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A tree is defined as a perennial plant 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 coppice, 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 modern cycads no longer appear as trees). 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.
Flowering plants (Magnoliophyta)
Aceraceae family
Norway maple, Acer platanoides
European Sycamore, Acer pseudoplatanus
Red maple, Acer rubrum
Aquifoliaceae family
Common Holly, Ilex aquifolium
Tarajo Holly, Ilex latifolia
Betulaceae family
Common Alder, Alnus glutinosa
Silver Birch, Betula pendula
Fagaceae family
Sweet Chestnut, Castanea sativa
American Beech, Fagus grandifolia
Oriental Beech, Fagus orientalis
Common Beech, Fagus sylvatica
Black Beech, Nothofagus solandri
English Oak, Quercus robur
Fouquieriaceae family
Boojum, Fouquieria columnaris
Juglandaceae family
Common Walnut, Juglans Regia
Black Walnut, Juglans nigra
Hickories, Carya species
Leguminosae family
Honey locust (Gleditsia triacanthos)
Pau Brasil, Brazilwood, South American Redwood, Caesalpinia echinata
Sappanwood, East Indian Redwood, Caesalpinia sappan
Meliaceae family (Mahogany family)
Neem, Azadirachta indica (A. Juss)
Thai Neem, Azadirachta siamensis
Marango, Azadirachta excelsa (Jack)
Persian Lilac, Melia azedarach
Melia toosendan
Gurke, Melia volkensii
Mahogany, Swietenia mahagoni
Myristicaceae family
Nutmeg, Mysristica fragrans
Myrtaceae family
Silver Gum, Eucalyptus cordata
Oleaceae family
Olive, Olea europaea
Palmae family, also called Palmaceae or Arecaceae, the palm family
Areca Nut, sometimes miscalled "Betel Nut", Areca catechu
Coconut Cocos nucifera
Canary Island Date Palm, Phoenix canariensis
date Palm, Phoenix dactylifera
Chusan Palm, Trachycarpus fortunei
Rhizophoraceae, the mangrove family
Red Mangrove, Rhizophora mangle
Rosaceae family
Common Hawthorn, Crataegus monogyna
apple, Malus domestica
Apricot, Prunus armeniaca
Almond, Prunus dulcis
Fuji Cherry, Prunus incisa
Peach, Prunus persica
Pear, Pyrus communis
Rowan, Sorbus aucuparia
Rubiaceae family
coffee, Coffea arabica
coffee, Coffea robusta
Rutaceae family
Lime, Citrus aurantiifolia
Sour Orange, Citrus aurantium, Citrus bigaradia, Citrus vulgaris
Sweet Orange, Citrus sinensis
Grapefruit, Citrus paradisi
Mandarin, also called Tangerine, Citrus reticulata, Citrus nobilis
Lemon, Citrus limon, Citrus limonum
Satsuma, Citrus unshiu, Citrus reticulata, Citrus nobilis
Salicaceae family
Aspen, Populus tremula
Sapotaceae family
Tambalacoque, or dodo tree, Sideroxylon grandiflorum, previously Calvaria major
Sterculiaceae family
Cacao, the source of cocoa, Theobroma cacao
Tiliaceae family. The Lime trees in this family are also known as Linden trees: see
Tilia.
Basswood (also known as American Lime or American Linden), Tilia americana
Common Lime, Tilia europaea
Mongolian Lime, Tilia mongolica
Silver Lime, Tilia tomentosa, Tilia argentea
Banana trees are not actually trees.
Conifers
Family Cupressaceae
Alerce or Patagonian cypress, Fitzroya cupressoides
Eastern redcedar, Juniperus virginiana
Family Pinaceae
Norway spruce, Picea abies
European larch, Larix decidua
Stone pine, Pinus pinea
Monterey pine, Pinus Radiata
White pine, Pinus strobus
Scots Pine, Pinus sylvestris
Douglas-fir, Pseudotsuga menziesii
Bigcone Douglas-fir, Pseudotsuga macrocarpa
Family Taxodiaceae
Japanese Cedar, Cryptomeria japonica
California coast redwood, Sequoia sempervirens
Giant sequoia or Giant redwood, Sequoiadendron giganteum
Chinese dawn redwood, Metasequoia glyptostroboides
Bald cypresses, Taxodium distichum, T. ascendens, T. mucronatum
Ginkgos
Ginkgoaceae family
Ginkgo, Ginkgo biloba
Ferns
Cyatheaceae and Dicksoniaceae families
Tree ferns, Cyathea, Alsophila, Dicksonia
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