stem from the insertion of a leaf is spoken of collectively as the "leaf-trace." Hence we see the leaf-trace of the oak consists of five bundles—one median, two lateral median, and two lateral; and since the phyllotaxis of the oak is two fifths, there will be twenty-five bundles in various stages of separation or conjunction coming down in the five internodes between any one leaf and the leaf vertically above it, as well as the parts of bundles from other leaves which are still continuing their course for a short time.
Now, since the main lengths of the course (in the stem) of these bundles is nearly vertically downward, with slight swerves to one side or another as the strands join, it is obvious that on the transverse section of the stem the bundles will appear arranged in a series round the center—in fact, they will form on the whole a more or less regular ring of bundles dividing off the pith from the cortical portions of the stem. Even in the very young condition (Fig. 9) we see bundles or groups of strands thus surrounding the pith, only the "ring" which they make is a sinuous one, so that the pith is five-rayed—a characteristic point in the oak. At a slightly later stage, as we shall see, this ring of bundles becomes more nearly circular from the gradual filling up of irregularities.
Before proceeding further it is necessary to make clear one or two other points. Since all the vascular bundles in the oak-stem are bundles which are common to the stem and leaf, they are termed "common bun-