Heartwood differs from sapwood in
a)	Presence of rays and fibres	b)	Absence of vessels and parenchyma
c)	Having dead and non-conducting elements	d)	Being susceptible to pests and pathogens

(c)

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 heartwood.