From Bvio
:''This article is about the biological organisms known as trees. For other meanings of the word see tree (disambiguation).'' A '''tree''' can be defined as a large perennial woody plant. Though there is no set definition of size, it is generally at least 6 m (20 ft) high at maturity, and with branches supported on a single main stem. 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. Trees are important components of the natural landscape and significant elements in landscaping. Trees also play an important role in many of the world's mythologies. See Tree (mythology) for more information. ==Classification== Trees occur in many diverse orders and families of plants, and thus show a wide variety of growth form, leaf types and shapes, bark, reproductive organs, etc. The earliest trees were tree ferns and horsetails, which grew in vast forests in the Carboniferous Period; tree ferns still survive, but the only surviving horsetails are not of tree form. Later, in the Triassic Period, conifers, ginkgos, cycads and other gymnosperms appeared, and subsequently flowering plants in the Cretaceous Period. 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. ==Morphology== 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). Wood consists of ''xylem'' cells, and bark is primarily made of ''phloem''. Trees may be broadly grouped into ''exogenous'' and ''endogenous'' trees according to the way in which their stem diameter grows. Exogenous trees, which comprise the great majority of modern trees (all conifers, and all broadleaf trees), grow by the addition of new wood outwards, immediately under the bark. Endogenous trees, mainly in the monocotyledons (e.g. palms), grow by addition of new material inwards. As an exogenous tree grows, it creates growth rings. In temperate climates, these are commonly visible due to changes in the rate of growth with the temperature variation over the year. These can be counted to determine the age of the tree, and used to date cores or even wood taken from trees in the past; this is known as the science of dendrochronology. In tropical climates with near-constant climate, growth is continuous and does not form distinct rings, so age determination is impossible. Age determination is also impossible in endogenous trees. 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 optimizes exposure of the leaves to sunlight. Not all trees have all the plant organs mentioned above. For examples: most palm trees 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 size or shape of individual specimens. A spruce seedling does not fit the definition of a tree, but all spruces are trees. Bamboos by contrast, ''do'' show most of the characteristics of trees, yet are perhaps strangely rarely called trees. A small group of trees growing together is called a grove or copse, and a landscape covered by a large area of trees is called a forest. Several biotopes are defined largely by the trees that inhabit them, for example, rainforest and taiga; see ecozones. Large, but scattered trees with grassland (usually grazed or burned over periodically) in between is called savanna. ==Major tree genera== === Flowering plants (Magnoliophyta) === ====Dicotyledons (Magnoliopsida; broadleaf or hardwood trees)==== * Anacardiaceae (Cashew family) ** Cashew, ''Anacardium occidentale'' ** Mango, ''Mangifera indica'' ** Pistachio, ''Pistacia vera'' ** Lacquer tree, ''Toxicodendron verniciflua'' * Aquifoliaceae (Holly family) **Holly, ''Ilex'' species * Araliaceae (Ivy family) ** Kalopanax, ''Kalopanax pictus'' * Betulaceae (Birch family) ** Alder, ''Alnus'' species ** Birch, ''Betula'' species * Cactaceae (Cactus family) ** Saguaro, ''Carnegiea gigantea'' * Cornaceae (Dogwood family) ** Dogwood, ''Cornus'' species * Corylaceae (Hazel family) ** Hornbeam, ''Carpinus'' species ** Hazel, ''Corylus'' species * Dipterocarpaceae family ** Garjan ''Dipterocarpus'' species ** Sal ''Shorea'' species * Ericaceae (Heath family) ** Arbutus, ''Arbutus'' species * Fabaceae (Pea family) ** Honey locust, ''Gleditsia triacanthos'' ** Black locust, ''Robinia pseudoacacia'' ** Laburnum, ''Laburnum'' species ** Pau Brasil, Brazilwood, ''Caesalpinia echinata'' * Fagaceae (Beech family ) ** Chestnut, ''Castanea'' species ** Beech, ''Fagus'' species ** Southern beech, ''Nothofagus'' species ** Tanoak, ''Lithocarpus densiflorus'' ** Oak, ''Quercus'' species * Fouquieriaceae (Boojum family) ** Boojum, ''Fouquieria columnaris'' * Hamamelidaceae (Witch-hazel family) ** Sweetgum, ''Liquidambar'' species ** Persian ironwood, ''Parrotia persica'' * Juglandaceae (Walnut family) ** Walnut, ''Juglans'' species ** Hickory, ''Carya'' species * Lauraceae (Laurel family) ** Cinnamon ''Cinnamomum zeylanicum'' ** Bay laurel ''Laurus nobilis'' ** Avocado ''Persea americana'' * Lecythidaceae (Paradise nut family) ** Brazil nut ''Bertholletia excelsa'' * Lythraceae Loosestrife family ** Crape-myrtle ''Lagerstroemia'' species * Magnoliaceae (Magnolia family) ** Tulip tree, ''Liriodendron'' species ** Magnolia, ''Magnolia'' species * Malvaceae (Mallow family; including Tiliaceae and Bombacaceae)Image:Baobab.jpg
Baobab tree in South-Africa
Image:Yellowmaple.jpg
Yellow maple in fall
Image:Coastredwood.jpg
The coniferous Coast Redwood, the tallest tree species on earth
Categories: Plants | Forestry | Trees
