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 sapwood is the part of the wood that is located between the bark and the heartwood. In contrast to heartwood that is found at the inner region of the wood, the sapwood is at the outermost part. The sapwood can also be distinguished from the heartwood in being light in color. However, the sapwood may become dark and discoloured such as when decaying. The sapwood is also softer and not as durable since the heartwood is more resistant to decay.
The sapwood consists of the living wood that primarily functions in the conduction of sap. Hence, all wood in a tree would initially be sapwood, then may form heartwood (from the center outwards) as the tree ages.
The volume of the sapwood may be used as an indication of the growth history of a tree. For instance, a tree that is able to grow rapidly in the open would have thicker sapwood. In contrast, a tree that grows in dense forest tends to have relatively thinner sapwood.