In Vascular Plants

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Invascular plants, the root is theorgan of a plantthat typically lies below the

surface of the soil. This is not always the case, however, since a root can also

beaerial(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 (seerhizome). So, it is better to define

rootas 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 aplantis called theradicle. The four major

functions of roots are 1)absorptionof water and inorganic nutrients, 2)

anchoring of the plant body to the ground, andsupportingit, 3) storage of

food andnutrients, 4)vegetative reproduction. In response to the

concentration of nutrients, roots also synthesisecytokinin, 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 certainfungito formmycorrhizas, and a large range of other

organisms includingbacteriaalso closely associate with roots

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 capcells (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 ofair,

mineralnutrientsandwaterexists 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. Atgermination, roots grow

downward due togravitropism, the growth mechanism of plants that also

causes the shoot to grow upward. In some plants (such asivy), 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 ofwoody planttissues and many nonwoody plants. For example,

storage roots ofsweet potatohave secondary growth but are not woody.

Secondary growth occurs at thelateral meristems, namely thevascular

cambiumandcork cambium. The former formssecondary xylemand

secondary phloem, while the latter forms theperiderm.

In plants with secondary growth, the vascular cambium, originating between thexylem and the phloem, forms acylinderof tissue along thestemand root. The
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vascular cambium 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

protectivecorkcells containing suberin. In roots, the cork cambium originates

in thepericycle, 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.

Vascular plants (also known as tracheophytes or higher plants) form a large

group of plants that are very roughly defined. Vascular plants are thoseplants

that havelignifiedtissuesfor conductingwater, minerals, and photosynthetic

products through the plant. Vascular plants include theclubmosses,

Equisetum,ferns,gymnosperms(includingconifers) andangiosperms

(flowering plants). Scientific names for the group include Tracheophyta[2]

and

Tracheobionta.[3]

and water from the soil and the organic compounds produced in leaves are

distributed to specific areas in the plant through thexylemand phloem. The

xylem draws water and nutrients up from the roots to the upper sections of

the plant's body, and the phloem conducts other materials, such as thesucrose

produced duringphotosynthesis, which gives the plant energy to keep growing

and seeding.

The xylem consists oftracheids, which are dead hard-walled hollow cells arranged to

form tiny tubes to function in water transport. A tracheid cell wall usually

contains the polymerlignin. The phloem however consists of living cells called

sieve-tube members. Between the sieve-tube members are sieve plates, which

have pores to allow molecules to pass through. Sieve-tube members lack such

organs asnucleiorribosomes, but cells next to them, the companion cells,

function to keep the sieve-tube members alive.

The movement of nutrients, water and sugars is affected by transpiration,

conduction and absorption of water.

[edit] Transpiration

The most abundantcompoundin all plants, as in all known life, iswaterwhich serves

an important role in the various processes taking place.Transpirationis the
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main process a plant can call upon to move compounds within its tissues. The

basic minerals and nutrients a plant is composed of remain, generally, within

the plant. Water is constantly lost from the plant through itsstomatato the

atmosphere.

Water is transpired from the plants leaves viastomata, carried there via leafveins

and vascular bundles within the plantscambiumlayer. The movement of water

out of the leaf stomata creates, when the leaves are considered collectively, atranspiration pull. The pull is created through watersurface tensionwithin the

plant cells. The draw of water upwards is assisted by the movement of water

into the roots viaosmosis. This process also assists the plant in absorbing

nutrients from the soil as solublesalts, a process known as absorption.

Surprisingly, the movement of water upwards requires very little or no energy

from the plant.Hydrogen bondsexist betweenwatermolecules, causing them

to line up; as the molecules at the top of the plant evaporate, each pulls the

next one up to replace it, which in turn pulls on the next one in line.

[edit] Absorption

Xylemvessels allow the movement of water and nutrients upwards towards the

shootsandleavesthrough therootsand fineroot hairsfrom thesoil. Living

root cells passively absorb water in the absence of transpiration pull via

osmosiscreating root pressure. It is possible for there to be no

evapotranspirationand therefore no pull of water towards the shoots and

leaves. This is usually due to high temperatures, highhumidity, darkness or

drought.

[edit] Conduction

Xylem andphloemtissues are involved in the conduction processes within plants.

Sugars are conducted throughout the plant in the phloem and other nutrients

through the xylem. Conduction occurs from a source to a sink for each

separate nutrient. Sugars are produced in the leaves (a source) by

photosynthesisand transported to the roots (a sink) for use incellular

respirationor storage. Minerals are absorbed in the roots (a source) and

transported to the shoots to allow cell division and growth.[11]

Non-vascular plants is a general term for thoseplantswithout avascular system

(xylemandphloem). Although non-vascular plants lack these particular tissues,

a number of non-vascular plants possess tissues specialized for internal

transport ofwater.

Non-vascular plants do not have a wide variety of specialized tissue.Liverwortshave

structures that look likeleaves, but they are not true leaves because they have

noxylemorphloem. Likewise, mosses and algae have no such tissues.
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All plants have a life cycle with analternation of generationsbetween a diploid

sporophyteand a haploidgametophyte, but only nonvascular plants can

potentially have a dominant gametophyte generation. In these plants, the

sporophytes grow from and are dependent on gametophytes for taking in

water and other materials.

Lignin or lignen is a complexchemical compoundmost commonly derived fromwood, and

an integral part of the secondarycell wallsofplants

[1]

and somealgae.

