The root• The prolongation of the radicle (first root axis arises from cells laid down in the seed)of the embryo is ROOT.• Principally, the underground organ of the plant body which absorbs water and minerals from the soil transporting them to other parts of the plant body.• Due to this, root tends to grow downwards, away from light and towards water.• As a general rule, they bear neither leaves nor buds.• The primary roles are anchorage, absorption and transport.• However, it has adapted to fulfil a variety of other functions including storage, support and aeration.• Compared with stem, root is relatively simple and uniform in structure.
Root Structure• The main root, primary root or Tap root is formed from the radicle. The lateral branches of main root are called secondary roots which are further branched to form tertiary roots. Roots are absent in some angiosperms, e.g. genus utricularia ( free- floating, aquatic, canivorous plants that trap and digest very thin animal).
Four zones or regions are commonly recognized in developing roots namely:1. the root cap,2. the meristematic zone (zone of cell division),3. the elongation zone, and4. the maturation zone.
Zone of elongation: This root is covered by point occurringMeristematic tip ofRoot cap: The zone: the zone is a growing a cap (rootZone of maturation:Newsthat is shaped likemitosiscap.specific cell meristemscap) cells produced by arootinto the is composed of twoElongated cells differentiate by It primaryimmediately behind the thimble. Meristematic regionelongate of meristematiccolumella (they look likeby the rootconsists rapidly and become several times longertypes (e.g. xylem and phloem) in the zone of than wide.types,of cells, the inner tissue and is protected columns)Thus, their width also increases slightly. The small vacuolescap. and theThe the apical rootlateral that arecells It consists ofcells of the andcap cellsmeristems. Most ofmaturation. outer, lateral root surfacepresent merge and grow until they occupy 90% or more of thethe activity in this zone of cell divisionapical place towardcontinuously replenished by thecells, which meristem. As mature into epidermal root takescylinderof each cell. No further increase in cell size occursvolumethe edges ofcells dome, Just abovecells a strong protectivetheseaouter thin cuticle. where the form divide every 12 tohave the zone of disintegrateand the mature parts of the root, the they theabove very elongation,36 hours, often rhythmically, remain stationaryof division ofcover which protects in is a part offrom a peak and damage aselongation zone, there girth, reaching injury for the lifeexcept for an increase the root tip theonce or twice causes the calleddifferentiate to form morethe plant.pushes day. Thethroughelongate and root provides amaturation zoneits way root topiliferous Thethe root This a with is cells the soil.specialized supply of the soil in search cells are providedcontinuous root tissues and additionalfor water andthat lieforregion. deeper into expendale parenchyma cells mineralpenetrate is where the epidermal cells produce manyThiszone meristematic tissues.over region elongation.the the ofsalts. unicellular outgrowth called hair roots. Roottubular,hairs, which can number over 35,000 per squarecentimeter of root surface and many billions perplant, greatly increase the surface area and therefore theabsorptive capacity of the root. Thus, Water absorptionmostly takes place through this area. The root hairs usuallyare alive and functional for only a few days before they aresloughed off at the older part of the zone ofmaturation, while new ones are being produced towardthe zone of elongation. Above the root hair region, the rootbecomes thicker and secondary or lateral roots aredeveloped. The secondary roots in turn rebranch toform tertiary roots. Each lateral branch has its owncap, root hairs, meristematic, elongation and matureregions. The roots in this region are covered by a protective
Types of rootsTap root• Generally found in dicotyledons• Main or primary root develop from the radicle.• Grows vertically down into the soil• Later lateral or secondary roots grow from this at an acute angle outwards and downwards, and from Lateral these other branches may arise roots for absorption of water and nutrients.• When tap root is associated with many branched roots, it forms the tap root system. e.g. pastinaca sativa (parsnip) and taraxacum officinale (Dandelion)
Adventitious roots• Commonly found in monocots• Growth of the radicle is usually arrested at an early stage and is replaced by numerous roots that develop from the stem.• These adventitious roots are slender and equal in size.• Adventitious roots associated with branched or lateral roots and form the adventitious root system.• Also known as fibrous roots.
Modifications in roots• Root modification occurs when there is a permanent change in the structure of tap or adventitious roots. This is to perform additional specific functions to those of anchorage and absorption for adaptation to their surrounding environment.
