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In botany, a tree is a perennial plant with an elongated stem, or trunk, supporting leaves or branches. In some usages, the definition of a tree may be narrower, including only woody plants, only plants that are usable as lumber or only plants above a specified height. At its broadest, trees include the taller palms, the tree ferns, bananas and bamboo. A tree typically has many secondary branches supported clear of the ground by the trunk. This trunk typically contains woody tissue for strength, and vascular tissue to carry materials from one part of the tree to another. For most trees it is surrounded by a layer of bark which serves as a protective barrier. Below the ground, the roots branch and spread out widely; they serve to anchor the tree and extract moisture and nutrients from the soil. Above ground, the branches divide into smaller branches and shoots. The shoots typically bear leaves, which capture light energy and convert it into chemical energy by photosynthesis, providing the food needed by the tree for its growth and development. Flowers and fruit may also be present, but some trees such as conifers instead have pollen cones and seed cones, and others such as tree ferns produce spores instead. Trees tend to be long-lived,[1] some reaching several thousand years old. The tallest known specimen on Earth is 115.6 m (379 ft) and they have a theoretical maximum height of 130 m (426 ft).[2] Trees have been in existence on the Earth for 370 million years. Trees are not a taxonomic group but are a number of plant species that have independently evolved a woody trunk and branches as a way to tower above other plants and make full use of the sunlight. Trees play a significant role in reducing erosion and moderating the climate. They remove carbon dioxide from the atmosphere and store large quantities of carbon in their tissues. Trees and forests provide a habitat for many species of animals and plants. Tropical rainforests are one of the most biodiverse habitats in the world. Trees provide shade and shelter, timber for construction, fuel for cooking and heating, and fruit for food as well as having many other uses. In parts of the world, forests are shrinking as trees are cleared to increase the amount of land available for agriculture. Because of their longevity and usefulness, trees have always been revered and they play a role in many of the world's mythologies. Contents [hide] 1 Definition 2 Overview 3 Distribution 4 Parts and function 4.1 Roots 4.2 Trunk 4.3 Buds and growth 4.4 Leaves 4.5 Reproduction 4.6 Seeds 5 Evolutionary history 6 Tree ecology 7 Uses 7.1 Food 7.2 Fuel 7.3 Timber 7.4 Bark 7.5 Other uses 8 Care 9 Mythology 10 Superlative trees 11 See also 12 References Definition

Although "tree" is a term of common parlance, there is no universally recognised precise definition what a tree is, neither botanically nor in common language.[3][4][5] In its broadest sense, a tree is any plant with the general form of an elongated stem, or trunk, which supports the photosynthetic leaves or branches at some distance above the ground.[6][7] Trees are also typically defined by height,[8][9][10] with smaller plants being classified as shrubs,[11] however the minimum height which defines a tree varies widely, from 10 m to 0.5 m.[10] By these broadest definitions, large herbaceous plants such as papaya and bananas are trees, despite not being considered as trees under more rigorous definitions.[3][5][12][13][14][15] Another criterion often added to the definition of a tree is that it has a woody trunk.[10][16][17] Such a definition excludes herbaceous trees such as bananas and papayas. Monocots such as bamboo and palms may be considered trees under such a definition.[18] Despite being herbaceous[19][20] and not undergoing secondary growth and never producing wood,[21][22][22][23] palms and bamboo may produce "pseudo-wood" by lignifying cells produced through primary growth. Aside from structural definitions, trees are commonly defined by use. Trees may be defined as plants from which lumber can be produced.[5] Overview

Trees are an evolutionary adaptation to competition for space. By growing taller trees are able to compete better for sunlight.[24] They have modified structures that allow them to grow much taller and spread out their foliage, such as thicker stems that are composed of specialized cells that add structural strength and durability. They are long-lived perennial plants that can increase their size each year by producing woody stems. They differ from shrubs, which are also woody plants, by usually growing larger and having a single main stem;[11] but the distinction between a small tree and a large shrub is not always clear,[25] made more confusing by the fact that trees may be reduced in size under harsher environmental conditions such as on mountains and subarctic areas. The tree form has evolved separately in unrelated classes of plants in response to similar environmental challenges, making it a classic example of parallel evolution. With an estimated 100,000 species, the number of trees worldwide might total twenty-five percent of all living plant species.[26] Their greatest number grow in tropical regions and many of these areas have not yet been fully surveyed by botanists, making tree diversity and ranges poorly known.[27] Trees exist in two different groups of vascular or higher plants, the gymnosperms and the angiosperms[clarification needed]. Both groups are seed plants. The gymnosperm trees include conifers, cycads, ginkgophytes and gnetales. Angiosperm trees are also known as broad-leaved trees. Most angiosperm trees are eudicots, the "true dicotyledons", so named because the seeds contain two cotyledons or seed leaves. A relatively smaller number of other angiosperm trees are paleodicots; these include Amborella, Magnolia, nutmeg, avocado, and others. Wood gives structural strength to a tree stem which is used to support the plant as it grows larger. The vascular system of trees allows water, nutrients and other chemicals to be distributed around the plant, and without it trees would not be able to grow as large as they do. The three main parts of trees include the root, stem, and leaves; they are integral parts of the vascular system which interconnects all the living cells. In trees and other plants that develop wood, the vascular cambium allows the expansion of vascular tissue that produces woody growth. Because this growth ruptures the epidermis of the stem, woody plants also have a cork cambium that develops among the phloem. The cork cambium gives rise to thickened cork cells to protect the surface of the plant and reduce water loss. Both the production of wood and the production of cork are forms of secondary growth. Trees are either evergreen, having foliage that persists and remains green throughout the year,[28] or deciduous, shedding their leaves at the end of the growing season and then having a dormant period without foliage.[29] Most conifers are evergreens but larches (Larix and Pseudolarix) are deciduous, dropping their needles each autumn, and some species of cypress (Glyptostrobus, Metasequoia and Taxodium) shed small leafy shoots annually in a process known as cladoptosis.[11] The crown is a name for the upper part[clarification needed] of a tree including the branches and leaves [30] and the uppermost layer in a forest, formed by the crowns of the trees, is known as the canopy.[31] A sapling is a young tree.[32] Tree-like plants include some palms which are not trees but herbaceous[19][20] monocots that do not undergo secondary growth and never produce wood,[21][22][22][23] and hence do not meet the definition of tree used in this article. In many tree-like palms, the terminal bud on the main stem is the only one to develop so they have tall, unbranched trunks with spirally arranged large leaves. Some of the tree ferns, order Cyatheales, have tree-like growth forms, growing up to 20 metres (66 ft) but they are structurally very different from other trees: their trunks are composed of rhizomes which grow vertically and which are covered by numerous adventitious roots.[33] Distribution