[2]

The term wasintroduced in 1819 byde Candolleand is derived from the Latin wordlignum,

[3]meaning

wood. It is one of the most abundantorganic polymersonEarth, exceeded only bycellulose,

employing 30% of non-fossilorganic carbon,[4]

and constituting from a quarter to a third of

the dry mass of wood. As abiopolymer, lignin is unusual because of itsheterogeneityand

lack of a defined primary structure. Its most commonly noted function is the support

through strengthening of wood (xylemcells) in trees.[5][6][7]

Global production of lignin is around 1.1 million tonnes per year and is used in a wide range

of low volume

Xylem is one of the two types of transport tissue invascular plants(phloemis the other).

The wordxylem is derived from theGreekword (xylon), meaning "wood"; the best-

known xylem tissue iswood, though it is found throughout the plant. Its basic function is to

transport water, but it also transports somenutrientsthrough the plant.

Contents

[hide]

1 Structure 2 Primary and secondary xylem 3 Main function upwards water transport

o 3.1 Cohesion-tension theoryo 3.2 Measurement of pressure

4 Evolution 5 Development

o 5.1 Protoxylem and metaxylemo 5.2 Patterns of protoxylem and metaxylem

6 See also

7 Referenceso 7.1 General references

[edit] Structure

The most distinctive xylemcellsare the long tracheary elements that transport water.

Tracheidsandvessel elementsare distinguished by the multitude of spots that occur on the

vessel elements. In these spots both outer and innerwallsare missing. The vessel elements

are connected together into long tubes that are called vessels.[2]

Xylem also contains other cell types, for exampleparenchyma.[3]
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Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Tracheidhttp://en.wikipedia.org/wiki/Tracheidhttp://en.wikipedia.org/wiki/Vessel_elementhttp://en.wikipedia.org/wiki/Vessel_elementhttp://en.wikipedia.org/wiki/Vessel_elementhttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Xylem#cite_note-1http://en.wikipedia.org/wiki/Xylem#cite_note-1http://en.wikipedia.org/wiki/Xylem#cite_note-1http://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Xylem#cite_note-2http://en.wikipedia.org/wiki/Xylem#cite_note-2http://en.wikipedia.org/wiki/Xylem#cite_note-2http://en.wikipedia.org/wiki/Xylem#cite_note-2http://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Xylem#cite_note-1http://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Vessel_elementhttp://en.wikipedia.org/wiki/Tracheidhttp://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/w/index.php?title=Xylem&action=edit&section=1http://en.wikipedia.org/wiki/Xylem#General_referenceshttp://en.wikipedia.org/wiki/Xylem#Referenceshttp://en.wikipedia.org/wiki/Xylem#See_alsohttp://en.wikipedia.org/wiki/Xylem#Patterns_of_protoxylem_and_metaxylemhttp://en.wikipedia.org/wiki/Xylem#Protoxylem_and_metaxylemhttp://en.wikipedia.org/wiki/Xylem#Developmenthttp://en.wikipedia.org/wiki/Xylem#Evolutionhttp://en.wikipedia.org/wiki/Xylem#Measurement_of_pressurehttp://en.wikipedia.org/wiki/Xylem#Cohesion-tension_theoryhttp://en.wikipedia.org/wiki/Xylem#Main_function_.E2.80.93_upwards_water_transporthttp://en.wikipedia.org/wiki/Xylem#Primary_and_secondary_xylemhttp://en.wikipedia.org/wiki/Xylem#Structurehttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Plant_nutritionhttp://en.wikipedia.org/wiki/Woodhttp://en.wikipedia.org/wiki/Greek_languagehttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Lignin#cite_note-6http://en.wikipedia.org/wiki/Lignin#cite_note-4http://en.wikipedia.org/wiki/Lignin#cite_note-4http://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Heterogeneityhttp://en.wikipedia.org/wiki/Biopolymerhttp://en.wikipedia.org/wiki/Lignin#cite_note-boerjan-3http://en.wikipedia.org/wiki/Organic_chemistryhttp://en.wikipedia.org/wiki/Fossilhttp://en.wikipedia.org/wiki/Cellulosehttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Organic_polymerhttp://en.wikipedia.org/wiki/Lignin#cite_note-eero-2http://en.wiktionary.org/wiki/lignumhttp://en.wikipedia.org/wiki/A._P._de_Candollehttp://en.wikipedia.org/wiki/Lignin#cite_note-martone2009-1http://en.wikipedia.org/wiki/Algaehttp://en.wikipedia.org/wiki/Planthttp://en.wikipedia.org/wiki/Planthttp://en.wikipedia.org/wiki/Cell_wallhttp://en.wikipedia.org/wiki/Woodhttp://en.wikipedia.org/wiki/Chemical_compoundhttp://en.wikipedia.org/wiki/Gametophytehttp://en.wikipedia.org/wiki/Sporophytehttp://en.wikipedia.org/wiki/Alternation_of_generations


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Xylem can be found:

invascular bundles, present in non-woody plants and non-woody parts of woodyplants

in secondary xylem, laid down by ameristemcalled thevascular cambiumin woodyplants

as part of astelar arrangementnot divided into bundles, as in manyferns.

In transitional stages of plants with secondary growth, the first two categories are notmutually exclusive, although usually a vascular bundle will containprimary xylem only.

The branching pattern exhibited by xylem followsMurray's law.[4]

Schematic cross section of part of leaf, xylem shown as red circles at figure 8.

[edit] Primary and secondary xylem

Primary xylem is the xylem that is formed during primary growth fromprocambium. It

includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but beforesecondary xylem. It is distinguished by wider vessels and tracheids.Xylem development

occurs in a number of patterns, which vary in the relative position of the protoxylem and

metaxylem, e.g. endarch in which the protoxylem is towards the centre of the stem or root,

or exarch in which the metaxylem is towards the centre.

