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Primary and secondary roots in a cotton plant

In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial (growing above the ground) or aerating (growing up above the ground or especially above water). Furthermore, 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 nodes. There are also important internal structural differences between stems and roots.

The first root that comes from a plant is called the radicle. The three major functions of roots are 1) absorption of water and inorganic nutrients, 2) anchoring of the plant body to the ground and 3) storage of food and nutrients. In response to the concentration of nutrients, roots also synthesise cytokinin, which acts as a signal as to how fast the shoots can grow. Roots often function in storage of food and nutrients. The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizas, and a large range of other organisms including bacteria also closely associate with roots. The parts of a root are the xylem, the epidermis, the cortex, the root cap, the root hairs, the phloem, and the cambium.

Contents

Root growth

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 are sacrificed to protect the meristem), and undifferentiated root cells. The latter 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 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 meristems, 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 of tissue along the 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 cells containing suberin. In roots, the cork cambium originates in the pericycle, a component of the vascular cylinder.

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 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

Aerating roots of a mangrove
The growing tip of a fine root
The stilt roots of Socratea exorrhiza

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 out-of-sequence from the more usual root formation of branches of a primary root, and instead originate from the stem, branches, leaves, or old woody roots. They commonly occur in monocots and pteridophytes, but also in many dicots, such as clover (Trifolium), ivy (Hedera), strawberry (Fragaria) and willow (Salix). Most aerial roots and stilt roots are adventitious. In some conifers adventitious roots can form the largest part of the root system.
  • Aerating roots (or knee root or knee or pneumatophores or Cypress knee): 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 epiphytic orchids. They function as prop roots, as in maize or anchor roots or as the trunk in strangler fig.
  • Contractile roots: they pull bulbs or corms of monocots, 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.
  • Coarse roots: Roots that have undergone secondary thickening and have a woody structure. These roots have some ability to absorb water and nutrients, but their main function is transport and to provide a structure to connect the smaller diameter, fine roots to the rest of the plant.
  • Fine roots: Primary roots usually <2 mm diameter that have the function of water and nutrient uptake. They are often heavily branched and support mycorrhizas. These roots may be short lived, but are replaced by the plant in an ongoing process of root 'turnover'.
  • 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 suckers, which form new plants, as in Canada thistle, cherry and many others.
  • Proteoid roots 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, Elaeagnaceae, Moraceae, Fabaceae and Myricaceae.
  • Stilt roots: these are adventitious support roots, common among mangroves. They grow down from lateral branches, branching in the soil.
  • Storage roots: these roots are modified for storage of food or water, such as carrots and beets. They include some taproots and tuberous roots.
  • Structural roots: large roots that have undergone considerable secondary thickening and provide mechanical support to woody plants and trees.
  • Surface roots: These proliferate close below the soil surface, exploiting water and easily available nutrients. Where conditions are close to optimum in the surface layers of soil, the growth of surface roots is encouraged and they commonly become the dominant roots.
  • Tuberous roots: A portion of a root swells for food or water storage, e.g. sweet potato. A type of storage root distinct from taproot.

Rooting depths

Cross section of a mango tree

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.

Rooting Depth Records

Species Location Maximum rooting depth (m) Reference[1]
Boscia albitrunca Kalahari desert 68 Jennings (1974)
Juniperus monosperma Colorado Plateau 61 Cannon (1960)
Eucalyptus sp. Australian forest 61 Jennings (1971)
Acacia erioloba Kalahari desert 60 Jennings (1974)
Prosopis juliflora Arizona desert 53.3 Phillips (1963)

Root architecture

The pattern of development of a root system is termed root architecture, and is important in providing a plant with a secure supply of nutrients and water as well as anchorage and support. The architecture of a root system can be considered in a similar way to above-ground architecture of a plant—i.e. in terms of the size, branching and distribution of the component parts. In roots, the architecture of fine roots and coarse roots can both be described by variation in topology and distribution of biomass within and between roots. Having a balanced architecture allows fine roots to exploit soil efficiently around a plant, but the “plastic” nature of root growth allows the plant to then concentrate its resources where nutrients and water are more easily available. A balanced coarse root architecture, with roots distributed relatively evenly around the stem base, is necessary to provide support to larger plants and trees.


Tree roots normally grow outward to about three times the branch spread. Only half of a tree's root system occurs between the trunk and the circumference of its canopy. Roots on one side of a tree normally supply the foliage on that same side of the tree. Thus when roots on one side of a tree are injured, the branches and leaves on that same side of the tree may die or wilt. For some trees however, such as the maple family, the effect of a root injury may show itself anywhere in the tree canopy.

