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{{short description|Sugar transport tissue in vascular plants}}
[[File:Xylem and phloem diagram.svg|thumb|323x323px|Phloem (orange) transports products of photosynthesis to various parts of the plant.|alt=]]
[[File:Stem-histology-cross-section-tag.svg|thumb|right|250px|Cross-section of a [[flax]] plant stem: {{ordered list
    |[[Pith]]
    |[[Xylem#Protoxylem and metaxylem|Protoxylem]]
    |[[Xylem#Protoxylem and metaxylem|Metaxylem]]
    |'''Phloem'''
    |[[Sclerenchyma]] ([[bast fibre]])
    |[[Cortex (botany)|Cortex]]
    |[[Epidermis (botany)|Epidermis]]
}}]]

'''Phloem''' ({{IPAc-en|'|f|l|oʊ|.|əm}}, {{respell|FLOH|əm}}) is the living [[biological tissue|tissue]] in [[vascular plant]]s that transports the soluble [[organic compound]]s made during [[photosynthesis]] and known as ''photosynthates'', in particular the sugar [[sucrose]],<ref>
{{cite journal |author=Lalonde S. Wipf D., Frommer W. B. |journal=[[Annual Review of Plant Biology|Annu Rev Plant Biol]] |year=2004 |volume=55 |pages=341–72 |title=Transport mechanisms for organic forms of carbon and nitrogen between source and sink |issue=1 |doi=10.1146/annurev.arplant.55.031903.141758 |pmid=15377224|bibcode=2004AnRPB..55..341L }}</ref> to the rest of the plant. This transport process is called translocation.<ref>Collins Edexcel International GCSE Biology, Student Book ({{ISBN|978-0-00-745000-8}}) p.124</ref> In [[tree]]s, the phloem is the innermost layer of the [[bark (botany)|bark]], hence the name, derived from the [[Ancient Greek]] word {{lang|grc|[[wikt:φλοιός|φλοιός]]}} (''phloiós''), meaning "bark".<ref>{{OEtymD|phloem}}</ref><ref>{{LSJ|floio/s|φλοιός|ref}}.</ref> The term was introduced by [[Carl Nägeli]] in 1858.<ref>{{cite journal |last1=Nägeli |first1=Carl |title=Das Wachstum des Stammes und der Wurzel bei den Gefäßpflanzen und die Anordnung der Gefäßstränge im Stengel |journal=Beiträge zur Wissenschaftlichen Botanik (Contributions to Scientific Botany) |date=1858 |volume=1 |pages=1–156 |url=https://www.biodiversitylibrary.org/item/91249#page/5/mode/1up |trans-title=The growth of the stem and of the root among vascular plants and the arrangement of the vascular strands in the stalk |language=de}}  From p. 9:  ''"Ich will die beiden Partien Dauergewebe, welche von dem Cambium nach aussen und nach innen gebildet werden, Phloëm und Xylem nennen."'' (I will call the two parts of the permanent tissue, which are formed by the cambium outwardly and inwardly, "phloëm" and "xylem".)</ref><ref>{{cite book |doi=10.1007/978-3-642-73635-3_10 |chapter=Phloem |title=Ontogeny, Cell Differentiation, and Structure of Vascular Plants |pages=287–368 |year=1989 |last=Buvat |first=Roger |isbn=978-3-642-73637-7}}</ref> Different types of phloem can be distinguished. The early phloem formed in the growth apices is called protophloem. Protophloem eventually becomes obliterated once it connects to the durable phloem in mature organs, the metaphloem.<ref>{{Cite journal |last1=Graeff |first1=Moritz |last2=Hardtke |first2=Christian S. |date=2021 |title=Metaphloem development in the Arabidopsis root tip |url=https://journals.biologists.com/dev/article/148/18/dev199766/270978/Metaphloem-development-in-the-Arabidopsis-root-tip |journal=Development|volume=148 |issue=18 |doi=10.1242/dev.199766 |pmid=34224570 }}</ref><ref>{{Cite book |last=Evert |first=Ray F. |url=https://onlinelibrary.wiley.com/doi/book/10.1002/0470047380 |title=Esau's Plant Anatomy: Meristems, Cells, and Tissues of the Plant Body: Their Structure, Function, and Development |date=2006-08-25 |publisher=Wiley |isbn=978-0-471-73843-5 |edition=1 |language=en |doi=10.1002/0470047380}}</ref> Further, secondary phloem is formed during the thickening of stem structures.<ref>{{Cite journal |last1=Nieminen |first1=Kaisa |last2=Blomster |first2=Tiina |last3=Helariutta |first3=Ykä |last4=Mähönen |first4=Ari Pekka |date=May 2015 |title=Vascular Cambium Development |journal=The Arabidopsis Book |volume=2015 |issue=13 |article-number=e0177 |doi=10.1199/tab.0177 |pmid=26078728 |pmc=4463761 |issn=1543-8120}}</ref>

