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Rh Medullary rays of two kinds, a smaller number of very broad, shining ones, from ½ to 1 mm., or even a centimetre or more apart, and very numerous (about



38.—Transverse section of wood of oak (magnified five diameters), showing five annual rings, as denoted by the large vessels of the spring wood; the vessels become smaller in the summer and autumn wood, and are arranged in tongue-like groups. Nine broad medullary rays are shown, the rest are very narrow (cf. Fig. 27). The rest of the section is filled with tracheids, fibers, and wood-parenchyma. (Müller.)