Evolutionary history

The fossil record of roots - or rather, infilled voids where roots rotted after death - spans back to the late Silurian,[2] but their identification is difficult, because casts and molds of roots are so similar in appearance to animal burrows - although they can be discriminated on the basis of a range of features.[3]

Economic importance

Roots can also protect the environment by holding the soil to prevent soil erosion

The term root crops 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, sarsaparilla 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, 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 Mayan temples 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 shrubs and many houseplants.

Roots can also protect the environment by holding the soil to prevent soil erosion.

Root

See also

Notes

  1. ^ Source: Generated in part from data in Stone and Kalisz (1991) and Canadell et al. (1996)
  2. ^ Retallack, G. J. (1986). Wright, V. P.. ed. Paleosols: their Recognition and Interpretation. Oxford: Blackwell. 
  3. ^ Hillier, R, Edwards, D;Other, A.N. (2008). "Sedimentological evidence for rooting structures in the Early Devonian Anglo–Welsh Basin (UK), with speculation on their producers". Palaeogeography Palaeoclimatology Palaeoecology 270: 366. doi:10.1016/j.palaeo.2008.01.038. 

References

  • Brundrett, M. C. 2002. Coevolution of roots and mycorrhizas of land plants. New phytologist 154(2): 275-304. (Available online: DOI | Abstract | Full text (HTML) | Full text (PDF))
  • Chen, R., E. Rosen, P. H. Masson. 1999. Gravitropism in Higher Plants. Plant Physiology 120 (2): 343-350. (Available online: Full text (HTML) | Full text (PDF)) - article about how the roots sense gravity.
  • Clark, Lynn. 2004. Primary Root Structure and Development - lecture notes
  • Coutts, M.P. 1987. Developmental processes in tree root systems. Canadian Journal of Forest Research 17: 761-767.
  • Raven, J. A., D. Edwards. 2001. Roots: evolutionary origins and biogeochemical significance. Journal of Experimental Botany 52 (Suppl 1): 381-401. (Available online: Abstract | Full text (HTML) | Full text (PDF))
  • Schenk, H.J., and R.B. Jackson. 2002. The global biogeography of roots. Ecological Monographs 72 (3): 311-328.
  • Sutton, R.F., and R.W. Tinus. 1983. Root and root system terminology. Forest Science Monograph 24 pp 137.
  • Phillips, W.S. 1963. Depth of roots in soil. Ecology 44 (2): 424.

External links



1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

ROOT (late O.E. rOt, adopted from Scand., cf. Norw. and Saved. rot, Dan. rod; the true O.E. word was wyrt, plant, represented in Ger. Wurz or Wurzel; the ultimate root is the same in both words, and is seen in Lat. radix), the underground part of a plant. This is the popular meaning of the word. In its botanical use the term is more restricted (see below). The various other meanings have all developed from this, its primary, significance. Of these the principal are: the source or origin Of a condition, state, quality, &c.; the base or embedded part of A structure of the body, such as a nail, tooth, the hair, &c.; in mathematics, a number, quantity or dimension which produces a given expression when multiplied by itself a requisite number of times; and in philology an ultimate 'element of language, incapable of further analysis. A particular extension of the primary meaning is that which applies the word generally to a class of plants, such as the turnip 'or carrot, whose root is fleshy, and edible either by man or domestic animals.

The embryo of a. typical plant, for instance a pea plant (fig. 1), has an ascending axis which will grow into the shoot, and a descending axis or radicle which will grow into the root. When the seed germinates, the radicle is the first to appear; it grows downwards, and its primary function is to act as a holdfast for the plant; its most important function, however, is the absorption of water and dissolved nutrient substances from the soil, and it also frequently serves for storage of foodstuffs. The root is distinguished from underground shoots by not bearing leaves and by having its apex (growing point) protected by a cap (root-cap), which can be clearly seen by making a median vertical section through the root-tip; the cap;protects it in its passage through the soil. The root also generally bears root-hairs, slender unicellular outgrowths of the outer layer, borne in the region a little behind the roottip. It is by means of the root - hairs especially that the root is brought into close relation with the soil particles and absorbs the nutrient materials in solution in the water which surrounds these particles. The older root-hairs are continually dying off, so that they are borne only on a small part of the area behind the apex. Branches of the root, which repeat the form and structure of the main root, are developed in regular succession from above downwards (acropetal), and owing to the fact that they originate in a definite position in the interior of the root (endogenous) they develop in longitudinal rows and have to break through the overlying tissue of the parent root (fig. 2). True forking of the root (dichotomy) occurs in the Lycopodiaceae (the shoots of which also branch dichotomously), but is unknown in the higher;plants.

