Pith

Pith, the central conjunctive tissue of the root and stem, known technically as the medulla. It varies considerably in amount, being relatively largest in such underground structures as potato tubers, in herbaceous stems, and in the elder. Its cells may become prosenchymatous; but seldom become sclerotic. They generally lose their protoplasmic contents at an early stage, and either shrivel up, or are ruptured by rapid elongation of the organ. Thus, in the centre of a sound oak stem the pith remains as a line of dust-like cells; whilst in the internodes of most grasses (straw, bamboo, etc.), and throughout the aerial stems of many herbaceous perennials among the Umbelliferoe (q.v.) and Composite (q.v.) it is so ruptured as to leave a hollow stem.