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xylem
xylem Transport tissue of a plant, which conducts water and minerals from the roots to the rest of the plant and provides support. The most important cells are long, thin tapering cells called xylem vessels. These cells are dead and have no cross-walls; they are arranged in columns to form long tubes, up which water is drawn. As water evaporates from the leaves (transpiration), water is drawn across the leaf by osmosis to replace it, drawing water out of the xylem. This suction creates a tension in the xylem vessels, and the side walls are reinforced with rings or spirals of lignin, a rigid substance, to prevent them collapsing. Tiny holes in the walls of the xylem vessels (pits) allow water to cross from one tube to another. In trees, the xylem becomes blocked with age, and new xylem forms towards the outside of the trunk to replace it. The core of dead, non-functioning xylem remains an essential part of the support system. See also phloem; vascular bundle
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"xylem." World Encyclopedia. 2005. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "xylem." World Encyclopedia. 2005. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O142-xylem.html "xylem." World Encyclopedia. 2005. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O142-xylem.html |
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xylem
xylem A tissue that transports water and dissolved mineral nutrients in vascular plants. In flowering plants it consists of hollow vessels that are formed from cells (vessel elements) joined end to end. The end walls of the vessel elements are perforated to allow the passage of water. In less advanced vascular plants, such as conifers and ferns, the constituent cells of the xylem are called tracheids. In young plants and at the shoot and root tips of older plants primary xylem is formed by the apical meristems (see protoxylem; metaxylem). In plants showing secondary growth this xylem is replaced in most of the plant by secondary xylem, formed by the vascular cambium. The walls of the xylem cells are thickened with lignin, the extent of this thickening being greatest in secondary xylem. Xylem contributes greatly to the mechanical strength of the plant: wood is mostly made up of secondary xylem. See also fibre. Compare phloem.
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"xylem." A Dictionary of Biology. 2004. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "xylem." A Dictionary of Biology. 2004. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O6-xylem.html "xylem." A Dictionary of Biology. 2004. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O6-xylem.html |
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xylem
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MICHAEL ALLABY. "xylem." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. MICHAEL ALLABY. "xylem." A Dictionary of Plant Sciences. 1998. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O7-xylem.html MICHAEL ALLABY. "xylem." A Dictionary of Plant Sciences. 1998. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O7-xylem.html |
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xylem
xy·lem / ˈzīləm/ • n. Bot. the vascular tissue in plants that conducts water and dissolved nutrients upward from the root and also helps to form the woody element in the stem. Compare with phloem. |
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"xylem." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "xylem." The Oxford Pocket Dictionary of Current English. 2009. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1O999-xylem.html "xylem." The Oxford Pocket Dictionary of Current English. 2009. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O999-xylem.html |
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xylem
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"xylem." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. 28 May. 2012 <http://www.encyclopedia.com>. "xylem." The Columbia Encyclopedia, 6th ed.. 2011. Encyclopedia.com. (May 28, 2012). http://www.encyclopedia.com/doc/1E1-X-xylem.html "xylem." The Columbia Encyclopedia, 6th ed.. 2011. Retrieved May 28, 2012 from Encyclopedia.com: http://www.encyclopedia.com/doc/1E1-X-xylem.html |
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