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[Image:Primary and secondary cotton roots.jpg|thumb|right|250px|Primary and secondary roots in a cotton plant]]
In [[vascular plant]]s, the '''root''' is that organ of a [[plant]] body that typically lies below the surface of the [[soil]] (compare with [[plant stem|stem]]). However, this is not always the case, since a root can also be [[aerial root|'''aerial''']] (that is, growing above the ground) or '''aerating''' (that is, growing up above the ground or especially above water). On the other hand, a stem normally occurring below ground is not exceptional either (see [[rhizome]]). So, it is better to define ''root'' as a part of a plant body that bears no leaves, and therefore also lacks [[Node (botany)|nodes]]. There are also important internal structural differences between stems and roots. The two major functions of roots are 1) absorption of water and inorganic nutrients and 2) anchoring the plant body to the ground. Roots also function in [[cytokinin]] synthesis, which supplies some of shoot needs. They often function in storage of food.
==Root structure==
[[Image:Roots.JPG|thumb|Roots of a [[Hydroponics|hydroponically]] grown plant]]
At the tip of every growing root is a conical covering of tissue called the '''root cap'''. It usually is not visible to the naked eye. It consists of undifferentiated soft tissue (parenchyma) with unthickened walls covering the '''[[Meristem|apical meristem]]'''. The root cap provides mechanical protection to the meristem cells as the root advances through the soil, its cells worn away but quickly replaced by new cells generated by cell division within the meristem. The root cap is also involved in the production of '''[[mucigel]]''', a sticky mucilage that coats the new formed cells. These cells contain '''statoliths''', starch grains that move in response to gravity and thus control root orientation.
The outside surface of the primary root is the [[epidermis (botany)|epidermis]]. Recently produced ''epidermal'' cells absorb water from the surrounding environment and produce outgrowths called '''[[trichome#root hairs|root hairs]]''' that greatly increase the cell's absorptive surface. Root-hairs are very delicate and generally short-lived, remaining functional for only a few days. However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die. The process by which water is absorbed into the epidermal cells from the soil is known as '''''[[osmosis]]'''''. For this reason, water that is [[salinity|saline]] is more difficult for most plant species to absorb.
[[Image:Root(cross section).jpg|thumb|left|325px|Cross section of the root of a [[dicotyledon]]]]
Beneath the epidermis is the '''cortex''', which comprises the bulk of the primary root. Its main function is storage of [[starch]]. Intercellular spaces in the cortex aerate cells for [[cellular respiration|respiration]]. An '''[[endodermis]]''' is a thin layer of small cells forming the innermost part of the cortex and surrounding the [[vascular tissue]]s deeper in the root. The tightly packed cells of the endodermis contain a substance known as [[suberin]] in their cell walls. This suberin layer is the [[Casparian strip]], which creates an impermeable barrier of sorts. Mineral nutrients can only move passively within root cell walls until they reach the endodermis. At that point, they must be actively transported across a cell membrane to continue further into the root. This allows the plant to accumulate mineral nutrients in the [[Stele (biology)|stele]].
The vascular cylinder, or stele, consists of the cells inside the endodermis. The outer part, known as the '''pericycle''', surrounds the actual [[vascular]] tissue. In [[monocotyledon]]ous plants, the [[xylem]] and [[phloem]] cells are arranged in a circle around a [[pith]] or center, whereas in [[dicotyledon]]s, the xylem cells form a central "hub" with lobes, and phloem cells fill in the spaces between the lobes.
===Secondary growth===
All roots have primary growth or growth in length. Roots of many vascula
r plants, especially [[dicot]]s and [[gymnosperm]]s, often undergo [[secondary growth]], which is an increase in diameter. A [[vascular cambium]] forms in the stele to produce secondary [[phloem]] and secondary [[xylem]]. The epidermis is replaced by a [[periderm]]. As the stele increases in diameter, the cortex, pericycle and endodermis are lost. Even nonwoody roots often undergo secondary growth, including those of [[tomato]] and [[alfalfa]].