Roots modified for the storage of food:• Conical roots: modified tap root of conical shape throughout which they store food. It is broad at the base and gradually tapers towards its apex - e.g. Carrot (Daucas carota).• Napiform roots: modified tap root, fleshy with the upper portion inflated or swollen and abrupt narrowing of the base into a tail-like portion. - e.g. Turnip (Brassica rapa) and Beetroot (Beta vulgaris) .• Fusiform roots modified tap root with thickened middle portion containing food and tapered towards both ends – e.g. radish (Raphanus sativus) Turnip
• Tuberous/storage root is • Nodulated roots are modified fibrous root with fibrous/adventitious roots irregular shape. Many single of the plants of the roots are modified to form Leguminosae family. several similar swollen roots Nodules like structures are for the storage of food. They present on branches of root occur in a bunch - e.g. in which nitrogen fixing Mirabilis jalapa bacteria can be found.The apex of these roots become swollen because of the accumulation of food e.g. Curcuma amada, Ginger.
Roots modified for mechanical support:• Prop roots – the name is related to the pillar like appearance. They are aerial adventitious roots that Maize develop from branches and give roots support to the branches. They grow vertically downward, penetrate the soil and become thick to provide additional support to the plant. they brace the plant against wind - e.g. Ficus Screw pine bengalensis, banyan tree (Ficus roots macrophylla) and maize.• Stilt roots - roots develop from nodes of the lowermost portion of the stem and provide mechanical support to the plant by fixing it in soil firmly. e.g. sugarcane, Pandanus Tectorius (screw pine). Decumaria barbara• Climbing roots - in some weak stemmed plants roots develop from nodes which are useful to climb on the hard object.
• Contractile roots widely distributed among monocotyledons and herbaceous perennial dicotyledons. Bulbs and The roots from the bulbs of lilies roots of and of several other plants such as lilies dandelions and colocassia contract by spiraling to pull the plant a little deeper into the soil each year until they reach an area of relatively stable temperatures. The roots may contract to a third of their original length as they spiral like a corkscrew due to cellular thickening and constricting.• Floating roots - in some aquatic plants, the roots will float on the surface of the water. These roots store air, become inflated and Water spongy, project above the level of primrose water and make the plant light. (Jussiaea ) They also help in exchange of gases. E.g. Jussiaea
Roots modified for vital function:• Pneumatophores or Respiratory roots - The roots of some aquatic plants and plants which grow on marshy areas, such as mangroves, develop outgrowths (pneumatophores). This type of roots arises from underground branches of tap root, grows in upwards direction and usually extends several centimeters above water, facilitating the oxygen supply to the roots beneath. In aquatic plants, floating roots are acting as respiratory roots. In marshy area, the roots are not getting sufficient oxygen and hence they grow upwards from the ground.
• Epiphytic roots are aerial roots of plants such as orchids. A distinguishing feature is that these roots absorb moisture from atmosphere with the help of velamen tissue (outer layer of Orchids roots dead cells). Most of the cells are water absorbing while the others are filled with air and thus facilitate the exchange of gases with the inner cortex. Epiphytic roots have only physical contact and cause no harm to the host plant. Taeniophyllum• Photosynthetic or Assimilatory roots: The roots are green, flattened, and ribbon like. The root tip cells contain chloroplasts and thus, perform photosynthesis. In some cases such as in the case of leafless orchids of the genera Taeniophyllum and Chiloschista, they are the only Trapa photosynthetic tissues. E.g. Tinospora (- aerial roots) , Trapa (-Hydrophyte is with submerged green roots)
• Parasitic roots: The stems of certain plants that lack chlorophyll, such as dodder (Cuscuta), produce peg-like Cuscuta roots called haustoria that on host penetrate the host plants plant around which they are twined. The haustoria establish contact with the conducting tissues of the host and effectively parasitize their host. Thus, Electromacrograph they are food sucking roots. to show the• Root thorns: roots of some haustoria. plants arise from the stem and change into thorns performing the protective function e.g. Pothos (money plant) Thorns of Pothos
Other modifications of roots:• Fasciculated roots: from the bas or lower nodes of the stem, these tuberous roots arise in groups. E.g • Buttress roots. Certain Dahlia species of fig and other• Moniliform roots: these are also tropical trees produce called beaded roots because of their huge buttress roots bead-like appearance e.g. Momordica (bitter gourd) toward the base of the• Annulated roots: these thickened trunk, which provide roots look as if formed by a number considerable stability. of discs placed on above another. Ipecac (Cephalis)• Water storage roots. Some members of the pumpkin family (Cucurbitaceae), especially those that grow in arid regions, may produce water-storage roots weighing 50 or more kilograms.
