pith

Pith is the soft, spongy tissue found at the center of many vascular plant stems and, less commonly, in roots. In stems it lies at the core, surrounded by the cortex and the vascular tissue, and is mainly composed of parenchyma cells. The pith stores nutrients such as water and starch and can participate in radial transport of substances through cell connections. Its presence and size vary among species and developmental stages; it is well developed in many herbaceous stems but is often reduced or eliminated by secondary growth in woody plants. In many monocots, a distinct pith is absent, with ground tissue distributed around scattered vascular bundles rather than forming a centralized core. In some species, the central cells die or degrade, leaving a hollow pith.

The term pith also has figurative uses. It denotes the essential or central part of something, as in the pith of the matter, and pithy describes concise, meaningful expression. Etymologically, pith derives from Old English pīþ and is related to terms in other Germanic languages referring to the core or essence.

Historically, the material called pith gave its name to pith helmets, a lightweight headgear traditionally made from plant pith. The term remains common in botanical descriptions, while its metaphorical sense persists in everyday language.