==Root growth==
[[Image:Root-system.web.jpg|thumb|right|350px|Root systems of [[prairie]] plants]]
Early root growth is one of the functions of the '''apical meristem''' located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells (these sacrificed to protect the meristem), and undifferentiated root cells. The latter will become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues.
Roots will generally grow in any direction where the correct environment of [[Earth's atmosphere|air]], mineral [[nutrients]] and [[water]] exists to meet the plant's needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. At [[germination]], roots grow downward due to [[gravitropism]], the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as [[ivy]]), the "root" actually clings to walls and structures.
Growth from apical meristems is known as '''primary growth''', which encompasses all elongation. '''Secondary growth''' encompasses all growth in diameter, a major component of [[woody plant]] tissues and many nonwoody plants. For example, storage roots of [[sweet potato]] have secondary growth but are not woody. Secondary growth occurs at the [[lateral meristem]]s, namely the [[vascular cambium]] and [[cork cambium]]. The former forms [[secondary xylem]] and [[secondary phloem]], while the latter forms the [[periderm]].
In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a [[cylinder (geometry)|cylinder]] of tissue along the [[Plant stem|stem]] and root. The cambium layer forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem (including the epidermis and cortex, in many cases) tend to be pushed outward and are eventually "sloughed off" (shed).
At this point, the cork cambium begins to form the periderm, consisting of protective [[cork (material)|cork]] cells containing suberin. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.
[[Image:Roots by cesarpb.jpg|250px|thumb|Stilt roots in the [[Amazon Rainforest]] support a tree in very soft, wet soil conditions]]
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity but are responsible for most water transport through the vascular tissue in stems and roots.
==Types of roots==
A true root system consists of a '''primary root''' and '''secondary roots''' (or [[lateral roots]]).
The primary root originates in the [[radicle]] of the seedling. During its growth it rebranches to form the lateral roots. Generally, two categories are recognized:
*the [[taproot]] system: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward. Usually allows for deeper roots capable of reaching low water tables. Most common in [[dicots]]. The main function of the [[taproot]] is to store food.
* the diffuse root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. Most common in [[monocots]]. The main function of the fibrous root is to anchor the plant.
===Specialized roots===
[[Image:Mangroves.jpg|thumb|250px|right|Aerating roots of a [[mangrove]]]]
[[Image:Ceiba pentandra MS4104.JPG|thumb|250px|right|Buttress roots of ''[[Ceiba pentandra]]'']]
The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described in the introduction.
* '''Adventitious roots''' arise from the stem and not from another root. They usually occur in [[monocot]]s and pteridophytes, but also in many [[dicot]]s, such as [[clover]] (''Trifolium''), [[ivy]] (''Hedera''), [[strawberry]] (''Fragaria'') and [[willow]] (''Salix''). Most aerial roots and stilt roots are adventitious.
* '''Aerating roots''' (or '''pneumatophores'''): roots rising above the ground, especially above water such as in some [[mangrove]] genera (''[[Avicennia]], [[Sonneratia]]''). In some plants like ''Avicennia'' the erect roots have a large number of breathing pores for exchange of gases.
* '''Aerial roots''': roots entirely above the ground, such as in ivy (''Hedera'') or in [[epiphyte|epiphytic]] [[orchid]]s. They function as prop roots, as in [[maize]] or anchor roots or as the trunk in [[strangler fig]].
* '''Buttress roots''' or tabular roots: support roots for many tropical tree species
* '''Contractile roots''': they pull bulbs or corms of [[monocot]]s, such as [[hyacinth]] and [[lily]], and some taproots, such as [[dandelion]], deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.
* '''Haustorial roots''': roots of parasitic plants that can absorb water and nutrients from another plant, such as in [[mistletoe]] (''Viscum album'') and [[dodder]].
* '''Propagative roots''': roots that form adventitious buds that develop into aboveground shoots, termed [[Basal shoot|suckers]], which form new plants, as in [[Canada thistle]], [[cherry]] and many others.
