ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
Cross-section through the heartwood of an elm tree, Ulmus sp., showing the annual growth rings. The heartwood occupies the central core of a mature tree & is surrounded by a layer of sapwood. The sapwood is saturated with water and forms the tree's conducting tissue, whereas the heartwood is a completely lifeless, dry tissue which functions only as a structural support.
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