Correct Answer - c
As a result of continued secondary growth in subsequent years, the older part of secondary xylem or wood becomes non - functional (deal) s it loese 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 of activities, non - functional , secondary xylem becomes hard , durable and blaskish in colour , called heartwood Now , the function of secondary xylem (water and mineral conduction from roots) is performed by outer younger - rings of xylem which is called sapwood.
