Pith

Pith, or MEDULLA, is the central cylinder of tissue in the stems of Dicotyledons and Gymnosperms. In all plants where it is found it is continuous in the young state. In older plants it may be continuous, as in the elder, oak, &c., in the form of transverse discs, as in the walnut, or wanting in the internodes, as in hemlock, &c. In very young plants it is composed of thin-walled cells filled with protoplasm and cell-sap, and takes part in the conduction of nutritive substances throughout the plant. The cell-walls usually remain very thin, the protoplasm is soon all used up within the cells, and their further growth ceases. Examined microscopically, pith cells are usually polygonal in transverse section, while they are rectangular in longitudinal section, and not much longer than broad. The ring of wood immediately surrounding the pith consists largely of spiral and annular vessels, and is known as the medullary sheath. The pith is connected with the cortex and bast by the medullary rays, which are composed of cells similar to those of the pith, and which convey sap to the inner parts of the stem. In the early life of most trees the pith serves as a storehouse for starch and other reserve substances; but as the tree becomes older the pith is crushed inwards, communication with the cortex and bast is physiologically obstructed, and the pith cells become dry and full of air.