Page:Darwin - On the movements and habits of climbing plants.djvu/21

 course, and, consequently, the majority, as is well known, ascend their supports from left to right. Occasionally, though rarely, plants of the same order twine in opposite directions, of which Mohl (S. 125) gives a case in the Leguminosæ, and we have in the table another in the Acanthaceæ. At present no instance is known of two species of the same genus twining in opposite directions; and this is a singular fact, because different individuals of Solanum dulcamara (Dutrochet, tom. xix. p. 299) revolve and twine in both directions: this plant, however, is a most feeble twiner. Loasa aurantiaca (Léon, p. 351) offers a much more striking case: I raised seventeen plants: of these eight revolved in opposition to the sun, and ascended from left to right; five followed the sun, and ascended from right to left; and four revolved and twined first in one direction, and then reversed their course, the petioles of the opposite leaves affording a point d'appui for the reversal of the spire. One of these four plants made seven spiral turns from right to left, and five turns from left to right. These individuals of the Loasa are interesting, as showing how almost every change is effected most gradually. For another plant in the same family, the Scyphanthus elegans, habitually twines in this manner. I raised many plants of it, and the stems of all took one turn, or occasionally two or even three turns in one direction, and then, ascending for a short space straight, reversed their course and took one or two turns in an opposite direction. The reversal of the curvature occurred at any point in the stem, even in the middle of an internode. Had I not seen this case, I should have thought its occurrence most improbable. It could hardly occur with any plant which ascended above a few feet in height, or which lived in an exposed situation; for the stem could be easily pulled from its support with little unwinding; nor could it have adhered at all, had not the internodes soon become moderately rigid. With leaf-climbers, as we shall soon see, analogous cases frequently occur; but these present no difficulty, as the stem is secured by the clasping petioles.

In the many other revolving and twining plants observed by me, I never but twice saw the movement reversed; once, and only for a short space, in Ipomæa jucunda; but frequently with Hibbertia dentata. This plant at first much perplexed me, for I continually observed its long and flexible shoots, evidently well fitted for twining, make a whole or half or quarter circle in one direction