Page:Darwin - On the movements and habits of climbing plants.djvu/66

 .—Pisum sativum.—The common Pea was the subject of a valuable memoir by Dutrochet, who discovered that both the internodes and tendrils revolved in ellipses. The ellipses are generally very narrow, but sometimes approach to circles: I several times observed that the longer axis slowly changed its direction, which is of importance, as the tendril thus sweeps a much wider circuit. Owing to this change of direction, and likewise to the movement of the stem towards the light, the successive irregular ellipses generally form an irregular spire. I have thought it worth while to annex a tracing of the course pursued by the upper internode (the movement of the tendril being neglected) of a young plant from 8.40 A.M. to 9.15 P.M. The course was traced on a hemispherical glass placed over the plant, and the dots with figures give the hours of observation; each dot was joined by a straight line: no doubt these lines, if the course had been observed at shorter intervals, would have been all curvilinear. The extremity of the petiole, where the young tendril arises, was 2 inches from the glass, so that if a pencil 2 inches long had been in imagination affixed to the petiole, it would have traced the annexed figure on the under side of the glass; but it must be remembered that the figure is here reduced one-half. Neglecting the first great sweep towards the light or window, the end of the petiole swept a space 4 inches across in one direction, and 3 inches in another. As a full-grown tendril is considerably above 2 inches in length, and as the