The darker part of annual growth rings in woody stems is formed by

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The darker part of annual growth rings in woody stems is formed by summer wood, which is the denser, darker wood that is produced at the end of the growing season. In contrast, spring wood is the lighter, less dense wood that is produced early in the growing season. The terms sap wood and heart wood refer to the outer and inner layers of wood in a tree trunk, respectively, while secondary phloem refers to the layer of tissue that transports sugars and other nutrients throughout the plant.

The Darker Part of Annual Growth Rings in Woody Stems