What defines heartwood in a tree?

Heartwood is identified as the dense, central part of a tree. It consists of older xylem that has become non-conductive and typically darkens with age due to the accumulation of various substances, such as resins and tannins. This section of the tree is important for structural stability and strength, contributing to the overall durability of the tree. While heartwood does not serve in the transportation of water or nutrients like the outer layers do, its density and composition play a critical role in the tree's long-term longevity and health.

In contrast, the other options refer to different parts or functions of the tree: the outer layer of bark serves as a protective barrier; living cells that transport nutrients are found in the phloem; and photosynthesis occurs in the leaves and green parts of the plant where chlorophyll is present. Understanding the role of heartwood offers insight into tree anatomy and the importance of different woody components in a tree's growth and development.