Root growth• Most individual root growth can be divided into two main phases; the indeterminate growth phase and the termination growth phase. The indeterminate growth phase is one where growth is maintained for an undefined period of time. Growth is usually accomplished by division of RAM while the terminate phase is a phase where growth stops usually after a certain period of time, size or length of roots or when appropriated conditions are unavailable. According to Wilcox 1962, the RAM can become dormant for example during droughts but can restart its division and growth after some time.
Primary growth• Primary growth is a longitudinal growth occurring in all vascular plants and involves all elongation of the roots. Primary growth takes place only in the apical meristem.
Secondary growth• Secondary growth occurs mainly in dicots but rarely in monocots. It is the result of division in the lateral meristems, the cork cambium and the vascular cambium. Secondary growth takes place in woody as well as in non woody plants. It involves all growth in diameter of the roots.• 1. Vascular cambium
3. One of the new cells remains2. The vascular cambium cells vascular cambium and the otherdivides longitudinally. becomes xylem. Vascular cambium
4 & 5. The cells can be seen enlarging in this diagram VascularVascular cambiumcambium
6. Occasionally, the inner cells remain vascular cambium and the outer ones become phloem. Vascular phloem cambium
Root meristem• Meristems are areas in plants where mitosis occurs, and due to this cell division, it is also where growth occurs. Apical meristems are responsible for vertical growth and they can be found at the root tips.• The planes of cell division in the root meristem are strictly ordered, and are primarily transverse divisions that provide growth of the root in length.• A primary root meristem generates two tissues simultaneously, the main root axis extending proximally towards the shoot, and the root cap pushing relentlessly forward into the soil. Primary roots arise through controlled cell divisions in the apical meristem and subsequent expansion and differentiation of these cells.
Apical meristem and development• Divisions apical meristem is covered• The root can be in any of three planes, either anticlinal which provides root by the root cap (normal to the a axis), periclinal lubricative function as protective and (tangential to the root axis) or takes place through soil growth radial to the axis. These divisions will give rise, respectively, to particles. Root caps advance at a increasedspeed: a rootincreased root dramatic root length, might thicknessby 5 cm per day cells new elongate (more layers of and through the root), or increased root in root cap cells can be pushed circumference. apex of the primary advance of the• The apical meristem supplies all the axis at about the same rate.• cells for the primary root axisapices is A remarkable feature of root and the consequencescentre,planes of cell the the quiescent of the a paradox at division are evident long after heart of the meristem. The quiescent meristematic activity ceases.inactive, centre is a zone of relatively• Separate cell divisions at the leading slowly dividing cells, numbering about 500–600 meristem maize edge of the rootin a maturegenerate a Diagram showing longitudinal root. root cap which extends forward as a section of a maise (Zea mays) protective structure. root tip. The quiescent centre is shaded dark green.
• The Picture shows the Root Cap (Thimble-like covering which protects the delicate apical meristem), the Apical Meristem (Region of rapid cell division of undifferentiated cells), the Quiescent Center (Populations of cells in apical meristem which reproduce much more slowly than other meristematic cells), the Zone of Cell Division - Primary Meristems (Three areas just above the apical meristem that continue to divide for some time), the Zone of Elongation (Cells elongate up to ten times their original length )and the Zone of Maturation (Region of the root where completely functional cells are found) of a root.
Lateral meristem and development• Lateral root formation is a major determinant of root systems architecture. The degree of root branching impacts the efficiency of water uptake, acquisition of nutrients and anchorage by plants.• Lateral roots extend horizontally from the primary root and serve to anchor the plant securely into the soil. This branching of roots also contributes to water uptake, and facilitates the extraction of nutrients required for the growth and development of the plant.• Many different factors are involved in the formation of lateral roots. Regulation of root formation is tightly controlled by plant hormones such as auxin, and by the precise control of aspects of the cell cycle. Such control can be particularly useful: increased auxin levels, which help to promote lateral root development, occur when young leaf primordia form and are able to synthesise the hormone. This allows coordination of root development with leaf development, enabling a balance between carbon and nitrogen metabolism to be established.
