Page:The Oak.djvu/58

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chief points of difference from the root are that the xylem and phloëm of these vascular bundles of the stem do not alternate on the section, as they did in the root, but the phloëm of each bundle is on the same radius as the xylem; and that there is no pericycle, for branches

Fig. 9.—Transverse sections through very young twigs of oak, showing the vascular bundles of the stem (P and X), arranged in a ring round the pith, and joined by the cambium ring—the fine line passing through the bundles; M and s, the vascular bundles passing down from the leaves—M the median bundles and s the lateral bundles. The external outline is the epidermis; the letters P P stand in the primary cortex; the letters X X stand in the pith; the primary medullary rays separate the bundles. (After Müller.)

are not developed endogenously as rootlets are. Then there are some important differences in the mode of origin of these vascular bundles in space. We saw that in the root the first-formed spiral vessels are developed at the outer parts of the axis-cylinder, nearest the cortex, and the succeeding vessels are formed in centripetal order from these points. In the young stem the exact