Secondary xylem is the xylem that is formed during secondary growth fromvascular

cambium. Although secondary xylem is also found in members of the "gymnosperm" groups

GnetophytaandGinkgophytaand to a lesser extent in members of theCycadophyta, the

two main groups in which secondary xylem can be found are:

1. conifers(Coniferae): there are some six hundred species of conifers. All species havesecondary xylem, which is relatively uniform in structure throughout this group.

Many conifers become tall trees: the secondary xylem of such trees is marketed as

softwood.

2. angiosperms(Angiospermae): there are some quarter of a million to four hundredthousand species of angiosperms. Within this group secondary xylem has not been

found in themonocots. In the remainder of the angiosperms, this secondary xylem

may or may not be present; this may vary even within a species, depending on

growing circumstances. In view of the size of this group, it will be no surprise that no

absolutes apply to the structure of secondary xylem within the angiosperms. Manynon-monocot angiosperms become trees, and the secondary xylem of these is

marketed ashardwood.

[edit] Main function upwards water transport

The xylem transports water and soluble mineral nutrients from the roots throughout the

plant. It is also used to replace water lost duringtranspirationand photosynthesis. Xylem

sapconsists mainly of water and inorganic ions, although it can contain a number of organic

chemicals as well. This transport is not powered by energy spent by thetrachearyelements

themselves, which are dead by maturity and no longer have living contents. Twophenomena cause xylem sap to flow:
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Transpirational pull: the most important cause of xylem sap flow is theevaporationofwaterfrom the surfaces ofmesophyllcells to the atmosphere. Thistranspiration

causes millions of minutemeniscito form in the mesophyll cell wall. The resulting

surface tensioncauses a negative pressure ortensionin the xylem that pulls the

water from the roots and soil.

Root pressure: If thewater potentialof the root cells is more negative than that ofthesoil, usually due to high concentrations ofsolute, water can move byosmosis

into the root from the soil. This causes a positive pressure that forces sap up the

xylem towards the leaves. In some circumstances, the sap will be forced from theleaf through ahydathodein a phenomenon known asguttation. Root pressure is

highest in the morning before the stomata open and allow transpiration to begin.

Different plant species can have different root pressures even in a similar

environment; examples include up to 145 kPa inVitis ripariabut around zero in

Celastrus orbiculatus.[5]

The primary force that creates thecapillary actionmovement of water upwards in plants is

the adhesion between the water and the surface of the xylem conduits.[6][7]

Capillary action

provides the force that establishes an equilibrium configuration, balancing gravity. When

transpiration removes water at the top, the flow is needed to return to the equilibrium.

Transpirational pull results from the evaporation of water from the surfaces ofcellsin the

leaves. This evaporation causes the surface of the water to recess into theporesof thecell

wall. Bycapillary action, the water forms concavemenisciinside the pores. The high surface

tension of water pulls theconcavityoutwards, generating enoughforceto lift water as high

as a hundred meters from ground level to atree's highest branches.

Transpirational pull requires that the vessels transporting the water are very small in

diameter, otherwisecavitationwould break the water column. And as waterevaporates

from leaves, more is drawn up through the plant to replace it. When the water pressure

within the xylem reaches extreme levels due to low water input from the roots (if, for

example, the soil is dry), then the gases come out of solution and form a bubble an

embolismforms, which will spread quickly to other adjacent cells, unlessbordered pitsare

present (these have a plug-like structure called a torus, that seals off the opening between

adjacent cells and stops the embolism from spreading).

Development

Patterns of xylem development: xylem in brown; arrows show direction of

development from protoxylem to metaxylem

Xylem development can be described by four terms: centrarch, exarch, endarch and

mesarch. As it develops in young plants, its nature changes fromprotoxylem to metaxylem

(i.e. fromfirst xylem to after xylem). The patterns in which protoxylem and metaxylem are

arranged is important in the study of plant morphology.

[edit] Protoxylem and metaxylem

As a youngvascular plantgrows, one or more strands of primary xylem form in its stems and

roots. The first xylem to develop is called 'protoxylem'. In appearance protoxylem is usuallydistinguished by narrower vessels formed of smaller cells. Some of these cells have walls
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iki/Solutionhttp://en.wikipedia.org/wiki/Soilhttp://en.wikipedia.org/wiki/Water_potentialhttp://en.wikipedia.org/wiki/Root_pressurehttp://en.wikipedia.org/wiki/Tension_(physics)http://en.wikipedia.org/wiki/Surface_tensionhttp://en.wikipedia.org/wiki/Meniscihttp://en.wikipedia.org/wiki/Transpirationhttp://en.wikipedia.org/wiki/Mesophyll_tissuehttp://en.wikipedia.org/wiki/Waterhttp://en.wikipedia.org/wiki/Evaporation


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which contain thickenings in the form of rings or helices. Functionally, protoxylem can

extend: the cells are able to grow in size and develop while a stem or root is elongating.

Later, 'metaxylem' develops in the strands of xylem. Metaxylem vessels and cells are usually

larger; the cells have thickenings which are typically either in the form of ladderlike

transverse bars (scalariform) or continuous sheets except for holes or pits (pitted).

Functionally, metaxylem completes its development after elongation ceases when the cells

no longer need to grow in size.[23][24]

[edit] Patterns of protoxylem and metaxylem

There are four main patterns to the arrangement of protoxylem and metaxylem in stems

and roots.

Centrarch refers to the case in which the primary xylem forms a single cylinder in the centre

of the stem and develops from the centre outwards. The protoxylem is thus found in the

central core and the metaxylem in a cylinder around it.[25]

This pattern was common in early

land plants, such as "rhyniophytes".

The other three terms are used where there is more than one strand of primary xylem.