• Lateral root primordia originate After the lateral root primordium is formed, it becomes a mature from the mature pericycle of the lateral root. One of stage parent root by a twothe first events process. First, the primodium is a periclinal division that emerges through the overlaying generates a double layer of tissues by cell expansion. The pericycle-derived cells. Cells of the increase in cell size is particularly primordia begin to differentiate apparent in cells near the base of almost immediately after initiation, the primordium, while cell as evidenced by differential gene number remains inner and expression in therelatively outer unchanged. Second, the new layers.• lateral root begins to elongate, Lateral root primordia develop and cell numbers increase at the of through a characteristic program rootdivisions and expansions to cell tip. This is characteristic of maturearoot elongation via create fully patterned structure division of cells the primaryapical that resembles in the root root meristem. tip.
• It was found that lateral root formation can be divided into four stages:1) Differentiation of pericycle cells2) First morphological event is series of asymmetric transverse divisions of pericycle cells in three cell files positioned opposite the xylem pole. Although all pericycle cells are morphologically identical, these three files are already fated differently from their neighbors.Followed by ordered cell divisions and differentiation that generates a lateral root primordium3) Emergence via cell expansion4) Activation of lateral root primordia to form a functional root.
Root-stem transition• The root transition zone concept states that root cells leaving the apical meristem need to accomplish a transitional stage of cyto- architectural rearrangement, especially of the actin cytoskeleton, in order to perform rapid cell elongation. Cells of this zone also have unique functional and sensorial properties.• The root and the stem make a continuous structure called the axis of the plant. The vascular bundles are continuous from the root to the stem, but the arrangement of vascular bundles is quite different in the two organs; the stems possess collateral bundles with endarch xylem, whereas the roots possess radial bundles with exarch xylem. The change of position involving inversion and twisting of xylem strands from exarch to endarch type is referred to as vascular transition, and the part of the axis where these changes occur is called transition region.
There exist four types of root-stem transition.1. In Fumaria, Mirabilis and Dipsacus, and others, each xylem strand of the root divides by radial division forming branches, they swing in their lateral direction; one towards right and the other goes to the left. These branches join the phloem strands on the inside. The phloem strands, do not change their position and also remain unchanged in their orientation. They remain in the form of straight strands continuously from the root into the stem. In this type as many primary bundles are formed in the stem as many phloem strands are formed in the root. Dipsacus
2. In Cucurbita, Phaseolus, Acer and Trapaeolum and others, the xylem and phloem strands fork, the branches of the strands of both swing in lateral direction and join in pairs. After joining in the pairs they remain in the alternate position of the strands in the root. The xylem strands become inverted in their position and the phloem strands do not change their orientation. This way, in the stem, the number of bundles becomes double of the phloem strands found in the root. This type of transition is more commonly found. Phaseolus vulgaris
3. In Lathyrus, Medicago and Phoenix, the xylem strands do not fork and continue their direct course into the stem. These strands twist through 180 degrees. The phloem strands divide soon and the resulting halves swing in the lateral direction to the xylem positions. The phloem strands join the xylem strands on the outside. In this type as many bundles are formed as there are phloem strands in the root (as in type 1). Lathyrus tuberosus
4. This type is rarely found and is known in only a few monocotyledons (e.g., Anemarrhena). In this type half of the xylem strands fork and the branches swing in their lateral direction to join the other undivided strands of xylem which become inverted. The phloem strands do not divide but they become united in pairs. These united phloem strands unite with the triple strands of the xylem. Thus, a single bundle of the stem consists of five united strands. In this type half as many bundles are formed in the stem as there are phloem strands in the root. Zhi-Mu (Anemarrhena asphodeloides)
• The transition zone of root apices is a unique part of the whole plant body. Apart from tip-growing cells, such as root hairs and pollen tubes, the cells of the transition zone have the highest rate of vesicle recycling activity, and their auxin transport shows the highest degree of activity. In this root apex zone synchronized electrical activity has been reported. The activity of auxin-secreting domains of the transition zone is sensitive not only to internal developmental cues but, importantly, also to environmental inputs such as light and gravity.
