Page:Darwin - On the movements and habits of climbing plants.djvu/81

 which diverge equally from it. One of the branches (B) has a scale at its base, and is always, as far as I have seen, longer than the other, and very often bifurcates. The several branches when rubbed become curved, and subsequently straighten themselves. After a tendril has clasped any object by its extremity, it contracts spirally; but this does not occur (Palm, S. 56) when no object has been seized. The tendrils move spontaneously from side to side; and on a very hot day one made two elliptical revolutions at an average rate of 2 h. 15 m. During these movements a coloured line, painted along the convex surface, became first lateral and then concave. The separate branches have independent movements; after a tendril has spontaneously revolved for a time, it bends from the light towards the dark: I do not give this latter statement on my own authority, but on that of Mohl and Dutrochet; Mohl (S. 77) says that in a vine planted against a wall the tendrils point towards it, and in a vineyard generally more or less to the north.

The young internodes spontaneously revolve; but in hardly any other plant have I seen so slight a movement. A shoot faced a window, and I traced its course on the glass during two perfectly calm and hot days; during ten hours on one day it described a spire, representing two and a half ellipses. I likewise placed a bell-glass over a young muscat grape in a hothouse, and it made three or four extremely minute oval revolutions each day: the shoot moved less than half an inch from side to side; and had it not made at least three revolutions during the same day when the sky was uniformly overcast, I should have attributed the motion to the varying action of the light. The extremity of the shoot is more or less bent downwards; but the extremity never reverses its curvature, as so generally occurs with twining plants.

Various authors (Palm, S. 55; Mohl, S. 45; Lindley, &c.) believe that the tendrils of the vine are modified flower-peduncles. I here give a drawing (fig. 10) of the ordinary state of a flower-peduncle in bud: it consists of the "common peduncle" (A); of the "flower-tendril" (B), which is represented as having caught a twig; and of the "sub-peduncle" (C) bearing the flower-buds. The whole peduncle moves spontaneously, like a true tendril, but in a less degree, and especially when the sub-peduncle (C) does not bear many flower-buds. The common peduncle (A) has not the power of clasping a support, nor has the corresponding part in the true tendril. The flower-tendril (B) is always longer than the sub-peduncle (C), and has a scale at its base; it sometimes