Exarch is used when there is more than one strand of primary xylem in a stem or root, and

the xylem develops from the outside inwards towards the centre, i.e. centripetally. The

metaxylem is thus closest to the centre of the stem or root and the protoxylem closest to

the periphery. The roots ofvascular plantsare normally considered to have exarch

development.[23]

Endarch is used when there is more than one strand of primary xylem in a stem or root, and

the xylem develops from the inside outwards towards the periphery, i.e. centrifugally. The

protoxylem is thus closest to the centre of the stem or root and the metaxylem closest to

the periphery. The stems ofseed plantstypically have endarch development.[23]

Mesarch is used when there is more than one strand of primary xylem in a stem or root, and

the xylem develops from the middle of a strand in both directions. The metaxylem is thus on

both the peripheral and central sides of the strand with the protoxylem between the

metaxylem (possibly surrounded by it). The leaves and stems of manyfernshave mesarch

development

Invascular plants,phloem is the livingtissuethat carries organicnutrients(known as

photosynthate), in particular,sucrose,[1]

a sugar, to all parts of the plant where needed. In

trees, the phloem is the innermost layer of thebark, hence the name, derived from the

Greekword(phloos) "bark". The phloem is concerned mainly with the transport of

soluble organic material made duringphotosynthesis. This is called translocation.

Phloem tissue consists of: conducting cells, generally called sieve elements;parenchyma

cells, including both specialized companion cells or albuminous cells and unspecialized cells;

and supportive cells, such asfibresandsclereids).

[edit] Conducting cells (Sieve elements)

Sieve elements are the group of cells that are responsible for actually moving sugar-laden

fluids through the plant.[3]

At maturity they lack anucleusand have very few organelles, so
http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/w/index.php?title=Xylem&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Xylem&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Xylem&action=edit&section=9http://en.wikipedia.org/wiki/Xylem#cite_note-24http://en.wikipedia.org/wiki/Xylem#cite_note-24http://en.wikipedia.org/wiki/Xylem#cite_note-24http://en.wikipedia.org/wiki/Rhyniophytehttp://en.wikipedia.org/wiki/Rhyniophytehttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Seed_planthttp://en.wikipedia.org/wiki/Seed_planthttp://en.wikipedia.org/wiki/Seed_planthttp://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Fernhttp://en.wikipedia.org/wiki/Fernhttp://en.wikipedia.org/wiki/Fernhttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Nutrienthttp://en.wikipedia.org/wiki/Nutrienthttp://en.wikipedia.org/wiki/Nutrienthttp://en.wikipedia.org/wiki/Photosynthatehttp://en.wikipedia.org/wiki/Photosynthatehttp://en.wikipedia.org/wiki/Sucrosehttp://en.wikipedia.org/wiki/Sucrosehttp://en.wikipedia.org/wiki/Phloem#cite_note-0http://en.wikipedia.org/wiki/Phloem#cite_note-0http://en.wikipedia.org/wiki/Phloem#cite_note-0http://en.wikipedia.org/wiki/Treehttp://en.wikipedia.org/wiki/Treehttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Greek_languagehttp://en.wikipedia.org/wiki/Greek_languagehttp://en.wiktionary.org/wiki/%CF%86%CE%BB%CF%8C%CE%BF%CF%82http://en.wiktionary.org/wiki/%CF%86%CE%BB%CF%8C%CE%BF%CF%82http://en.wiktionary.org/wiki/%CF%86%CE%BB%CF%8C%CE%BF%CF%82http://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Cell_nucleushttp://en.wikipedia.org/wiki/Cell_nucleushttp://en.wikipedia.org/wiki/Cell_nucleushttp://en.wikipedia.org/wiki/Cell_nucleushttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=2http://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Photosynthesishttp://en.wiktionary.org/wiki/%CF%86%CE%BB%CF%8C%CE%BF%CF%82http://en.wikipedia.org/wiki/Greek_languagehttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Treehttp://en.wikipedia.org/wiki/Phloem#cite_note-0http://en.wikipedia.org/wiki/Sucrosehttp://en.wikipedia.org/wiki/Photosynthatehttp://en.wikipedia.org/wiki/Nutrienthttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Fernhttp://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Seed_planthttp://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Vascular_planthttp://en.wikipedia.org/wiki/Rhyniophytehttp://en.wikipedia.org/wiki/Xylem#cite_note-24http://en.wikipedia.org/w/index.php?title=Xylem&action=edit&section=9http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22http://en.wikipedia.org/wiki/Xylem#cite_note-FosterGifford1974-22


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they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve

tube cells do containvacuolesand other organelles, such asribosomes, before they mature,

but these generally migrate to the cell wall and dissolve at maturity; this ensures there is

little to impede the movement of fluids. One of the few organelles they do contain at

maturity is the smoothendoplasmic reticulum, which can be found at the plasma

membrane, often nearby theplasmodesmatathat connect them to their companion or

albuminous cells. All sieve cells have clusters of pores at their ends that grow from modified

and enlargedplasmodesmata, called sieve areas; the 'sieve' part of sieve cell is from these

groups of pores having a sieve-like appearance. The pores are reinforced by platelets of apolysaccharidecalledcallose.

[3]

[edit] Sieve cells

Sieve cells are the more primitive of the two main conducting cell types in phloem, and are

found in most seedless vascular plants (e.g., ferns, club mosses, horsetails) and

gymnosperms (conifers,Ginkgo, etc.). Sieve cells have relatively narrow, uniformly-sized

pores in the sieve areas.

