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12.—Transverse section of young stem, showing primary vascular bundles, etc., highly magnified. a and h, the pith; c, primary cortex; i, epidermis; h, periderm (cork); g, collenchyma. Two complete primary vascular bundles, and parts of two others, are shown, separated by the primary medullary rays. r, spiral vessels (protoxylem); k, bast-fibers (protophloëm); n, m, cambium, separating the phloëm from the xylem; p, wood-parenchyma. Secondary medullary rays are seen in the bundles, as also are pitted vessels of different sizes. (Th. Hartig.)