The outer, living portion of the xylem that conducts water is called?

• A. Heartwood
• B. Branch collar
• C. Sapwood
• D. Rays

Answer: (C)

Water moves through the sapwood, the living portion of the secondary xylem just under the bark. This outer layer contains functioning vessels and tracheids plus living parenchyma cells that actively transport water from roots to leaves. As trees age, this conducting tissue can convert to heartwood, the inner, nonconducting xylem filled with extractives that provide support and durability but no water transport. Rays are horizontal pathways that store and move nutrients radially, not the main water-conducting tissue, and the branch collar is a pruning-related area at the base of a branch, not part of the xylem’s water-conducting zone. So the outer living part that conducts water is the sapwood.