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28.—The various chief elements of the wood of the oak, isolated by maceration, and highly magnified: f, a fiber, distinguished by its thick walls, simple slit-like pits, and no contents; w.p, part of a row of wood-parenchyma cells, with simple pits, and containing starch in winter; tr, a tracheid, distinguished from the fiber especially by its bordered pits; p.v, part of a rather large pitted vessel, made up of communicating segments, each of which corresponds to a tracheid, and has bordered pits on its walls; sp, part of a spiral vessel.