Pith

Pith is a light substance that is derived from certain vascular plants. It consists of parenchyma cells, and is located within the center of the stem or root. It is encircled by a ring of vascular bundles. Mature woody structures typically do not possess a pith, because their secondary growth (in width), soon outpaces the pith and replaces it with woody tissue. Young structures within a tree may still have pith, even while the trunk and major branches no longer have any at all.