Dicot roots:• The parenchyma cells of the cortex• The Cortex contains ground tissue store starch and other substances. which stores spaces between the They have air photosynthetic cells which are active in the uptake products. It is essential for aeration of the root tissue as they are non of water and minerals. photosynthetic.• The vascular tissue, i.e.cylinder once• The Endodermis is a the xylem and the thick that forms a boundary cell phloem forms a central cylinder through the root and is surrounded by between the cortex and the cells the pericycle which is a ring of stele. It contains lateral roots arise. from which the casparian strip.• The primary xylem of Dicot roots• The Pericycle is found just inside forms a star shape in the center of the of the endodermis. It may become vascular cylinder with usually 3 or 4 meristematic. It is responsible no points, unlike monocots, there is for central pith of parenchyma cells. the formation of lateral roots.• The Epidermis of the dicot root• Vasculardermal tissue and the contains Tissue contains acts mostly and Phloem and forms an Xylem to protect the root Figure showing X-shaped pattern in very center of the cross section root of a dicot root
Monocot roots• In the Fibrous rootform bulbs, such as Many monocots system of Monocots, the primary root is and tulips. These are not onion, gladiolus, almost non-existent. The secondary roots are important in root structures, but rather modified absorption, but are not as deep as the primarymade of compact leaves. stems, root of most Dicot.• Because many monocots have shallow• The Epidermis containssystem has manyand root systems (the fibrous dermal tissue it protects the that spread more on top secondary roots root. than they grow deep into the ground),• The Cortex contains ground tissue which secondary or adventitious roots will be stores photosynthetic products. It is also produced.• Most monocotyledon plants such as grass active in the uptake of water and minerals and onions have fibrous foot systems. The• The Endodermis is a cylinder once cell actual root structure differs from the dicotsthat forms a tend to have parallel the thick as Monocots boundary between vein systems in their stems. cortex and the stele. It is even more• In monocots, the first root to emerge from distinct thanoff, andcounterpart. central the seed dies Dicot so no strong, It also tap root forms. Instead, monocots sprout contains the casparian strip. roots from shoot tissue near the base,• The Vascular Tissue contains the Xylem called adventitious roots. The familiar fibrous root systemIt forms a ring near and the Phloem. of grasses is an example of this rooting pattern. Figure showing the center of plant• The Pith is the center of most region of cross section of a root. monocot
References• http://faculty.clintoncc.suny.edu/faculty/michael.gregory/files/bio%20102/bio%20102%20lectures/plant%20structure/plant%20structure.htm [Accessed on 21.01.12]• http://plantsinaction.science.uq.edu.au/edition1/?q=content/7-1-1-root-apical-meristems [Accessed on 21.01.12]• http://www.cls.zju.edu.cn/sub/fulab/plant_Antomy/plant/mitosis/root1/038344.html [Accessed on 21.01.12]• http://www.public.iastate.edu/~bot.512/lectures/Roots.htm [Accessed on 21.01.12]• http://www.aensionline.com/jasr/jasr/2011/706-713.pdf [Accessed on 22.01.12]• http://hortsci.ashspublications.org/content/28/5/545.1 [Accessed on 22.01.12]• http://www.cell.com/trends/plant-science/abstract/S1360-1385(09)00146-0 [Accessed on 22.01.12]• http://www.gmi.oeaw.ac.at/documents/extranet-documents/publications/Sanchezetal.pdf [Accessed on 22.01.12]• Biology, Raven et al., 6th edition• introduction to plant structure and development, 2005, Charles Beck, Cambridge University Press An• http://www.northlakebiology.com/1411/lab/Botany%20Lab%20Unit%202%20-%20Root%20Structure%20and%20Function.PDF [Accessed on the 22.01.12]• http://www.northlakebiology.com/1411/lab/Botany%20Lab%20Unit%202%20-%20Root%20Structure%20and%20Function.PDF [Accessed on the 22.01.12]• http://prakashamarasooriya.wikispaces.com/file/view/Root+Structure.pdf [Accessed on the 22.01.12]• http://www.biologyreference.com/Mo-Nu/Monocots.html#b [Accessed on the 22.01.12]• http://people.bridgewater.edu/~lhill/Monocotdicot.htm [Accessed on the 22.01.12]• http://www.northlakebiology.com/1411/lab/Botany%20Lab%20Unit%202%20-%20Root%20Structure%20and%20Function.PDF [Accessed on the 22.01.12]• http://www.tutorvista.com/content/biology/biology-iii/angiosperm-morphology/roots-modifications.php• http://www.beta-theta.com/Biology/Root.html• http://www.mhhe.com/biosci/genbio/tlw3/eBridge/Chp17/17_3.pdf• http://www.sakshieducation.com/(S(2iwaqj3wm3ymsemwhqzaud55))/Inter(New)/..%5CEAMCET%5CQR%5CBotany%5Cjr_botanysynopsis%5C04Root.pdf• http://edudel.nic.in/PAHAL/biology_260309/biology_dt_260309.pdf