The layer of material found in the innermost part of wood is called heartwood. In all trees after a period of useful life, the living part of wood in each ring ultimately dies. This results in gradual accumulation of dead wood in the center of the tree called heartwood.
The heart wood does not take part in the transportation of food materials but is mainly concerned with providing mechanical stability and supporting the heavy top part of the tree.
Heartwood is more durable than sapwood and it attains a darker color.