What is heartwood?

Heartwood is indeed dead tissue that provides structural support for a tree. It comprises older xylem cells that have lost their function in conducting water and nutrients but still play a crucial role in the tree's strength and stability. As trees grow, the inner layers of the xylem become filled with resins and other compounds that enhance durability and resistance to decay. This contributes to the heartwood's characteristic darker color compared to the surrounding sapwood, which is still functional and contains living cells responsible for active water and nutrient transport.

The other options refer to different aspects of tree anatomy. Young tissue that grows in trees describes the cambium or new sapwood, which is vital for growth but not the same as heartwood. A nutrient-rich layer in trees is more akin to the cambium or inner bark, which helps in nutrient transport. The outer layer of a tree is known as the bark, serving as a protective layer but is not related to the structural support role that heartwood provides.