pith science definition

The soft, spongy tissue in the center of the stems of most flowering plants, gymnosperms, and ferns. Pith is composed of parenchyma cells. In plants that undergo secondary growth, such as angiosperms, the pith is surrounded by the vascular tissues and is gradually compressed by the inward growth of the vascular tissue known as xylem. In plants with woody stems, the pith dries out and often disintegrates as the plant grows older, leaving the stem hollow. See illustration at xylem.