Page:Darwin - On the movements and habits of climbing plants.djvu/20

 (, continued.)

In the foregoing table, which includes twining plants belonging to as widely different orders as is possible, we see that the contraction or turgescence of the cells circulating round the axis, on which the revolving movement depends, differs much in rate. As long as a plant remains under the same conditions, the rate is often remarkably uniform, as we see with the Hop, Mikania, Phaseolus, &c. The Scyphanthus made one revolution in 1 h. 17 m., and this is the quickest rate observed; but we shall afterwards see a tendril-bearing Passiflora revolving even more rapidly. A shoot of the Akebia quinata made a revolution in 1 h. 30 m., and three revolutions at the average rate of 1 h. 38 m.; a Convolvulus made two revolutions at the average of 1 h. 42 m., and Phaseolus vulgaris three at the average of 1 h. 57 m. On the other hand, some plants take 24 h. for a single revolution, and the Adhadota sometimes required 48 h.; yet this latter plant is an efficient twiner. Species of the same genus move at different rates. The rate does not seem governed by the thickness of the shoots: those of the Sollya are as thin and flexible as string, but move slower than the thick and fleshy shoots of the Ruscus, which seems so little fitted for movement of any kind; the shoots of the Wistaria, which become woody, move faster than those of the Ipomæa or Thunbergia.

We know that the internodes, whilst very young, do not acquire their proper rate of movement; hence several shoots on the same plant may sometimes be seen revolving at different rates. The two or three, or even more, internodes which are first formed above the cotyledons, or above the perennial root-stock, do not move; these first-formed shoots can support themselves, and nothing superfluous is granted them.

A greater number of twiners revolve in a course opposed to that of the sun, or to the hands of a watch, than in the reversed