[edit]Sieve tube cells

The sieve-tube cells, also known as sieve-tube members, are the more advanced type of

conducting cell, and are the only sieve element found in the phloem of angiosperms.[3]

The

sieve tube is an elongated rank of individual cells, arranged end to end, and functioning to

conduct food materials throughout the plant. The sieve areas of these cells are calledsieve

plates; the pores in sieve plates are generally larger and more variable in size than those in

sieve cells.[3]

[edit] Parenchyma cells

[edit]Companion cells

The survival of sieve-tube members depends on a close association with the companion

cells, a specialized form ofparenchymacell. All of the cellular functions of a sieve-tube

element are carried out by the (much smaller) companion cell, a typicalplant cellexcept the

companion cell usually has a larger number ofribosomesandmitochondria. This is because

the companion cell is more metabolically active than a 'typical' plant cell. Thecytoplasmof a

companion cell is connected to the sieve-tube element by plasmodesmata.[3]

There are three types of companion cell.

1. Ordinary companions cells, which have smooth walls and few or noplasmodesmata connections to cells other than the sieve tube.

2. Transfer cells, which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialised in

scavenging solutes from those in the cell walls that are actively

pumped requiring energy.

3. Intermediary cells, which have smooth walls and numerousplasmodesmata connecting them to other cells.
http://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Polysaccharidehttp://en.wikipedia.org/wiki/Polysaccharidehttp://en.wikipedia.org/wiki/Callosehttp://en.wikipedia.org/wiki/Callosehttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=3http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=3http://en.wikipedia.org/wiki/Ginkgohttp://en.wikipedia.org/wiki/Ginkgohttp://en.wikipedia.org/wiki/Ginkgohttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=4http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=4http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Sieve_platehttp://en.wikipedia.org/wiki/Sieve_platehttp://en.wikipedia.org/wiki/Sieve_platehttp://en.wikipedia.org/wiki/Sieve_platehttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=5http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=6http://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Plant_cellhttp://en.wikipedia.org/wiki/Plant_cellhttp://en.wikipedia.org/wiki/Plant_cellhttp://en.wikipedia.org/wiki/Ribosomeshttp://en.wikipedia.org/wiki/Ribosomeshttp://en.wikipedia.org/wiki/Ribosomeshttp://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Transfer_cellshttp://en.wikipedia.org/wiki/Transfer_cellshttp://en.wikipedia.org/wiki/Transfer_cellshttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Mitochondriahttp://en.wikipedia.org/wiki/Ribosomeshttp://en.wikipedia.org/wiki/Plant_cellhttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=6http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=5http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Sieve_platehttp://en.wikipedia.org/wiki/Sieve_platehttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=4http://en.wikipedia.org/wiki/Ginkgohttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=3http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Callosehttp://en.wikipedia.org/wiki/Polysaccharidehttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Plasmodesmatahttp://en.wikipedia.org/wiki/Endoplasmic_reticulumhttp://en.wikipedia.org/wiki/Ribosomehttp://en.wikipedia.org/wiki/Vacuole


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The first two types of cell collect solutes throughapoplastic(cell wall) transfers, whilst the

third type can collect solutes via thesymplastthrough the plasmodesmata connections.

[edit]Albuminous cells

Albuminous cells have a similar role to companion cells, but are associated with sieve cells

only and are therefore found only in seedless vascular plants and gymnosperms.[3]

[edit]Other parenchyma cells

Otherparenchymacells within the phloem are generally undifferentiated and used for food

storage.[3]

[edit] Supportive cells

Although its primary function is fluid transport, phloem may also contain supportive cells.

These generally fall into two categories:fibresandsclereids. Both cell types have a

secondary cell walland are therefore dead at maturity. The secondary cell wall makes them

particularly rigid and resistant to damage.

[edit]Fibres

Fibres are the long, narrow supportive cells that providetensionstrength without limiting

flexibility. They are also found in xylem, and are the main component of many textiles such

as paper, linen, and cotton.[3]

[3]

[edit]Sclereids

Sclereids are irregularly-shaped cells that add compression strength[3]but may reduceflexibility to some extent. They also serve as anti-herbivory structures, as their irregular

shape and hardness will increase wear on teeth as the herbivores chew. For example, they

are responsible for the gritty texture in pears.

[edit] Function

Unlikexylem(which is composed primarily of dead cells), the phloem is composed of still-

living cells that transportsap. The sap is a water-based solution, but rich insugarsmade by

the photosynthetic areas. These sugars are transported to non-photosynthetic parts of the

plant, such as the roots, or into storage structures, such astubersor bulbs.

The Pressure flow hypothesis was a hypothesis proposed byErnst Mnchin 1930 that

explained the mechanism of phloem translocation.[4]

A high concentration of organic

substance insidecellsof the phloem at a source, such as aleaf, creates adiffusion gradient

that draws water into the cells. Movement occurs by bulk flow; phloem sap moves from

sugar sources to sugar sinks by means ofturgorpressure gradient. A sugar source is any part

of the plant that is producing or releasing sugar.

