Page:Darwin - On the movements and habits of climbing plants.djvu/22

 and then in the opposite direction; consequently, when I placed the shoots near thin or thick sticks, or stretched string, they seemed perpetually to be trying to ascend these supports, but failed. I then surrounded the plant with a mass of branched twigs; the shoots ascended, and passed through them, but several came out laterally, and their depending extremities seldom turned upwards as is usual with twining plants. Finally, I surrounded another plant with many thin upright sticks, and placed this plant near the other plant with the twigs; and now the Hibbertia had got what it liked, for it twined up the parallel sticks, sometimes winding round one and sometimes round several; and the shoots travelled laterally from one to the other plant; but as the plants grew older, some of the shoots twined regularly up a thin upright stick. Though the revolving movement was sometimes in one direction and sometimes in the other, the twining was invariably from left to right; so that the more potent or persistent movement of revolution must have been in opposition to the course of the sun. It would appear that this Hibbertia is adapted to ascend by twining, and to ramble laterally over the thick Australian scrub.

I have described this case in some detail, because, as far as I have seen, it is rare to find with twining plants any especial adaptations, in which respect they differ much from the more highly organized tendril-bearers. The Solanum dulcamara, as we shall presently see, can twine only round such stems as are both thin and flexible. Most twining plants apparently are adapted to ascend supports of different thicknesses. Our English twiners, as far as I have seen, never twine round trees, excepting the Honeysuckle (Lonicera periclymenum), which I have observed twining up a young beech-tree nearly 4½ inches in diameter. Mohl (S. 131) found that the Phascolus multiflorus and Ipomæa purpurea could not, when placed in a room with the light entering on one side, twine round sticks between 3 and 4 inches in diameter; for this interfered, in a manner presently to be explained, with the revolving movement. In the open air, however, the Phaseolus twined round a support of the above thickness, but failed in twining round one 9 inches in diameter. Nevertheless, some twiners of the warmer temperate regions can manage this latter degree of thickness; for I hear from Dr. Hooker that at Kew the Ruscus androgynus ascends a column 9 inches in diameter; and although a Wistaria grown by me in a small pot tried in vain for weeks to get round a post between 6 and 6 inches in