ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY ADAM HART-DAVIS / SCIENCE PHOTO LIBRARY
Cross section through an old & weathered log from an elm tree. The difference between the heart wood (dark) and the sap wood (light) is seen clearly. Sap wood is saturated with water and contains the living cells of the conducting tissue. After 10-15 years sap wood is converted into heart wood, which is a completely lifeless, dry tissue. Because it is dry and less likely to warp or crack, heart wood is preferred in the timber trade. Hardwood trees such as oak contain more heart wood than the softwoods, such as pine, and are preferred as timber. The concentric rings here correspond to annual growth rings, a count of which would indicate the age of the tree.
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