English: 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.)