== Structure ==
{{plain image with caption|Phloem cells.svg|Cross section of some phloem cells|440px}}

Phloem tissue consists of conducting [[Cell (biology)|cells]], generally called sieve elements, [[Ground tissue#Parenchyma|parenchyma]] cells, including both specialized companion cells or albuminous cells and unspecialized cells and supportive cells, such as [[fibres]] and [[sclereid]]s.{{cn|date=January 2025}}

===Conducting cells (sieve elements)===
[[File:Phloem and Xylem in stem.svg|thumb|Simplified phloem and companion cells: {{ordered list |[[Xylem]] |Phloem |[[Cambium]] |[[Pith]]|Companion cells}}]]

[[Sieve tube element]]s are the type of cell that are responsible for transporting sugars throughout the plant.<ref name="Raven et al. 1992" /> At maturity they lack a [[Cell nucleus|nucleus]] and have very few [[organelle]]s, so they rely on companion cells or albuminous cells for most of their metabolic needs. Sieve tube cells do contain [[vacuole]]s and other organelles, such as [[ribosome]]s, 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 rough [[endoplasmic reticulum]], which can be found at the plasma membrane, often nearby the [[plasmodesmata]] that connect them to their companion or albuminous cells. All sieve cells have groups of pores at their ends that grow from modified and enlarged plasmodesmata, called ''sieve areas''. The pores are reinforced by platelets of a [[polysaccharide]] called [[callose]].<ref name="Raven et al. 1992" />

===Parenchyma cells===
Other [[Ground tissue#Parenchyma|parenchyma]] cells within the phloem are generally undifferentiated and used for food storage.<ref name="Raven et al. 1992" />

====Companion cells====
The metabolic functioning of sieve-tube members depends on a close association with the ''companion cells'', a specialized form of parenchyma cell. All of the cellular functions of a sieve-tube element are carried out by the (much smaller) companion cell, a typical nucleate [[plant cell]] except the companion cell usually has a larger number of ribosomes and [[mitochondria]]. The dense cytoplasm of a companion cell is connected to the sieve-tube element by plasmodesmata.<ref name="Raven et al. 1992">{{cite book |last=Raven |first=Peter H. |author2=Evert, R.F. |author3=Eichhorn, S.E. |title=Biology of Plants |year=1992 |publisher=Worth Publishers |location=New York, NY, U.S.A. |page=791 |isbn=978-1-4292-3995-0}}</ref> The common sidewall shared by a sieve tube element and a companion cell has large numbers of plasmodesmata.

There are three types of companion cells.
# ''Ordinary companion cells'', which have smooth walls and few or no plasmodesmatal connections to cells other than the sieve tube.
# ''[[Transfer cell]]s'', which have much-folded walls that are adjacent to non-sieve cells, allowing for larger areas of transfer. They are specialized in scavenging solutes from those in the cell walls that are actively pumped requiring energy.
# ''Intermediary cells'', which possess many vacuoles and plasmodesmata and synthesize raffinose family [[oligosaccharide]]s.<ref>{{Cite journal|last1=Slewinski|first1=Thomas L.|last2=Zhang|first2=Cankui|last3=Turgeon|first3=Robert|date=2013-07-05|title=Structural and functional heterogeneity in phloem loading and transport|journal=Frontiers in Plant Science|volume=4|page=244|doi=10.3389/fpls.2013.00244|issn=1664-462X|pmc=3701861|pmid=23847646|doi-access=free|bibcode=2013FrPS....4..244S }}</ref><ref>{{Cite book|last=Bhatla|first=Satish C.|title=Plant physiology, development and metabolism|date=2018|others=Manju A. Lal|isbn=978-981-13-2023-1|location=Singapore|oclc=1077622456}}</ref>

====Albuminous cells====
Albuminous cells have a similar role to companion cells, but are associated with sieve cells only and are hence found only in seedless vascular plants and [[gymnosperm]]s.<ref name="Raven et al. 1992"/>

===Supportive cells===
Although its primary function is transport of sugars, phloem may also contain cells that have a mechanical support function. These are sclerenchyma cells which generally fall into two categories: fibres and sclereids. Both cell types have a [[secondary cell wall]] and are dead at maturity. The secondary cell wall increases their rigidity and tensile strength, especially because they contain [[lignin]].{{cn|date=January 2025}}

====Fibres====
[[Bast fibre]]s are the long, narrow supportive cells that provide [[Tension (physics)|tension]] strength without limiting flexibility. They are also found in [[xylem]], and are the main component of many textiles such as paper, linen, and cotton.<ref name="Raven et al. 1992" />

