A tree can be defined as a large perennial woody plant. Though there is no set definition of minimum size, it is generally at least 6 m (20 ft) high at maturity, and with branches supported on a single main stem. Compared with most other forms of plants, trees are long-lived. A few species of trees grow to 100 m tall, and some can live for several millennia.

Trees occur in many diverse orders and families of plants, and thus show a wide variety of growth form, leaf types and shapes, bark, reproductive organs, etc. The earliest trees were tree ferns and horsetails, which grew in vast forests in the Carboniferous Period; tree ferns still survive, but the only surviving horsetails are not of tree form. Later, in the Triassic Period, conifers, ginkgos, cycads and other gymnosperms appeared, and subsequently flowering plants in the Cretaceous Period. Most species of trees today are flowering plants and conifers. The list below gives some examples of well known trees and how they are typically classified.

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