THE TREE tree woody plant that regularly renews its growth (perennial). Most plants classified as trees have a single self-supporting trunk containing woody tissues, and in most species the trunk produces secondary limbs, called branches.
To many, the word tree evokes images of such ancient, powerful, and majestic structures as oaks andsequoias, the latter being among the most massive and longest-living organisms in the world. Although the majority of Earth’s terrestrialbiomass is represented by trees, the fundamental importance of these seemingly ubiquitous plants for the very existence and diversity of life on Earth is perhaps not fully appreciated. The biosphere is dependent on the metabolism, death, and recycling of plants, especially trees. Their vast trunks and rootsystems store carbon dioxide, move water, and produce oxygen that is released into the atmosphere. The organic matter of the soil develops primarily from decayed leaves, twigs, branches, roots, and fallen trees, all of which recycle nitrogen,carbon, oxygen, and other important nutrients. There are few organisms as important as trees for maintaining Earth’s ecology.
This article discusses the historical, popular, and botanical classifications of trees, their evolution, their importance to humans, and their general structure and patterns of growth. For more information on the three botanical groups that include trees, see fern (e.g., the tree ferns),gymnosperm (including conifers), and angiosperm (the flowering plants). For general information on plants, see plant.
Classification of trees
The ancient Greeks developed a classification about 300 BCE in which plants were grouped according to their general form—that is, as trees, shrubs, undershrubs, and vines. This classification was used for almost 1,000 years. Modern classifications of plants attempt to assign a plant to a particular taxon and establish relationships with other plants based on genetics, cytology, ecology, behaviour, and probable evolutionary lineages, in addition to gross morphology. Popular classifications, however, remain useful tools for studying the common stresses that the environment exerts on all plants and the general patterns of adaptation that are shown no matter how distantly plants are related.
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, andconifers), and angiosperms(flowering plants).
Although tree ferns account for only a small percentage of ferns, many are conspicuous members of a forest, attaining heights of 7 to 10 metres (23 to 33 feet); some are 15, 18, or occasionally 24 metres 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 theCarboniferous 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.