Page:The Oak.djvu/67

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49
THE SEEDLING AND YOUNG PLANT.

those primary medullary rays which happen to have been cut running between the bundles.

If we now trace the vascular bundles of the leaf-trace in the other direction—that is, up into the leaf—their course is simple enough, as shown in Figs. 10 and 11. The five bundles run through the midrib and the stronger lateral ribs to the tips and edges of the leaf, first breaking up into several strands in the petiole and midrib, and then becoming finer and finer as they give off the lateral strands. The median bundle does little more than run directly through the leaf as the midrib, becoming finer and finer as it nears the apex. The two lateral median bundles behave in a somewhat curious way. We have already seen how large and flat they are at the leaf insertion (Fig. 10). Soon after entering the petiole they break up into several strands, two of which converge and take a course along the dorsal side of the midrib, thus nearly completing a cylinder of bundles inclosing a pith; moreover, the xylem portions of these bundles are all turned inward towards the pith.

The lateral bundles, coming obliquely into the leaf insertion, pass up the midrib side by side with the above, and, like them, break up into parallel strands. Before entering the midrib they give off small bundles (fst in Fig. 10) to the pair of minute stipules which flank the petiole. As the strands pass along the midribs and chief lateral ribs they interosculate in various degrees, and give off smaller side branches into the mesophyll of the leaf (see Chapter VI).