* '''[[Proteoid root]]s''' or cluster roots: dense clusters of rootlets of limited growth that develop under low [[phosphate]] or low [[iron]] conditions in [[Proteaceae]] and some plants from the following families [[Betulaceae]], [[Casuarinaceae]], [[Eleagnaceae]], [[Moraceae]], [[Fabaceae]] and [[Myricaceae]].
* '''Stilt roots''': these are adventitious support roots, common among [[mangrove]]s. They grow down from lateral branches, branching in the soil.
* '''Storage roots''': these roots are modified for storage of food or water, such as [[carrot]]s and [[beet]]s. They include some taproots and tuberous roots.
* '''Tuberous roots''': A portion of a root swells for food or water storage, e.g. [[sweet potato]] and [[dahlia]]. A type of storage root distinct from taproot.
[[Image:Yosemite roots.jpg|thumb|left|300 px|<center>Roots from a fallen redwood at [[Yosemite National Park]].]]
==Rooting depths==
The distribution of vascular plant roots within soil depends on plant form, the spatial and temporal availability of water and nutrients, and the physical properties of the soil. The deepest roots are generally found in deserts and temperate coniferous forests; the shallowest in tundra, boreal forest and temperate grasslands. The deepest observed living root, at least 60 m below the ground surface, was observed during the excavation of an open-pit mine in Arizona, USA. Some roots can grow as deep as the tree is high. The majority of roots on most plants are however found relatively close to the surface where nutrient availability and aeration are more favourable for growth. Rooting depth may be physically restricted by rock or compacted soil close below the surface, or by anaerobic soil conditions.
==Economic importance==
The term [[root crop]]s refers to any edible underground plant structure, but many root crops are actually stems, such as [[potato]] tubers. Edible roots include [[cassava]], [[sweet potato]], [[beet]], [[carrot]], [[rutabaga]], [[turnip]], [[parsnip]], [[radish]], [[yam]] and [[horseradish]]. Spices obtained from roots include [[sassafras]],
[[angelica]], [[sasparilla]] and [[licorice]].
[[Sugar beet]] is an important source of sugar. [[Yam]] roots are a source of estrogen compounds used in birth control pills. The fish poison and insecticide [[rotenone]] is obtained from roots of ''Lonchocarpus'' spp. Important medicines from roots are [[ginseng]], [[aconite]], [[Syrup of ipecac|ipecac]], [[gentian]] and [[reserpine]]. Several legumes that have nitrogen-fixing root nodules are used as green manure crops, which provide nitrogen fertilizer for other crops when plowed under. Specialized [[bald cypress]] roots, termed knees, are sold as souvenirs, lamp bases and carved into folk art. Native Americans used the flexible roots of [[white spruce]] for basketry.
[[Tree]] roots can heave and destroy concrete sidewalks and crush or clog buried pipes. The aerial roots of [[strangler fig]] have damaged ancient [[Maya architecture|Mayan]] [[temple]]s in [[Central America]] and the temple of [[Angkor Wat]] in [[Cambodia]].
[[Vegetative propagation]] of plants via cuttings depends on adventitious root formation. Hundreds of millions of plants are propagated via cuttings annually including [[chrysanthemum]], [[poinsettia]], [[carnation]], ornamental [[shrub]]s and many [[houseplants]].
Roots can also protect the environment by holding the soil to prevent soil erosion.
==See also==
* [[Auxin|Rooting Powder]]
* [[Mycorrhiza]] - root symbiosis in which individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant.
* [[Fibrous root system]]
* [[Stolon]]
==References==
* Brundrett, M. C. 2002. Coevolution of roots and mycorrhizas of land plants. ''New phytologist'' '''154'''(2): 275-304. (Available online: [http://dx.doi.org/10.1046/j.1469-8137.2002.00397.x DOI] | [http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-8137.2002.00397.x/abs/ Abstract] | [http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-8137.2002.00397.x/full/ Full text (HTML)] | [http://www.newphytologist.org/Brundrett.pdf Full text (PDF)])
* Chen, R., E. Rosen, P. H. Masson. 1999. Gravitropism in Higher Plants. ''Plant Physiology'' '''120''' (2): 343-350. (Available online: [http://www.plantphysiol.org/cgi/content/full/120/2/343 Full text (HTML)] | [http://www.plantphysiol.org/cgi/reprint/120/2/343.pdf Full text (PDF)]) - article about how the roots sense gravity.
