Page:The Oak.djvu/70

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the seedling and young plant. (continued).

Structure of the Vascular Tissues, etc.

Before plunging into the intricacies of the vascular bundles it will be well to obtain some idea of the general plan of structure which they present on transverse section (Fig. 9). As already seen, each of the bundles of the ring consists of a xylem portion on the side next the center of the stem, and a phloëm portion on the side next the periphery, and these portions are separated by the cambium layer. The tissue in the center of the stem, and surrounded by the ring of bundles, is called the pith; the tissue outside the ring, and between it and the epidermis, is called the cortex; and the tissue left between the bundles is termed the primary medullary rays (Fig. 9).

It will, of course, be remembered that the term "ring," as used above, always expresses the fact that a cylinder is here viewed in section. Now, the cambium of the individual bundles soon unites across the primary medullary rays, and thus a complete hollow cylinder of cambium is formed throughout the stem, and, as we shall see later, throughout the root also. For the pres-