Roots which originate elsewhere than as acropetal outgrowths of a main root are known as adventitious, and may From Green's Vegetable Physiology, by permission.

FIG. 3 a and b. Root-hair in contact with parI Ultimate root-branches, showing tides of soil (highly magnified). position of root-hairs.

arise on any part of a plant. They are especially numerous on underground stems, such as the under side of rhizomes, and also develop from stem nodes under favourable conditions, such as moisture and absence of light; a young shoot or a cutting placed in moist soil quickly forms adventitious roots. They may also arise from leaves under similar conditions, as, for instance, from begonia leaves when planted in soil.

The forms of roots depend on their shape and mode of branching. When the central axis goes deep into the ground in a tapering manner, without dividing, a tap-root is produced. This kind of root is sometimes shortened, and becomes swollen by storage of food-stuffs, forming the conical root of carrot, or the fusiform or spindle-shaped root of radish, or the napiform root of turnip. In ordinary forest trees the first root protruded continues to elongate and forms a long primary root-axis, whence secondary axes come off. In primary plants, especially Monocotyledons, the primary axis soon dies and the secondary axes take its place. When the descending axis is very short, and at once divides into thin, nearly equal fibrils, the root is called fibrous, as in many grasses (fig. 4); when the fibrils are thick and succulent, the root is fasciculated, as in Ranunculus Ficaria, Asphodelus luteus, and Oenanthe crocata; when some of the fibrils are developed in the form of tubercules, the root is tubercular, as in dahlia (fig. 5); when the fibrils enlarge in certain FIG. 4. - Fibrous Root of a Grass. From Strasburger's Lehrbuch der Botanik, by per mission of Gustav Fischer.

Numerous fibrils FIG. 5. - Root-Tubers of Dahlia variabilis. coming off from s, the lower portions of the cut stems one point. (2, nat. size).

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parts only, the root is nodulose, as in Spiraea Filipendula, or moniliform, as in Pelargonium triste, or annulated, as in Ipecacuanha. Some of these so-called roots are formed of a stem and root combined, as in Orchis (fig. 6), where the tuber consists of a fleshy swollen FIG. I. - The Dicotyledonous Embryo of the Pea laid open. c, c, the two fleshy cotyledons, or seed-lobes, which remain under ground when the plant sprouts .; r, the radicular extremity of the axis which develops into the root; t, the axis bearing the young stalk and leaves g, which lie in a depression of the cotyledons f. From Vines's Student's Botany, by permission.

'FIG. 2. - Lateral Roots n arising endogenously from the pericycle of the Tap-Root of Vicia Faba (longitudinal section, X 5). f, axial cylinder (stele); r, cortex of main root; h, root-cap of lateral root.

root bearing at the apex a stem bud. As in the case of the stem, growth in length occurs only for a short distance behind the apex, but in long-lived roots increase in diameter occurs continually in a similar manner to growth in thickness in the stem.

Roots are usually underground and colourless, but in some cases where they arise from the stem they pass for some distance through the air before reaching the soil. Such roots are called aerial. They are well seen in the screw-pine (Pandanus), the Banyan (Ficus indica, fig. 7), and many other species of Ficus, where they assist in supporting the stem and branches. In the mangrove they often form the entire support of the stem, which has decayed at its lower part. In treeferns they form a dense coating around, and completely concealing, the stem; such is also the case in some Dracaenas and palms. In Epiphytes, or plants growing in the air, attached to the trunks of trees, such as orchids of warm climates, the aerial roots produced G. 6. - Base of do not reach the soil; they continue always FI plant of Orchis aerial and greenish, and they possess stomata. showing tuber' Delicate hairs are often seen on these epi les or tuberous phytal roots, as well as a peculiar spongy cu roots. investment formed by the cells of the epi dermis which have lost their succulent contents and are now filled with air. This layer is called the velanien, and serves to condense the. moisture contained in the air, on which FIG. 7. - Ficus indica, the Banyan tree, sending out numerous aerial roots, which reach the soil, and prop the branches.

the plant is dependent for its water-supply. The aerial roots of the ivy are not the nutritive roots of the plant, but are only intended for mechanical support. The climbing roots of many orchids, aroids and epiphytic ferns branch and form places of lodgment for humus into which absorbent branches of the climbing roots penetrate. Some leafless epiphytic orchids, such as species of Angraecum, depend entirely upon their aerial roots for nourishment; the roots, which are green, perform the functions both of leaves and roots. A respiratory or aerating function is performed by roots of certain mangroves growing in swampy soil or water and sending vertical roots up into the air which are provided with aerating passages by which the root system below can communicate with true outside air.

