Tree: Difference between revisions

Revision as of 12:18, 25 June 2007

Revision as of 12:19, 25 June 2007

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[[Image:Raunkiaer.jpg|200px|right|thumb|An [[oak]] tree in [[Denmark]]]]

[[Image:TicinoTree.jpg|200px|right|thumb|A [[Sweet Chestnut]] tree in [[Ticino]], [[Switzerland]]]]

A tree is a plant form that occurs in many different [[order (biology)|orders]] and [[family (biology)|families]] of plants. Trees show a wide variety of growth forms,my chesnuts are really wet leaf type and shape, barkThebark characteristics, and reproductive organs.

The earliest trees were [[tree fern]]s and [[horsetail]]s, which grew in vast [[forest]]s in the [[Carboniferous]] Period; tree ferns still survive, but the only surviving horsetails are not of tree form. Later, in the [[Triassic]] Period, [[Pinophyta|conifers]], [[ginkgo]]s, [[cycad]]s and other [[gymnosperm]]s appeared, and subsequently [[flowering plant]]s in the [[Cretaceous]] Period. Most species of trees today are flowering plants ([[Angiosperms]]) and conifers. The listing below gives examples of many well-known trees and how they are typically classified.

A small group of trees growing together is called a [[grove (nature)|grove]] or '''copse''', and a landscape covered by a dense growth of trees is called a '''[[forest]]'''. Several [[biotope]]s are defined largely by the trees that inhabit them; examples are [[rainforest]] and [[taiga]] (see [[ecozone]]s). A landscape of trees scattered or spaced across grassland (usually grazed or burned over periodically) is called a '''[[savanna]]'''.

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[[Image:Leavessnipedale.jpg||thumb|right|200px|Leaves are an important feature of trees. These are [[Beech]] leaves.]]

[[Image:WisconsinScenery.jpg|200px|right|thumb|Tree [[root]]s anchor the structure and provide [[water]] and [[nutrients]]. The ground has [[erosion|eroded]] away around the roots of this young [[Pine]] tree.]]The basic parts of a tree are the [[root]]s, [[trunk (botany)|trunk]](s), [[branch]]es, [[twig]]s and [[leaf|leaves]]. Tree stems consist mainly of support and transport tissues ([[xylem]] and [[phloem]]). [[Wood]] consists of ''xylem'' cells, and [[bark]] is made of ''phloem'' and other tissues external to the vascular cambium. Trees may be broadly grouped into ''exogenous'' and ''endogenous'' trees according to the way in which their [[stem]] diameter increases. Exogenous trees, which comprise the great majority of modern trees (all [[Pinophyta|conifer]]s, and all [[dicotyledon|broadleaf]] trees), grow by the addition of new wood outwards, immediately under the bark. Endogenous trees, mainly in the [[monocotyledon]]s (e.g., [[palm tree|palms]]), grow by addition hairyof ballsnew material inwards.

As an exogenous tree grows, it creates [[growth ring]]s. Also known as annual rings, each set of light/dark rings is equivalent to one year of growth. In temperate climates, these are commonly visible due to changes in the rate of growth with temperature variation over an annual cycle. These rings 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 practice is known as the science of [[dendrochronology]]. In some tropical regions with constant year-round climate, growth is continuous and distinct rings are not formed, so age determination is impossible. Age determination is also impossible in endogenous trees.