What is the term for the dead, inactive phloem located in the center of a tree?

Heartwood refers specifically to the central core of a tree trunk, where the wood is formed from dead and inactive phloem tissues. As trees grow, the outer layers of phloem and xylem are pushed outward, and the older phloem that is no longer functioning is transformed into heartwood. This heartwood serves as a support structure for the tree, providing strength and stability. Unlike the sapwood, which is the outer, active layer that transports nutrients and water, heartwood is primarily composed of non-conductive and often darker wood, reflecting its older, inactive state. This distinction is essential in understanding tree anatomy and the role each component plays in future growth and health.