Page:Darwin - On the movements and habits of climbing plants.djvu/13

 dozen had its leaves arranged in a perpendicular line: so I conclude that there is nothing remarkable in Palm's statement.

The leaves of twining-plants rise from the stem (before it has twined) either alternately, or oppositely, or in a spire; in this latter case the line of insertion of the leaves and the course of revolution or of twining coincide. This fact has been well shown by Dutrochet, who found different individuals of Solanum Dulcamara twining in opposite directions, and these had their leaves spirally arranged in opposite directions. A dense whorl of many leaves would apparently be incommodious for a twining plant, and some authors have supposed that none have their leaves thus arranged; but a twining Siphomeris has whorls of three.

If a stick which has arrested a revolving shoot, but has not as yet been wound round, be suddenly taken away, the shoot generally springs forward, showing that it has continued to press against the stick. If the stick, shortly after having been wound round, be withdrawn, the shoot retains for a time its spiral form, then straightens itself, and again commences to revolve. The long, much-inclined shoot of the Ceropegia previously alluded to offered some curious peculiarities. The lower and older internodes, which continued to revolve, had become so stiff that they were incapable, on repeated trials, of twining round a thin stick, showing that the power of movement was retained after flexibility had been lost. I then moved the stick to a greater distance, so that it was struck by a point 2½ inches from the extremity of the penultimate internode; and it was then neatly wound round by this part and by the ultimate internode. After leaving the spirally wound shoot for eleven hours, I quietly withdrew the stick, and in the course of the day the curled part straightened itself and recommenced revolving; but the lower and not curled portion of the penultimate internode did not move, a sort of hinge separating the moving and the motionless part of the same internode. After a few days, however, I found that the lower part of this internode had likewise recovered its revolving power. These several facts show that, in the arrested portion of a revolving shoot, the power of movement is not immediately lost, and that when temporarily lost it can be recovered. When a shoot has remained for a considerable time wound round its support, it permanently retains its spiral form even when the support is removed.