In vascular plants, the xylem is the vascular tissue that is responsible for the conduction of water and nutrients from the roots. It may be classified as primary or secondary, depending on the origin of growth. In particular, the primary xylem comes from the procambium whereas the secondary xylem arises from the vascular cambium. The secondary xylem may differentiate into sapwood and heartwood.
The heartwood is located at the inner part (center) of the wood. It is called as such because of being located at the center of the wood. Some trees, though, do not form heartwood but only sapwood. Compared with sapwood, the heartwood is darker, harder, and rather resistant to decay. This is due to the deposition of various chemical substances such as gums, tannins, or pigments. The darker colour of the heartwood can make it easier to distinguish from the lighter sapwood.
The heartwood does not conduct sap. Its vessels are blocked due to tyloses. Thus, the heartwood is involved in providing structural support.