Page:Darwin - On the movements and habits of climbing plants.djvu/23

 thickness, yet at Kew a plant ascended a trunk above 6 inches in diameter. The tropical twiners, on the other hand, can ascend thick trees. I hear from Drs. Thomson and Hooker that this is the case with the Butea parviflora, one of the Menispermaceæ, and with some Dalbergias and other Leguminosæ. This power would evidently be almost necessary for twining plants inhabiting tropical forests, as otherwise they could hardly ever reach the light. In our temperate countries twining plants which die down every year to the root would suffer if they were enabled to twine round trunks of trees, for they could not grow tall enough in a single season to reach the summit and gain the light.

By what means some twining plants are adapted to ascend only thin stems, whilst others can twine round thick trees, I do not know. It appeared to me probable that twining plants with very long revolving shoots might be able to ascend thick supports; accordingly I placed Ceropegia Gardnerii near a post 6 inches in diameter, but the shoots entirely failed to wind round it; their length and power of movement apparently serving merely to find some distant but thin stem round which to twine. The Sphærostemma marmoratum is a vigorous tropical twiner, and as it is a very slow revolver, I thought that this latter circumstance might aid it in ascending a thick support; but though it was able to wind round the 6-inch post, it could do this only on the same level or plane, and could not ascend in a spire. We can, however, see, in accordance with the views previously explained, that a revolving shoot, which, after coming into contact with any support, quickly lost its power of movement, would not again be drawn away from its support by the returning or opposite movement, and therefore remaining in contact with it, might thus ascend a thick support. But whether this slight difference in retaining for some time or in quickly losing the power of movement after coming into contact with a support alone determines how thick an object the stem can ascend I do not know.

As ferns differ so much from phanerogamic plants, it may be worth while here to show that twining ferns act in no respect differently from other twining plants. In Lygodium articulatum the two internodes first formed above the root-stock did not move; the third from the ground revolved, and at first very slowly. This species is a slow revolver: but L. scandens made five revolutions at an average rate of 5 h. 45 m.; and this represents fairly well the usual rate, taking quick and slow movers, amongst phanerogamic plants. The rate was accelerated by increased temperature. The