Blaser, H. Weston Department of Botany, University of Washington, Seattle, Washington.
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The central zone of tissue of an axis in which the vascular tissue is arranged as a hollow cylinder. Pith is present in most stems and in some roots (Fig. 1). Stems without pith rarely occur in angiosperms but are characteristic of psilopsids, lycopsids, Sphenophyllum, and some ferns. Roots of some ferns, many monocotyledons, and some dicotyledons include a pith although most roots have xylem tissue in the center. The pith may be present or absent in the same axis, depending upon size or vigor, the larger segments commonly containing pith.
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