Annual rings | definition of annual rings by Medical dictionary
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annual rings a series of concentric circles found in the heartwood of trees, indicating the approximate age of the tree. Each ring is formed by the contrast in texture between spring wood and autumn wood. Spring wood in ANGIOSPERMS contains a high proportion of large, thin-walled XYLEM vessels and few fibres, for maximum water transport when the leaves are newly formed. Autumn wood contains fewer, smaller, thicker-walled vessels with many fibres for greater mechanical support. GYMNOSPERMS show similar contrasting structures in spring wood and autumn wood, although their water-conducting tissues contain TRACHEIDS, not vessels. Microscopic examination of annual rings (a science called DENDROCHRONOLOGY) has revealed many interesting facts about past climates and also allows wooden structures (e.g. building timbers) to be accurately dated.