During the plant's growth period, usually during the spring, storage organs such as theroots

are sugar sources, and the plant's many growing areas are sugar sinks. The movement in

phloem is multidirectional, whereas, in xylem cells, it is unidirectional (upward).
http://en.wikipedia.org/wiki/Apoplasthttp://en.wikipedia.org/wiki/Apoplasthttp://en.wikipedia.org/wiki/Apoplasthttp://en.wikipedia.org/wiki/Symplasthttp://en.wikipedia.org/wiki/Symplasthttp://en.wikipedia.org/wiki/Symplasthttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=7http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=7http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=7http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=8http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=8http://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=9http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=9http://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Secondary_cell_wallhttp://en.wikipedia.org/wiki/Secondary_cell_wallhttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=10http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=10http://en.wikipedia.org/wiki/Tensionhttp://en.wikipedia.org/wiki/Tensionhttp://en.wikipedia.org/wiki/Tensionhttp://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=11http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=11http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=11http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=12http://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Sap_(plant)http://en.wikipedia.org/wiki/Sap_(plant)http://en.wikipedia.org/wiki/Sap_(plant)http://en.wikipedia.org/wiki/Sugarhttp://en.wikipedia.org/wiki/Sugarhttp://en.wikipedia.org/wiki/Sugarhttp://en.wikipedia.org/wiki/Tuberhttp://en.wikipedia.org/wiki/Tuberhttp://en.wikipedia.org/wiki/Tuberhttp://en.wikipedia.org/wiki/Ernst_M%C3%BCnchhttp://en.wikipedia.org/wiki/Ernst_M%C3%BCnchhttp://en.wikipedia.org/wiki/Ernst_M%C3%BCnchhttp://en.wikipedia.org/wiki/Phloem#cite_note-3http://en.wikipedia.org/wiki/Phloem#cite_note-3http://en.wikipedia.org/wiki/Phloem#cite_note-3http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Turgorhttp://en.wikipedia.org/wiki/Turgorhttp://en.wikipedia.org/wiki/Turgorhttp://en.wikipedia.org/wiki/Roothttp://en.wikipedia.org/wiki/Roothttp://en.wikipedia.org/wiki/Roothttp://en.wikipedia.org/wiki/Roothttp://en.wikipedia.org/wiki/Turgorhttp://en.wikipedia.org/wiki/Diffusionhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Phloem#cite_note-3http://en.wikipedia.org/wiki/Ernst_M%C3%BCnchhttp://en.wikipedia.org/wiki/Tuberhttp://en.wikipedia.org/wiki/Sugarhttp://en.wikipedia.org/wiki/Sap_(plant)http://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=12http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=11http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Tensionhttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=10http://en.wikipedia.org/wiki/Secondary_cell_wallhttp://en.wikipedia.org/wiki/Sclereidhttp://en.wikipedia.org/wiki/Fibreshttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=9http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/wiki/Parenchymahttp://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=8http://en.wikipedia.org/wiki/Phloem#cite_note-Raven_et_al._1992-2http://en.wikipedia.org/w/index.php?title=Phloem&action=edit&section=7http://en.wikipedia.org/wiki/Symplasthttp://en.wikipedia.org/wiki/Apoplast


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After the growth period, when themeristemsare dormant, theleavesare sources, and

storage organs are sinks. Developingseed-bearing organs (such asfruit) are always sinks.

Because of this multi-directional flow, coupled with the fact that sap cannot move with ease

between adjacent sieve-tubes, it is not unusual for sap in adjacent sieve-tubes to be flowing

in opposite directions.

While movement of water and minerals through the xylem is driven by negative pressures

(tension) most of the time, movement through the phloem is driven by positivehydrostatic

pressures. This process is termed translocation, and is accomplished by a process calledphloem loading and unloading. Cells in a sugar source "load" a sieve-tube element by

actively transportingsolute molecules into it. This causes water to move into the sieve-tube

element byosmosis, creating pressure that pushes the sap down the tube. In sugar sinks,

cells actively transport solutes outof the sieve-tube elements, producing the exactly

opposite effect.

Some plants, however, appear not to load phloem by active transport. In these cases a

mechanism known as thepolymer trap mechanismwas proposed byRobert Turgeon.[5]

In

this case small sugars such as sucrose move into intermediary cells through narrow

plasmodesmata, where they are polymerised toraffinoseand other largeroligosaccharides.Now they are unable to move back, but can proceed through wider plasmodesmata into the

sieve tube element.

The symplastic phloem loading (polymer trap mechanism above) is confined mostly to

plants in tropical rain forests and is seen as more primitive. The actively-transported

apoplastic phloem loading is viewed as more advanced, as it is found in the later-evolved

plants, and particularly in those in temperate and arid conditions. This mechanism may,

therefore, have allowed plants to colonise the cooler locations.

Organicmoleculessuch as sugars,amino acids, certainhormones, and evenmessenger

RNAsare transported in the phloem throughsieve tube elements

The vascular cambium (pl. cambia or cambiums) is a part of themorphologyofplants. It

consists of cells that are partly specialized, for the tissues that transport water solutions, but

have not reached any of the final forms that occur in their branch of the specialization

graph. When these cells have divided and specialized further they make up the secondary

vascular tissues, secondaryxylemand the secondaryphloem.

The vascular cambium is alateral meristemin thevascular tissueof plants. The vascular

cambium is the source of both thesecondary xylem(inwards, towards the pith) and the

secondaryphloem(outwards), and is located between these tissues in the stem and root. Afew leaf types also have a vascular cambium.

[2]

[edit] Origin

Vascular cambium arises from the primary meristem, procambium that remains

undifferentiated between the primary xylem and primary phloem. Upon maturity, this

region known as the fascicular cambium, and the area of cells between the vascular bundles

