The line followed by the apex of a leaf or leaflet, whilst describing one or more ellipses during the day, is often zigzag, either throughout its whole course or only during the morning or evening:  Robinia offered an instance of zigzagging confined to the morning, and a similar movement in the evening is shown in the diagram (Fig. 126) given under Sida.  The amount of the zigzag movement depends largely on the plant being placed under highly favourable conditions.  But even under such favourable conditions, if the dots which mark the position of the apex are made at considerable intervals of time, and the dots are then joined, the course pursued will still appear comparatively simple, although the number of the ellipses will be increased; but if dots are made every two or three minutes and these are joined, the result often is that all the lines are strongly zigzag, many small loops, triangles, and other figures being also formed.  This fact is shown in two parts of the diagram (Fig. 150) of the movements of Desmodium gyrans.  Strephium floribundum, observed under a high temperature, [page 406] made several little triangles at the rate of 43 m. for each.  Mimosa pudica, similarly observed, described three little ellipses in 67 m.; and the apex of a leaflet crossed 1/500 of an inch in a second, or 0.12 inch in a minute.  The leaflets of Averrhoa made a countless number of little oscillations when the temperature was high and the sun shining.  The zigzag movement may in all cases be considered as an attempt to form small loops, which are drawn out by a prevailing movement in some one direction.  The rapid gyrations of the little lateral leaflets of Desmodium belong to the same class of movements, somewhat exaggerated in rapidity and amplitude.  The jerking movements, with a small advance and still smaller retreat, apparently not exactly in the same line, of the hypocotyl of the cabbage and of the leaves of Dionaea, as seen under the microscope, all probably come under this same head.  We may suspect that we here see the energy which is freed during the incessant chemical changes in progress in the tissues, converted into motion.  Finally, it should be noted that leaflets and probably some leaves, whilst describing their ellipses, often rotate slightly on their axes; so that the plane of the leaf is directed first to one and then to another side.  This was plainly seen to be the case with the large terminal leaflets of Desmodium, Erythrina and Amphicarpaea, and is probably common to all leaflets provided with a pulvinus.

With respect to the periodicity of the movements of sleeping leaves, Pfeffer* has so clearly shown that this depends on the daily alternations of light and darkness, that nothing farther need be said on this

* ‘Die Periodischen Bewegungen der Blattorgane,’ 1875, p. 30, et passim. [page 407]