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Tree, woody perennial plant with a well-defined main stem, or trunk, that either dominates the form throughout the life cycle (giving a pyramidal shape) or is dominant only in the early stages, later forking to form a number of equally important branches (giving a rounded or flattened form to the tree). The trunk of a tree consists almost wholly of thick-walled water-conducting cells (xylem) that are renewed every year, giving rise to annual rings. The older wood in the center of the tree (the heartwood) is much denser and harder than the younger, outer sapwood. The outer skin, or bark, insulates and protects the trunk and often shows characteristic cracks or falls off, leaving a smooth skin. Trees belong to the two most advanced group of plants. The gymnosperms include the cone-bearing trees such as pine, spruce, and cedar; they are nearly all evergreens and most live in the cooler regions of the world. The angiosperms (flowering plants) have broader leaves and much harder wood; in tropical climates they are mostly evergreen, but in temperate regions they are deciduous.

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21st Century Webster's Family Encyclopedia21st Century Webster's Family Encyclopedia - Transcendentalism to United Church of Christ