MICHAEL MARTEN / SCIENCE PHOTO LIBRARY MICHAEL MARTEN / SCIENCE PHOTO LIBRARY
Close-up of the sawn end of the branch of an unidentified tree, showing the heart wood (dark, centre), sap wood (light) and bark (rusty brown). Sap wood is saturated with water and contains the living cells of the conducting tissue. After 10-15 years sap wood ceases to conduct and is converted into heart wood, which is a completely lifeless, dry tissue.
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