten years old, was still elastic and supported a weight of exactly two pounds. This tendril had five disk-bearing branches of equal thickness and of apparently equal strength; so that this one tendril, after having been exposed during ten years to the weather, would have resisted a strain of ten pounds!

.—Cardiospermum halicacabum.—In this family, as in the last, the tendrils are modified flower-peduncles. In our present plant there are no organs exclusively used for climbing like ordinary tendrils; but the two lateral branches of the main flower-peduncle have been converted into a pair of tendrils, corresponding with the single "flower-tendril" of the common vine. The main peduncle is thin, stiff, and from 3 to 4½ inches in length. Near the summit, above two little bracts, it divides into three branches. The middle one divides and redivides, and bears the flowers; ultimately it grows half as long again as the two other modified branches. These latter are the tendrils; they are at first thicker and longer than the middle branch, but never become more than an inch in length. They taper to a point and are flattened, with the lower clasping surface destitute of hairs. At first they project straight up; but soon diverging, they spontaneously curl downwards so as to become symmetrically and elegantly hooked, as represented in the diagram. They are now, whilst the flower-buds are still small, ready for action.

The two or three upper young internodes steadily revolve; those on one plant made two circles, against the course of the sun,