Tree | Definition, Structure, Uses, Importance, & Facts | Britannica

Phylogenetic classifications

Trees are represented in each of the major groups of the vascular plants: pteridophytes (seedless vascular plants that include the tree ferns), gymnosperms (cycads, ginkgoes, and conifers), and angiosperms (flowering plants).

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Although tree ferns account for only a small percentage of ferns, many are conspicuous members of a forest, attaining heights of 7 to 10 meters (23 to 33 feet); some are 15, 18, or occasionally 24 meters tall (49, 59, or 79 feet). These graceful trees, which are natives of humid montane forests in the tropics and subtropics and of warm temperate regions of the Southern Hemisphere, have huge lacy leaves; they are the remnants of a vastly more numerous flora that populated much of Earth during the Carboniferous Period (about 358.9 to 298.9 million years ago).

Cycads compose the Cycadophyta, a division of gymnospermous plants consisting of 4 families and approximately 140 species. Natives of warm regions of the Eastern and Western hemispheres, they also are remnants of a much larger number of species that in past geologic ages dominated Earth’s flora.

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The ginkgo is the only living representative of the gymnosperm division Ginkgophyta. It is a relic that has been preserved in cultivation around ancient Buddhist temples in China and planted elsewhere as an ornamental since the mid-18th century; the tree probably no longer exists in a wild state.

Conifers (division Pinophyta) are the largest group of gymnosperms and include trees and shrubs in 7 extant families and 545 species. Familiar representatives are araucarias, cedars, cypresses, Douglas firs, firs, hemlocks, junipers, larches, pines, podocarps, redwoods, spruces, and yews.

Dominating Earth’s present flora are the angiosperms, with about 352,000 described species, among which are the majority of the world’s trees. Angiosperms are sometimes divided on the basis of a group of characteristics into two major groups: the monocotyledons and the eudicotyledons. The most numerous of the monocotyledonous trees are palms; others include agaves, aloes, dracaenas, screw pines, and yuccas. By far the greatest number of tree species are eudicotyledons; they are represented by such familiar groups as birches, elms, hollies, maples, oaks, poplars, ashes, and willows.