* Clark, Lynn. 2004. ''[http://www.eeob.iastate.edu/classes/bot404/docs/404root104.pdf Primary Root Structure and Development]'' - lecture notes
* Raven, J. A., D. Edwards. 2001. Roots: evolutionary origins and biogeochemical significance. ''Journal of Experimental Botany'' '''52 (Suppl 1)''': 381-401. (Available online: [http://jxb.oupjournals.org/cgi/content/abstract/52/suppl_1/381 Abstract] | [http://jxb.oupjournals.org/cgi/content/full/52/suppl_1/381 Full text (HTML)] | [http://jxb.oupjournals.org/cgi/reprint/52/suppl_1/381.pdf Full text (PDF)])
* Schenk, H.J., and R.B. Jackson. 2002. The global biogeography of roots. ''Ecological Monographs'' '''72''' (3): 311-328.
* Phillips, W.S. 1963. Depth of roots in soil. ''Ecology'' '''44''' (2): 424.
==External links==
*[http://www.ualr.edu/~botany/roots.html Introduction to Botany - University of Arkansas]
[[Category:Plant physiology]]
[[Category:Plant morphology]]
[[Category:Soil biology]]
In [[vascular plant]]s, the '''root''' is that organ of a [[plant]] body that typically lies below the surface of the [[soil]] (compare with [[plant stem|stem]]). However, this is not always the case, since a root can also be [[aerial root|'''aerial''']] (that is, growing above the ground) or '''aerating''' (that is, growing up above the ground or especially above water). On the other hand, a stem normally occurring below ground is not exceptional either (see [[rhizome]]). So, it is better to define ''root'' as a part of a plant body that bears no leaves, and therefore also lacks [[Node (botany)|nodes]]. There are also important internal structural differences between stems and roots. The two major functions of roots are 1) absorption of water and inorganic nutrients and 2) anchoring the plant body to the ground. Roots also function in [[cytokinin]] synthesis, which supplies some of shoot needs. They often function in storage of food.
==Root structure==
[[Image:Roots.JPG|thumb|Roots of a [[Hydroponics|hydroponically]] grown plant]]
At the tip of every growing root is a conical covering of tissue called the '''root cap'''. It usually is not visible to the naked eye. It consists of undifferentiated soft tissue (parenchyma) with unthickened walls covering the '''[[Meristem|apical meristem]]'''. The root cap provides mechanical protection to the meristem cells as the root advances through the soil, its cells worn away but quickly replaced by new cells generated by cell division within the meristem. The root cap is also involved in the production of '''[[mucigel]]''', a sticky mucilage that coats the new formed cells. These cells contain '''statoliths''', starch grains that move in response to gravity and thus control root orientation.
The outside surface of the primary root is the [[epidermis (botany)|epidermis]]. Recently produced ''epidermal'' cells absorb water from the surrounding environment and produce outgrowths called '''[[trichome#root hairs|root hairs]]''' that greatly increase the cell's absorptive surface. Root-hairs are very delicate and generally short-lived, remaining functional for only a few days. However, as the root grows, new epidermal cells emerge and these form new root hairs, replacing those that die. The process by which water is absorbed into the epidermal cells from the soil is known as '''''[[osmosis]]'''''. For this reason, water that is [[salinity|saline]] is more difficult for most plant species to absorb.
[[Image:Root(cross section).jpg|thumb|left|325px|Cross section of the root of a [[dicotyledon]]]]
Beneath the epidermis is the '''cortex''', which comprises the bulk of the primary root. Its main function is storage of [[starch]]. Intercellular spaces in the cortex aerate cells for [[cellular respiration|respiration]]. An '''[[endodermis]]''' is a thin layer of small cells forming the innermost part of the cortex and surrounding the [[vascular tissue]]s deeper in the root. The tightly packed cells of the endodermis contain a substance known as [[suberin]] in their cell walls. This suberin layer is the [[Casparian strip]], which creates an impermeable barrier of sorts. Mineral nutrients can only move passively within root cell walls until they reach the endodermis. At that point, they must be actively transported across a cell membrane to continue further into the root. This allows the plant to accumulate mineral nutrients in the [[Stele (biology)|stele]].
