Page:The Oak.djvu/59

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41
THE SEEDLING AND YOUNG PLANT.

converse occurs—the first spiral vessels arise near the center of the stem, and development proceeds centrifugally from the first. We may begin our study of the shoot by tracing the course of the vascular bundles, which, it must be remembered, are the channels of communication between the water-supply at the roots below and the leaves and young parts of the shoot above.

If we cut a transverse section of the terminal bud of the oak, as close to the tip as possible, we shall obtain a preparation of the young axis consisting entirely of embryonic tissue, all the cells of which are practically alike—small, polygonal, thin-walled cells, with large nuclei and much protoplasm, but without sap-vacuoles; these cells are in a state of active division, those in the interior dividing successively in all planes. Those which form the peripheral layer, however, are already distinguished by only dividing in the two planes at right angles to the periphery, and they constitute the primitive epidermis. There is no structure corresponding to a root-cap.

Transverse sections a little lower down show differences of the following nature: In the first place, the outline of the section tends to be somewhat pentagonal, the points of origin of the very young leaves being at the angles of the pentagon in accordance with their phyllotaxis—i. e., the order in which the leaves are arranged on the stem. This is of such a nature that each leaf stands some distance above and to one side of its