(fascicles) called pith rays becomes what is called the interfascicular cambium. The fascicular

and interfascicular cambiums, therefore, represent a continuous ring which bisects the

primary xylem and primary phloem. The vascular cambium then produces secondary xylem
http://en.wikipedia.org/wiki/Meristemhttp://en.wikipedia.org/wiki/Meristemhttp://en.wikipedia.org/wiki/Meristemhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Seedhttp://en.wikipedia.org/wiki/Seedhttp://en.wikipedia.org/wiki/Seedhttp://en.wikipedia.org/wiki/Fruithttp://en.wikipedia.org/wiki/Fruithttp://en.wikipedia.org/wiki/Fruithttp://en.wikipedia.org/wiki/Hydrostatic_pressurehttp://en.wikipedia.org/wiki/Hydrostatic_pressurehttp://en.wikipedia.org/wiki/Hydrostatic_pressurehttp://en.wikipedia.org/wiki/Hydrostatic_pressurehttp://en.wikipedia.org/wiki/Active_transporthttp://en.wikipedia.org/wiki/Active_transporthttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/w/index.php?title=Polymer_trap_mechanism&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Polymer_trap_mechanism&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Polymer_trap_mechanism&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Robert_Turgeon&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Robert_Turgeon&action=edit&redlink=1http://en.wikipedia.org/wiki/Phloem#cite_note-4http://en.wikipedia.org/wiki/Phloem#cite_note-4http://en.wikipedia.org/wiki/Phloem#cite_note-4http://en.wikipedia.org/wiki/Raffinosehttp://en.wikipedia.org/wiki/Raffinosehttp://en.wikipedia.org/wiki/Raffinosehttp://en.wikipedia.org/wiki/Oligosaccharideshttp://en.wikipedia.org/wiki/Oligosaccharideshttp://en.wikipedia.org/wiki/Oligosaccharideshttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Sieve_tube_elementhttp://en.wikipedia.org/wiki/Sieve_tube_elementhttp://en.wikipedia.org/wiki/Sieve_tube_elementhttp://en.wikipedia.org/wiki/Morphology_(biology)http://en.wikipedia.org/wiki/Morphology_(biology)http://en.wikipedia.org/wiki/Morphology_(biology)http://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Lateral_meristemhttp://en.wikipedia.org/wiki/Lateral_meristemhttp://en.wikipedia.org/wiki/Lateral_meristemhttp://en.wikipedia.org/wiki/Vascular_tissuehttp://en.wikipedia.org/wiki/Vascular_tissuehttp://en.wikipedia.org/wiki/Vascular_tissuehttp://en.wikipedia.org/wiki/Secondary_xylemhttp://en.wikipedia.org/wiki/Secondary_xylemhttp://en.wikipedia.org/wiki/Secondary_xylemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Vascular_cambium#cite_note-1http://en.wikipedia.org/wiki/Vascular_cambium#cite_note-1http://en.wikipedia.org/wiki/Vascular_cambium#cite_note-1http://en.wikipedia.org/w/index.php?title=Vascular_cambium&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Vascular_cambium&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Vascular_cambium&action=edit&section=1http://en.wikipedia.org/w/index.php?title=Vascular_cambium&action=edit&section=1http://en.wikipedia.org/wiki/Vascular_cambium#cite_note-1http://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Secondary_xylemhttp://en.wikipedia.org/wiki/Vascular_tissuehttp://en.wikipedia.org/wiki/Lateral_meristemhttp://en.wikipedia.org/wiki/Phloemhttp://en.wikipedia.org/wiki/Xylemhttp://en.wikipedia.org/wiki/Plantshttp://en.wikipedia.org/wiki/Morphology_(biology)http://en.wikipedia.org/wiki/Sieve_tube_elementhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Messenger_RNAhttp://en.wikipedia.org/wiki/Hormonehttp://en.wikipedia.org/wiki/Amino_acidhttp://en.wikipedia.org/wiki/Moleculehttp://en.wikipedia.org/wiki/Oligosaccharideshttp://en.wikipedia.org/wiki/Raffinosehttp://en.wikipedia.org/wiki/Phloem#cite_note-4http://en.wikipedia.org/w/index.php?title=Robert_Turgeon&action=edit&redlink=1http://en.wikipedia.org/w/index.php?title=Polymer_trap_mechanism&action=edit&redlink=1http://en.wikipedia.org/wiki/Osmosishttp://en.wikipedia.org/wiki/Active_transporthttp://en.wikipedia.org/wiki/Hydrostatic_pressurehttp://en.wikipedia.org/wiki/Hydrostatic_pressurehttp://en.wikipedia.org/wiki/Fruithttp://en.wikipedia.org/wiki/Seedhttp://en.wikipedia.org/wiki/Leafhttp://en.wikipedia.org/wiki/Meristem


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on the inside of the ring, and secondary phloem on the outside, pushing the primary xylem

and phloem apart.

The vascular cambium usually consists of two types of cells:

Fusiform initials (tall cells,axiallyoriented) Ray initials (almost isodiametric cells - smaller and round to angular in shape)

The vascular cambium is a type ofmeristem- tissue consisting of embryonic (incompletelydifferentiated)cellsfrom which other (more differentiated) plant tissues originate. Primary

meristems are theapical meristemson root tips and shoot tips. Another lateral meristem is

thecork cambium, which produces cork, part of thebark. Together, the secondary vascular

tissues (produced by the vascular cambium) and periderm (formed by the cork cambium)

makes up the secondary plant body.

Vascular cambia are found indicotsandgymnospermsbut notmonocots, which usually lack

secondary growth. For successfulgrafting, the vascular cambia of the stock and scion must

be aligned so they can grow together. In wood, the vascular cambium is the obvious line

separating the bark and wood.[3]

.A meristem is thetissuein mostplantsconsisting of undifferentiated cells (meristematic

cells), found in zones of the plant where growth can take place.

The meristematic cells give rise to various organs of the plant, and keep the plant growing.

The Shoot Apical Meristem (SAM) gives rise to organs like the leaves and flowers. The cells

of the apical meristems - SAM and RAM (Root Apical Meristem) - divide rapidly and are

considered to be indeterminate, in that they do not possess any defined end fate. In that

sense, the meristematic cells are frequently compared to thestem cellsinanimals, that

have an analogous behavior and function.