The vascular cylinder, or stele, consists of the cells inside the endodermis. The outer part, known as the '''pericycle''', surrounds the actual [[vascular]] tissue. In [[monocotyledon]]ous plants, the [[xylem]] and [[phloem]] cells are arranged in a circle around a [[pith]] or center, whereas in [[dicotyledon]]s, the xylem cells form a central "hub" with lobes, and phloem cells fill in the spaces between the lobes.
===Secondary growth===
All roots have primary growth or growth in length. Roots of many vascula
r plants, especially [[dicot]]s and [[gymnosperm]]s, often undergo [[secondary growth]], which is an increase in diameter. A [[vascular cambium]] forms in the stele to produce secondary [[phloem]] and secondary [[xylem]]. The epidermis is replaced by a [[periderm]]. As the stele increases in diameter, the cortex, pericycle and endodermis are lost. Even nonwoody roots often undergo secondary growth, including those of [[tomato]] and [[alfalfa]].
==Root growth==
[[Image:Root-system.web.jpg|thumb|right|350px|Root systems of [[prairie]] plants]]
Early root growth is one of the functions of the '''apical meristem''' located near the tip of the root. The meristem cells more or less continuously divide, producing more meristem, root cap cells (these sacrificed to protect the meristem), and undifferentiated root cells. The latter will become the primary tissues of the root, first undergoing elongation, a process that pushes the root tip forward in the growing medium. Gradually these cells differentiate and mature into specialized cells of the root tissues.
Roots will generally grow in any direction where the correct environment of [[Earth's atmosphere|air]], mineral [[nutrients]] and [[water]] exists to meet the plant's needs. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack foundations, snap water lines, and lift sidewalks. At [[germination]], roots grow downward due to [[gravitropism]], the growth mechanism of plants that also causes the shoot to grow upward. In some plants (such as [[ivy]]), the "root" actually clings to walls and structures.
Growth from apical meristems is known as '''primary growth''', which encompasses all elongation. '''Secondary growth''' encompasses all growth in diameter, a major component of [[woody plant]] tissues and many nonwoody plants. For example, storage roots of [[sweet potato]] have secondary growth but are not woody. Secondary growth occurs at the [[lateral meristem]]s, namely the [[vascular cambium]] and [[cork cambium]]. The former forms [[secondary xylem]] and [[secondary phloem]], while the latter forms the [[periderm]].
In plants with secondary growth, the vascular cambium, originating between the xylem and the phloem, forms a [[cylinder (geometry)|cylinder]] of tissue along the [[Plant stem|stem]] and root. The cambium layer forms new cells on both the inside and outside of the cambium cylinder, with those on the inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem accumulates, the "girth" (lateral dimensions) of the stem and root increases. As a result, tissues beyond the secondary phloem (including the epidermis and cortex, in many cases) tend to be pushed outward and are eventually "sloughed off" (shed).
At this point, the cork cambium begins to form the periderm, consisting of protective [[cork (material)|cork]] cells containing suberin. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.
[[Image:Roots by cesarpb.jpg|250px|thumb|Stilt roots in the [[Amazon Rainforest]] support a tree in very soft, wet soil conditions]]
The vascular cambium produces new layers of secondary xylem annually. The xylem vessels are dead at maturity but are responsible for most water transport through the vascular tissue in stems and roots.
==Types of roots==
A true root system consists of a '''primary root''' and '''secondary roots''' (or [[lateral roots]]).
The primary root originates in the [[radicle]] of the seedling. During its growth it rebranches to form the lateral roots. Generally, two categories are recognized:
*the [[taproot]] system: the primary root is prominent and has a single, dominant axis; there are fibrous secondary roots running outward. Usually allows for deeper roots capable of reaching low water tables. Most common in [[dicots]]. The main function of the [[taproot]] is to store food.
