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| | [[Image:Bark nunniong.jpg|thumb|250px|Eucalypt bark]] | | [[Image:Bark nunniong.jpg|thumb|250px|Eucalypt bark]] |
| | [[Image:Pine bark.jpg|thumb|250px|right|[[Monterey Pine]] bark]] | | [[Image:Pine bark.jpg|thumb|250px|right|[[Monterey Pine]] bark]] |
| − | '''Bark''' is the outermost layer of [[Plant stem|stems]] and [[root]]s of [[woody plant]]s such as [[tree]]s. It overlays the [[wood]] and consists of three layers, the [[Cork (tissue)|cork]], the [[phloem]], and the [[vascular cambium]].
| + | [[Image:Pine_bark_tecpan_guatemala.JPG|thumb|200pix|right|Bark of a Pine tree in Tecpan, Guatemala.]] |
| | + | The word is often used in a general way to designate the softer outer envelope of a stem or root. In this sense, it includes all that peels readily, as the bark of the hemlock and oak, used for tanning leather. In a stricter sense, it is applied to the corky layers formed on the outer surface of woody plants. It is formed from an active layer of tissue,—the phellogen. The bark is developed in different ways on different trees. So distinct are the resulting tissues that species of trees may be readily recognized by their bark alone. Cork of commerce is the bark of the cork oak, a native of southwestern Europe. Inasmuch as the word covers so many structures, it is little used by botanists in technical descriptions. |
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| − | In young stems of woody plants like trees and shrubs and some perennial vines, the bark is
| + | {{glossary}} |
| − | made up of these tissues arranged from the outside surface to the inside:
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| − | * '''Cork''' - an external, secondary tissue impermeable to water and gases.
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| − | * '''Cork cambium''' - A layer of cells , normally one or two cell layers thick that is in a persistent meristematic state that produces cork.
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| − | * '''Phellederm''' - (not always present) A layer of cells formed in some plants from the inner cells of the cork cambium (Cork is produced from the outer layer).
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| − | * '''Cortex''' - The primary tissue of stems and roots. In stems the cortex is between the epidermis layer and the phloem, in roots the inner layer is not phloem but the [[pericycle]].
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| − | * '''Phloem''' - nutrient conducting tissue composed of sieve tub or sieve cells mixed with parenchym and fibers.
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| − | In old stems the epidermal layer, cortex, and primary phloem become separated from the inner tissues by thicker formations of cork. Due to the thickening cork layer these cells die because they do not receive water and nutrients. This dead layer is the rough corky bark that forms around tree trunks and other stems. In smaller stems and on typically non woody plants, sometimes a secondary covering forms called the periderm, which is made up of cork cambian, cork and phelloderm. It replaces the dermal layer and acts as a covering much like the corky bark, it too is made up of mostly dead tissue. The skin on the potato is a periderm.
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| − | The bark of some trees is edible.
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| − | Definitions of the term vary. In another usage, bark consists of the dead and protective tissue found on the outside of a woody stem, and does not include the vascular tissue. <!-- http://www.caf.wvu.edu/Bark/whatis.htm and many others, some further writing up needed -->
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| − | The vascular cambium is the only part of a woody stem where cell division occurs. It contains undifferentiated cells that divide rapidly to produce [[secondary]] [[xylem]] to the inside and [[secondary]] [[phloem]] to the outside.
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| − | Along with the [[xylem]], the phloem is one of the two tissues inside a plant that are involved with fluid transport. The phloem transports organic molecules (particularly sugars) to wherever they are needed.
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| − | Cork, sometimes confused with bark in colloquial speech, is the outermost layer of a woody stem, derived from the [[cork cambium]]. It serves as protection against damage, [[parasite]]s and [[disease]]s, as well as dehydration and extreme temperatures. Cork can contain [[antiseptic]]s like [[tannin]]s. Some cork is substantially thicker, providing further insulation and giving the bark a characteristic structure, in some cases thick enough to be harvestable as [[cork (material)|cork product]] without killing the tree.
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| − | Among the commercial products made from bark are [[cork (material)|cork]], [[cinnamon]], [[quinine]] (from the bark of [[Cinchona]]) and [[aspirin]] (from the bark of [[willow]] trees). The bark of some trees notably oak (''Quercus robur'') is a source of tannic acid, which is used in [[tanning]].
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| − | ==See also==
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| − | {{commonscat|Bark}} | |
| − | * [[Bark painting]]
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| − | * [[Bark beetle]]
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| − | * [[Trunk (botany)]]
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| − | [[Image:bark.jpg|thumb|200pix|right|Detail, bark of tree.]]
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| − | [[Image:Pine_bark_tecpan_guatemala.JPG|thumb|200pix|left|Bark of a Pine tree in Tecpan, Guatemala.]]
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| − | [[Category:Plant physiology]]
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