Parasitic plants, as the mistletoe (Viscum), broom-rape (Orobanche) and Rafflesia, send root-like processes into the substance of the plants whence they derive nourishment. In the dodder (Cuscuta), the tissue around the root swells into a kind of sucker (haustorium), which is applied flat upon the other plant, and ultimately becomes concave, so as to attach the plant by a vacuum. From the bottom of the sucker the root protrudes, and penetrates the tissue of the host plant. Leaf-buds are sometimes formed on roots, as in plum, cherry and other fruit trees; the common elm affords an excellent example, the young shoots which grow up in the neighbourhood of a tree arising from the roots beneath the soil. In some plants no roots are formed at all; thus in the orchid Corallorhiza, known as coral-root, a stem-structure, the shortly branched underground rhizome, performs all the functions of a true root which is absent. In aquatic plants the root acts merely as a holdfast or is altogether absent as in Salvinia, Utricularia and others.


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Wikispecies

Up to date as of January 23, 2010
(Redirected to Francis Metcalf Root article)

From Wikispecies

(1889-1934)


Simple English

[[File:|thumb|Coriander roots]]

This page is about roots of plants, if you want to know about roots in mathematics, read square root or nth root.

The roots of a plant is the part that is usually buried in the soil. Roots are not always under the soil, though—sometimes roots can be above the ground. This is called an aerial root. Also, stems can sometimes be under the soil (potatoes, for example). Roots do not have leaves, and are different from stems inside.

Plants need their roots especially for three reasons: The roots take water and nutrients, a sort of food, from the soil. They also often store this food. And they fix the plants to the ground.

There are two sorts of root systems:

  • the taproot system: there is one very big root that goes down into the ground, and many smaller roots that come out of it
  • the diffuse root system: there are many roots that go in all directions

Some roots go very deep into the ground. One root that was found in Arizona, USA, was 60 m below the surface.

Roots are also very strong. Some tree roots can destroy stones.Roots are not green because the cells have no chlorophyll.

Contents

Root growth

Roots grow through the whole life of the plant. They grow longer from the tip, adding cells to the end of each root.[1] The root adds cells to their tips, and they grow fatter as they add cells around their tube-like bodies.

At the tip of each root, there is a small group of tough, dead, hard cells called the root cap.[2] The root cap is the strongest part of the root tip, and its job is to push its way through the dirt to look for moisture and nutrients and protect the plant.[2]

Location

Roots are usually found underground, but in some cases, this is not true. In the rainforest, a lot of water is in the air because the rainforest is full of plants with big leaves that constantly do transpiration. Because there is so much rain and transpiration in a rainforest, the air is very humid(it has a lot of water). Because of this, rainforest plants often grow right on trees, with roots hanging down into the air or running into the moss that is also growing on the trees.[3] They do not even need the dirt: they have all they need floating around in the air or in the moss.[3]

Some plants have roots that are above the ground and underground. For example, the banyan tree has a root system that is underground, but it also has roots that start in its branches and grow down towards the ground.[3] These roots not only take in water and nutrients from the soil, but they also help to support the long branches of the banyan tree. Because of this extra support, banyan tree branches can be really long. A banyantree in Lahaina, Maui, which was planted in 1873 by a man named William Owen Smith, has branches that are so long that this single tree covers a full square block in the city.[3]

Root systems

There are two main kinds of roots systems: taproot systems and fibrous root systems. A taproot system has one thick main root growing down from the plant's stem, and lots of smaller secondary roots branching off from this one main root.[3] A taproot system is usually deeper than it is wide. Often, we eat taproots, like carrots and turnips.

A fibrous root system has lots of roots growing in many directions. There is not one main root. A fibrous root system is usually wider than deep.[3]

References

  1. Lewin, Benjamin (2007). "How plants grow". Cells. Jones & Bartlett Publishers. pp. 765. ISBN 0763739057. 
  2. 2.0 2.1 Raven, J.A.; Edwards, D. (2001). [Expression error: Unexpected < operator "Roots: evolutionary origins and biogeochemical significance"]. Journal of Experimental Botany 52 (90001): 381–401. doi:10.1093/jexbot/52.suppl_1.381. 
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Fulbright, Jeannie (2004). Exploring Creation with Botany. 1106 Meridian Plaza, Suite 220: Apologia Educational Ministries, Inc.. ISBN 1-932012-49-4. 








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