* the diffuse root system: the primary root is not dominant; the whole root system is fibrous and branches in all directions. Most common in [[monocots]]. The main function of the fibrous root is to anchor the plant.
===Specialized roots===
[[Image:Mangroves.jpg|thumb|250px|right|Aerating roots of a [[mangrove]]]]
[[Image:Ceiba pentandra MS4104.JPG|thumb|250px|right|Buttress roots of ''[[Ceiba pentandra]]'']]
The roots, or parts of roots, of many plant species have become specialized to serve adaptive purposes besides the two primary functions described in the introduction.
* '''Adventitious roots''' arise from the stem and not from another root. They usually occur in [[monocot]]s and pteridophytes, but also in many [[dicot]]s, such as [[clover]] (''Trifolium''), [[ivy]] (''Hedera''), [[strawberry]] (''Fragaria'') and [[willow]] (''Salix''). Most aerial roots and stilt roots are adventitious.
* '''Aerating roots''' (or '''pneumatophores'''): roots rising above the ground, especially above water such as in some [[mangrove]] genera (''[[Avicennia]], [[Sonneratia]]''). In some plants like ''Avicennia'' the erect roots have a large number of breathing pores for exchange of gases.
* '''Aerial roots''': roots entirely above the ground, such as in ivy (''Hedera'') or in [[epiphyte|epiphytic]] [[orchid]]s. They function as prop roots, as in [[maize]] or anchor roots or as the trunk in [[strangler fig]].
* '''Buttress roots''' or tabular roots: support roots for many tropical tree species
* '''Contractile roots''': they pull bulbs or corms of [[monocot]]s, such as [[hyacinth]] and [[lily]], and some taproots, such as [[dandelion]], deeper in the soil through expanding radially and contracting longitudinally. They have a wrinkled surface.
* '''Haustorial roots''': roots of parasitic plants that can absorb water and nutrients from another plant, such as in [[mistletoe]] (''Viscum album'') and [[dodder]].
* '''Propagative roots''': roots that form adventitious buds that develop into aboveground shoots, termed [[Basal shoot|suckers]], which form new plants, as in [[Canada thistle]], [[cherry]] and many others.
* '''[[Proteoid root]]s''' or cluster roots: dense clusters of rootlets of limited growth that develop under low [[phosphate]] or low [[iron]] conditions in [[Proteaceae]] and some plants from the following families [[Betulaceae]], [[Casuarinaceae]], [[Eleagnaceae]], [[Moraceae]], [[Fabaceae]] and [[Myricaceae]].
* '''Stilt roots''': these are adventitious support roots, common among [[mangrove]]s. They grow down from lateral branches, branching in the soil.
* '''Storage roots''': these roots are modified for storage of food or water, such as [[carrot]]s and [[beet]]s. They include some taproots and tuberous roots.
* '''Tuberous roots''': A portion of a root swells for food or water storage, e.g. [[sweet potato]] and [[dahlia]]. A type of storage root distinct from taproot.
[[Image:Yosemite roots.jpg|thumb|left|300 px|<center>Roots from a fallen redwood at [[Yosemite National Park]].]]
==Rooting depths==
The distribution of vascular plant roots within soil depends on plant form, the spatial and temporal availability of water and nutrients, and the physical properties of the soil. The deepest roots are generally found in deserts and temperate coniferous forests; the shallowest in tundra, boreal forest and temperate grasslands. The deepest observed living root, at least 60 m below the ground surface, was observed during the excavation of an open-pit mine in Arizona, USA. Some roots can grow as deep as the tree is high. The majority of roots on most plants are however found relatively close to the surface where nutrient availability and aeration are more favourable for growth. Rooting depth may be physically restricted by rock or compacted soil close below the surface, or by anaerobic soil conditions.