The term meristem was first used in 1858 byKarl Wilhelm von Ngeli(18171891) in his

book Beitrge zur Wissenschaftlichen Botanik.[1]

It is derived from the Greek word merizein

(), meaning to divide, in recognition of its inherent function.

In general, differentiated plant cells cannot divide or produce cells of a different type.

Therefore,cell divisionin the meristem is required to provide new cells for expansion and

differentiation of tissues and initiation of new organs, providing the basic structure of the

plant body.

Meristematic cells are incompletely or not at alldifferentiated, and are capable of continuedcellular division (youthful). Furthermore, the cells are small andprotoplasmfills the cell

completely. Thevacuolesare extremely small. Thecytoplasmdoes not contain

differentiatedplastids(chloroplastsorchromoplasts), although they are present in

rudimentary form (proplastids). Meristematic cells are packed closely together without

intercellular cavities. The cell wall is a very thinprimary cell wall.

Maintenance of the cells requires a balance between two antagonistic processes: organ

initiation and stem cell population renewal.
http://en.wiktionary.org/wiki/axialhttp://en.wiktionary.org/wiki/axialhttp://en.wiktionary.org/wiki/axialhttp://en.wikipedia.org/wiki/Meristemhttp://en.wikipedia.org/wiki/Meristemhttp://en.wikipedia.org/wiki/Meristemhttp://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Cell_(biology)http://en.wikipedia.org/wiki/Apical_meristemhttp://en.wikipedia.org/wiki/Apical_meristemhttp://en.wikipedia.org/wiki/Apical_meristemhttp://en.wikipedia.org/wiki/Cork_cambiumhttp://en.wikipedia.org/wiki/Cork_cambiumhttp://en.wikipedia.org/wiki/Cork_cambiumhttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Dicothttp://en.wikipedia.org/wiki/Dicothttp://en.wikipedia.org/wiki/Dicothttp://en.wikipedia.org/wiki/Gymnospermhttp://en.wikipedia.org/wiki/Gymnospermhttp://en.wikipedia.org/wiki/Gymnospermhttp://en.wikipedia.org/wiki/Monocothttp://en.wikipedia.org/wiki/Monocothttp://en.wikipedia.org/wiki/Monocothttp://en.wikipedia.org/wiki/Graftinghttp://en.wikipedia.org/wiki/Graftinghttp://en.wikipedia.org/wiki/Graftinghttp://en.wikipedia.org/wiki/Vascular_cambium#cite_note-2http://en.wikipedia.org/wiki/Vascular_cambium#cite_note-2http://en.wikipedia.org/wiki/Vascular_cambium#cite_note-2http://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Planthttp://en.wikipedia.org/wiki/Planthttp://en.wikipedia.org/wiki/Planthttp://en.wikipedia.org/wiki/Stem_cellshttp://en.wikipedia.org/wiki/Stem_cellshttp://en.wikipedia.org/wiki/Stem_cellshttp://en.wikipedia.org/wiki/Animalhttp://en.wikipedia.org/wiki/Animalhttp://en.wikipedia.org/wiki/Animalhttp://en.wikipedia.org/wiki/Karl_Wilhelm_von_N%C3%A4gelihttp://en.wikipedia.org/wiki/Karl_Wilhelm_von_N%C3%A4gelihttp://en.wikipedia.org/wiki/Karl_Wilhelm_von_N%C3%A4gelihttp://en.wikipedia.org/wiki/Lateral_meristem#cite_note-0http://en.wikipedia.org/wiki/Lateral_meristem#cite_note-0http://en.wikipedia.org/wiki/Lateral_meristem#cite_note-0http://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Cellular_differentiationhttp://en.wikipedia.org/wiki/Cellular_differentiationhttp://en.wikipedia.org/wiki/Cellular_differentiationhttp://en.wikipedia.org/wiki/Protoplasmhttp://en.wikipedia.org/wiki/Protoplasmhttp://en.wikipedia.org/wiki/Protoplasmhttp://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Plastidhttp://en.wikipedia.org/wiki/Plastidhttp://en.wikipedia.org/wiki/Plastidhttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Chromoplasthttp://en.wikipedia.org/wiki/Chromoplasthttp://en.wikipedia.org/wiki/Chromoplasthttp://en.wikipedia.org/wiki/Proplastidhttp://en.wikipedia.org/wiki/Proplastidhttp://en.wikipedia.org/wiki/Proplastidhttp://en.wikipedia.org/wiki/Proplastidhttp://en.wikipedia.org/wiki/Chromoplasthttp://en.wikipedia.org/wiki/Chloroplasthttp://en.wikipedia.org/wiki/Plastidhttp://en.wikipedia.org/wiki/Cytoplasmhttp://en.wikipedia.org/wiki/Vacuolehttp://en.wikipedia.org/wiki/Protoplasmhttp://en.wikipedia.org/wiki/Cellular_differentiationhttp://en.wikipedia.org/wiki/Mitosishttp://en.wikipedia.org/wiki/Lateral_meristem#cite_note-0http://en.wikipedia.org/wiki/Karl_Wilhelm_von_N%C3%A4gelihttp://en.wikipedia.org/wiki/Animalhttp://en.wikipedia.org/wiki/Stem_cellshttp://en.wikipedia.org/wiki/Planthttp://en.wikipedia.org/wiki/Biological_tissuehttp://en.wikipedia.org/wiki/Vascular_cambium#cite_note-2http://en.wikipedia.org/wiki/Graftinghttp://en.wikipedia.org/wiki/Monocothttp://en.wikipedia.org/wiki/Gymnospermhttp://en.wikipedia.org/wiki/Dicothttp://en.wikipedia.org/wiki/Barkhttp://en.wikipedia.org/wiki/Cork_cambiumhtt

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