This article contains weasel words: vague phrasing that often accompanies biased or unverifiable information. Such statements should be clarified or removed. (September 2012) In places where the climate is suitable, trees are the climax vegetation[citation needed]. In some of the cool temperate regions, conifers tend to predominate, but in much of the southern hemisphere, the tropics, or in warm-temperate climates, broad-leaved trees are more common. Shade tolerance in young trees varies between species, and may determine the pattern of forest succession. More than half the species of terrestrial plants and animals on the Earth are thought to live in tropical rainforests even though these occupy just five percent of the land area.[34] In tropical regions with a monsoon climate, where a drier part of the year alternates with a wet period, different species of broad-leaved trees dominate the forest, some of them being deciduous.[35] Tropical regions with a drier savanna climate have insufficient rainfall to support dense forests[citation needed]; the canopy is not closed and plenty of sunshine reaches the ground which is covered with grass and scrub. Acacia and baobab are well adapted to living in such areas.[36] In cool temperate parts of the world, particularly in the northern hemisphere, deciduous broad-leaved trees tend to be replaced by conifers. The long cold winter is unsuitable for plant growth and trees must grow rapidly in the short summer season when the temperature rises and the days are long. Light is very limited under their dense cover and there may be little plant life on the forest floor although fungi may abound.[37] Similar woodland is found on mountains where the altitude causes the average temperature to be lower thus reducing the length of the growing season.[38] Parts and function

A young red pine (Pinus resinosa) with spread of roots visible, as a result of soil erosion. Roots The roots of a tree serve to anchor it to the ground and gather water and nutrients to transfer to all parts of the tree, and for reproduction defense, survival, energy storage and many, many other purposes. The first root produced by a newly germinated seedling is a taproot which goes straight downwards. Within a few weeks lateral roots branch out of the side of this and grow horizontally through the upper layers of the soil. In most trees, the tap root eventually withers away and the wide-spreading laterals remain. Near the tip of the finer roots are single cell root hairs. These are in immediate contact with the soil particles and can absorb water and nutrients such as potassium in solution. The roots require oxygen to respire and only a few species such as the mangrove and the pond cypress (Taxodium ascendens) can live in permanently waterlogged soil.[39] In the soil, the roots encounter the hyphae of fungi. Many of these are known as mycorrhiza and form a mutualistic relationship with the tree roots. Some are specific to a single tree species, which will not flourish in the absence of its mycorrhizal associate. Others are generalists and associate with many species. The tree acquires minerals such as phosphorus from the fungus while it obtains the carbohydrate products of photosynthesis from the tree.[40] The hyphae of the fungus can link different trees and a network is formed, transferring nutrients from one place to another. The fungus promotes growth of the roots and helps protect the trees against predators and pathogens. It can also limit damage done to a tree by pollution as the fungus accumulate heavy metals within its tissues.[41] Fossil evidence shows that roots have been associated with mycorrhizal fungi since the early Paleozoic, four hundred million years ago, when the first vascular plants colonised dry land.[42] Some trees such as the alders (Alnus spp.) have a symbiotic relationship with Frankia sp,, a filamentous bacterium that can fix nitrogen from the air, converting it into ammonia. They have actinorhizal root nodules on their roots in which the bacteria live. This process enables the tree to live in low nitrogen habitats where they would otherwise be unable to thrive.[43] Researchers have discovered that certain plant hormones called cytokinins initiate root nodule formation and that this process is closely related to the mechanisms involved in mycorrhizal association. [44] It has been demonstrated that some trees are interconnected through their root system, forming a colony. The interconnections are made by the inosculation process, a kind of natural grafting or welding of vegetal tissues. The tests to demonstrate this networking are performed injecting chemicals, sometimes radioactive, in a tree, and then checking for its presence in neighbor trees.[45]

Buttress roots of the kapok tree (Ceiba pentandra) The roots are, generally, a subterranean part of the tree, but some tree species have evolved roots that are aerial. The common purposes for aerial roots may be of two kinds, to contribute to the mechanical stability of the tree, and to obtain oxygen from air. An instances of mechanical stability enhancement is the red mangrove that develops prop roots that loop out of the trunk and branches and descend vertically into the mud.[46] A similar structure is developed by the Indian banyan.[47] Many large trees have buttress roots which flare out from the lower part of the trunk. These brace the tree rather like angle brackets and provide stability, reducing sway in high winds. They are particularly prevalent in tropical rainforests where the soil is poor and the roots are close to the surface.[48] Some tree species have developed root extensions that pop out of soil, in order to get oxygen, when it is not available in the soil because of excess water. These root extensions are called pneumatophores, and are present, among others, in black mangrove and pond cypress.[46]

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