Tree
- This article is about the biological meaning of tree. For other meanings of the word see tree (graph theory) and tree data structure. A family tree can also be a chart of one's ancestors in genealogy.
Trees
 | | Baobab tree in South-Africa |
A tree can be defined as a large woody perennial plant. Though there is no set definition of size, it is generally at least 4.5 m (15 ft) high at maturity, and with branches supported on a single main stem. Trees are important components of the natural landscape and significant elements in landscaping. Compared with most other forms of plants, trees are long-lived. A few species of trees grow to over 100 m (300 ft) tall and some live for several millennia.
The component parts of a tree are the roots, trunk(s), branches, twigs and leaves. Tree stems consist mainly of support and transport tissues (xylem and phloem). In fact, wood consists of xylem cells, and the bark is primarily made of phloem. As a tree grows, it creates growth rings, which can be counted in temperate climates to determine the age of the tree, and used to date cores or even lumber taken from trees in the past, using the science of dendrochronology. The roots of a tree are generally embedded in earth, providing anchorage for the above-ground biomass and absorbing water and nutrients from the soil. Above ground, the trunk gives height to the leaf-bearing branches, aiding in competition with other plant species for sunlight. In many trees the arrangement of the branches optimize exposure of the leaves to sunlight.
A small group of trees growing together is called a grove or copse, and a landscape covered of many trees is called a forest. Several biotopes are defined largely by the trees that inhabit them, for example, the rainforest and the taiga. Large, but scattered trees with grassland (usually burned over periodically) in between is called a savanna.
Not all trees have the plant organs mentioned above. For examples: most palms are not branched, the saguaro cactus of North America has no functional leaves, tree ferns do not have bark, etc. Based on their rough shape and size, all of these are nonetheless generally regarded as trees. Indeed, sometimes size is the most important consideration. A plant form that is similar to a tree, but generally having smaller, multiple trunks and/or branches that arise near the ground, is called a shrub. However, no sharp differentiation between shrubs and trees is possible. Given their small size, Bonsai plants would not technically be 'trees', but one should not confuse reference to the form of a species with the the size or shape of individual specimens. A pine seedling does not fit the definition of a tree, but all pines are trees.
Trees often serve as important symbols in mythologies and religions. Examples are Yggdrasil in the Norse Mythology, the Christmas Tree that is derived from Germanic mythology, the Tree of Knowledge of Judaism and Christianity, and the Bodhi tree in Buddhism. In some religions, such as Hinduism, trees are said to be the homes of tree spirits.
Trees occur in many diverse families of plants, and thus show a wide variety of leaf types and shapes, bark, flowers, fruit, etc. The earliest trees were probably tree ferns, which grew in vast forests. Later the gymnosperms, ginkgos and cycads appeared. 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.
List of sample tree species
See also
Tree is the digit 3 in the NATO phonetic alphabet. It is pronounced as such, to prevent possible accidental confusion with other digits.
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