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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. 


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 


Ginkgoaceae family 
Ginkgo, Ginkgo biloba 


Cyatheaceae and Dicksoniaceae families 
Tree ferns, Cyathea, Alsophila, Dicksonia 

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