Page:The Oak.djvu/61

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43
THE SEEDLING AND YOUNG PLANT.

Now, the important point to apprehend first is that these strands at the corners (m, s) are the strands which pass directly into the leaves through the petioles, and it is necessary to be perfectly clear on this subject in order to understand much of what follows. For instance, the three strands marked m in Fig. 9, a (mm, ms, and ms in Fig. 10), pass directly into a given leaf, mm, in the middle, flanked by ms on either side; but this group is also accompanied on each side by another strand (marked s, s' in Fig. 9, a, and l, l in Fig. 10), so that five strands may be regarded as contributing to each corner of the section, the three middle ones running side by side up the midrib of the leaf and then branching out in a manner to be described subsequently.

It can be shown, moreover, that the larger curved strands, occupying the sides of the pentagon, are simply formed by the union of several of the smaller strands at different levels.

If, now, successively lower sections are cut of the very young shoot, and compared, or if the shoot is softened and dissected, it is possible to make out the course of these vascular bundle strands lower down; the course is somewhat complex, but the diagrammatic sketches in Fig. 11 will enable the reader to apprehend the chief points.

In the first place, the middle strand from a leaf, mm, passes vertically down in the angle of the young stem through five internodes (marked by the horizontal