نخاع

250px|right|thumb|Sambucus|Elder shoot cut longitudinally to show the broad, solid pith (rough textured, white) inside the wood (smooth, yellow tinged). Scale in millimeters. 250px|right|thumb|Walnut shoot cut longitudinally to show the chambered pith found in this genus. Scale in millimeters. Pith, or medulla, is a tissue in the stems of vascular plants. Pith is composed of soft, spongy parenchyma cells, which in some cases can store starch. In eudicotyledons, pith is located in the center of the stem. In monocotyledons, it extends only into roots. The pith is encircled by a ring of xylem; th