==Economic importance==
The term [[root crop]]s refers to any edible underground plant structure, but many root crops are actually stems, such as [[potato]] tubers. Edible roots include [[cassava]], [[sweet potato]], [[beet]], [[carrot]], [[rutabaga]], [[turnip]], [[parsnip]], [[radish]], [[yam]] and [[horseradish]]. Spices obtained from roots include [[sassafras]],
[[angelica]], [[sasparilla]] and [[licorice]].
[[Sugar beet]] is an important source of sugar. [[Yam]] roots are a source of estrogen compounds used in birth control pills. The fish poison and insecticide [[rotenone]] is obtained from roots of ''Lonchocarpus'' spp. Important medicines from roots are [[ginseng]], [[aconite]], [[Syrup of ipecac|ipecac]], [[gentian]] and [[reserpine]]. Several legumes that have nitrogen-fixing root nodules are used as green manure crops, which provide nitrogen fertilizer for other crops when plowed under. Specialized [[bald cypress]] roots, termed knees, are sold as souvenirs, lamp bases and carved into folk art. Native Americans used the flexible roots of [[white spruce]] for basketry.
[[Tree]] roots can heave and destroy concrete sidewalks and crush or clog buried pipes. The aerial roots of [[strangler fig]] have damaged ancient [[Maya architecture|Mayan]] [[temple]]s in [[Central America]] and the temple of [[Angkor Wat]] in [[Cambodia]].
[[Vegetative propagation]] of plants via cuttings depends on adventitious root formation. Hundreds of millions of plants are propagated via cuttings annually including [[chrysanthemum]], [[poinsettia]], [[carnation]], ornamental [[shrub]]s and many [[houseplants]].
Roots can also protect the environment by holding the soil to prevent soil erosion.
==See also==
* [[Auxin|Rooting Powder]]
* [[Mycorrhiza]] - root symbiosis in which individual hyphae extending from the mycelium of a fungus colonize the roots of a host plant.
* [[Fibrous root system]]
* [[Stolon]]
==References==
* Brundrett, M. C. 2002. Coevolution of roots and mycorrhizas of land plants. ''New phytologist'' '''154'''(2): 275-304. (Available online: [http://dx.doi.org/10.1046/j.1469-8137.2002.00397.x DOI] | [http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-8137.2002.00397.x/abs/ Abstract] | [http://www.blackwell-synergy.com/links/doi/10.1046/j.1469-8137.2002.00397.x/full/ Full text (HTML)] | [http://www.newphytologist.org/Brundrett.pdf Full text (PDF)])
* Chen, R., E. Rosen, P. H. Masson. 1999. Gravitropism in Higher Plants. ''Plant Physiology'' '''120''' (2): 343-350. (Available online: [http://www.plantphysiol.org/cgi/content/full/120/2/343 Full text (HTML)] | [http://www.plantphysiol.org/cgi/reprint/120/2/343.pdf Full text (PDF)]) - article about how the roots sense gravity.
* Clark, Lynn. 2004. ''[http://www.eeob.iastate.edu/classes/bot404/docs/404root104.pdf Primary Root Structure and Development]'' - lecture notes
* Raven, J. A., D. Edwards. 2001. Roots: evolutionary origins and biogeochemical significance. ''Journal of Experimental Botany'' '''52 (Suppl 1)''': 381-401. (Available online: [http://jxb.oupjournals.org/cgi/content/abstract/52/suppl_1/381 Abstract] | [http://jxb.oupjournals.org/cgi/content/full/52/suppl_1/381 Full text (HTML)] | [http://jxb.oupjournals.org/cgi/reprint/52/suppl_1/381.pdf Full text (PDF)])
* Schenk, H.J., and R.B. Jackson. 2002. The global biogeography of roots. ''Ecological Monographs'' '''72''' (3): 311-328.
* Phillips, W.S. 1963. Depth of roots in soil. ''Ecology'' '''44''' (2): 424.
==External links==
*[http://www.ualr.edu/~botany/roots.html Introduction to Botany - University of Arkansas]
[[Category:Plant physiology]]
[[Category:Plant morphology]]
[[Category:Soil biology]]