Heartwood differs from sapwood in

As a result of continued secondary growth in subsequent years, the older part of secondary xylem or wood becomes non-functional as it loses the power of conduction. The cells of this wood are filled with resins or tannins produced by adjacent functional cells. The activities of vessels become blocked by tyloses. Due to these activities, non-functional, secondary xylem becomes hard, durable and blackish in colour, called heart-wood. Now, the function of secondary xylem (water and mineral conduction from roots) is performed by outer younger-rings of xylem, which is called sap wood.