Page:Darwin - On the movements and habits of climbing plants.djvu/99

 to those round the pencil, and consequently with a straight piece of string between the opposite spires. In short, we have given to the string the regular spiral arrangement of a tendril caught at both ends. The spiral contraction generally begins at the extremity which has clasped a support; and these first-formed spires give a twist to the axis of the tendril, which necessarily inclines the basal part into an opposite spiral curvature. I cannot resist giving one other illustration, though superfluous: when a haberdasher winds up ribbon for a customer, he does not wind it into a single coil; for, if he did, the ribbon would twist itself as many times as there were coils; but he winds it into a figure of eight on his thumb and little finger, so that he alternately takes turns in opposite directions, and thus the ribbon is not twisted. So it is with tendrils, with this sole difference, that they take several consecutive turns in one direction and then the same number in an opposite direction; but in both cases the self-twisting is equally avoided.

.—In the concluding remarks I shall have to allude to some points which may be here passed over. In the majority of tendril-bearing genera the young internodes revolve in more or less broad ellipses, like those made by twining plants; but the figures described, when carefully traced, generally form irregular ellipsoidal spires. The rate of revolution in different plants varies from one to five hours, and consequently in some cases is more rapid than with any twining plant, and is never so slow as with those many twiners, which take more than five hours for each revolution. The direction is variable even in the same individual plant. In Passiflora, the internodes of only one of the species have the power of revolving. The Vine is the weakest revolver observed by me, apparently exhibiting only a trace of a former power. In the Eccremocarpus the movement is interrupted by many long pauses. Some, but very few, tendril-bearing plants can spirally twine up an upright stick. Although the twining-power has generally been lost by tendril-bearers, either from the stiffness or shortness of the internodes, from the size of the leaves, or from other unknown causes, the revolving movement well serves to bring the tendrils into contact with surrounding objects.

The tendrils also have the power of revolving in the same manner and generally at the same rate with the internodes. The movement begins whilst the tendril is young, but is at first slow. In Bignonia littoralis even the mature tendrils moved much slower