====Sclereids====
{{Main|Sclereid}}
Sclereids are irregularly shaped cells that add compression strength<ref name="Raven et al. 1992" /> but may reduce flexibility 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, and in winter pears.<ref>{{Cite journal |last1=Martin-Cabrejas |first1=Maria A. |last2=Waldron |first2=Keith W. |last3=Selvendran |first3=Robert R. |last4=Parker |first4=Mary L. |last5=Moates |first5=Graham K. |date=1994 |title=Ripening-related changes in the cell walls of Spanish pear (Pyrus communis) |url=https://onlinelibrary.wiley.com/doi/10.1111/j.1399-3054.1994.tb03004.x |journal=Physiologia Plantarum |language=en |volume=91 |issue=4 |pages=671–679 |doi=10.1111/j.1399-3054.1994.tb03004.x |bibcode=1994PPlan..91..671M |issn=0031-9317|url-access=subscription }}</ref>

==Function==
[[File:Translocation from the source to the sink within the phloem.svg|thumb|The process of translocation within the phloem]]

Unlike xylem (which is composed primarily of dead cells), the phloem is composed of still-living cells that transport [[Sap (plant)|sap]]. The sap is a water-based solution, but rich in [[sugar]]s made by photosynthesis. These sugars are transported to non-photosynthetic parts of the plant, such as the roots, or into storage structures, such as [[tuber]]s or bulbs.<ref>{{Cite journal |last=Dinant |first=Sylvie |date=2008-05-01 |title=Phloème, transport interorgane et signalisation à longue distance |url=https://www.sciencedirect.com/science/article/pii/S1631069108000589 |journal=Comptes Rendus Biologies |language=fr |volume=331 |issue=5 |pages=334–346 |doi=10.1016/j.crvi.2008.03.001 |pmid=18472079 |issn=1631-0691|url-access=subscription }}</ref>

During the plant's growth period, usually during the spring, storage organs such as the [[root]]s 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).{{citation needed|date=November 2016}}<ref>{{Cite journal|last=Ciffroy, P. & Tanaka, T.|title=Modelling the Fate of Chemicals in Plants|journal=Modelling the Fate of Chemicals in the Environment and the Human Body 57 (2017): 167.|page=174}}</ref>

After the growth period, when the [[meristem]]s are dormant, the [[leaves]] are sources, and storage organs are sinks. Developing [[seed]]-bearing organs (such as [[fruit]]) 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.<ref>{{cite book|last1=Canny|first1=MJ|title=Phloem Translocation|page=124}}</ref>

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 positive [[hydrostatic pressure]]s. This process is termed ''translocation'', and is accomplished by a process called [[phloem loading]] and ''unloading''.{{cn|date=January 2025}}

[[Phloem sap]] is also thought to play a role in sending informational signals throughout vascular plants. "Loading and unloading patterns are largely determined by the [[Electrical resistivity and conductivity|conductivity]] and number of plasmodesmata and the position-dependent function of [[solute]]-specific, [[plasma membrane]] [[transport protein]]s. Recent evidence indicates that mobile proteins and [[RNA]] are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of [[macromolecules]] as they pass through plasmodesmata."<ref name="turgeon2009">{{cite journal |last=Turgeon|first=Robert |author2=Wolf, Shmuel |title=Phloem Transport: Cellular Pathways and Molecular Trafficking |journal=Annual Review of Plant Biology |year=2009 |doi=10.1146/annurev.arplant.043008.092045 |volume=60 |issue=1 |pages=207–21 |pmid=19025382 |bibcode=2009AnRPB..60..207T |s2cid=25574512 }}</ref>

Organic [[molecule]]s such as sugars, [[amino acid]]s, certain [[phytohormone]]s, and even [[messenger RNA]]s are transported in the phloem through sieve tube elements.<ref name="turgeon2009" />

Phloem is also used as a popular site for oviposition and breeding of insects belonging to the order Diptera, including the fruit fly ''[[Drosophila montana]]''.<ref>{{Cite journal|last=Aspi|first=Jouni|date=1996-01-01|title=Larval niche differences between the sibling species, Drosophila montana and D. littoralis(Diptera) in Northern Finland|url=https://www.researchgate.net/publication/236585435|journal=Entomologica Fennica|volume=7|pages=29–38|doi=10.33338/ef.83885|doi-access=free}}</ref>

=== Girdling ===
{{main|Girdling}}
Because phloem tubes are located outside the xylem in most plants, a tree or other plant can be killed by stripping away the bark in a ring on the trunk or stem. With the phloem destroyed, nutrients cannot reach the roots, and the tree/plant will die. Trees located in areas with animals such as beavers are vulnerable since beavers chew off the bark at a fairly precise height. This process is known as girdling, or ring-barking, and can be used for agricultural purposes. For example, enormous fruits and vegetables seen at fairs and carnivals are produced via girdling. A farmer would place a girdle at the base of a large branch, and remove all but one fruit/vegetable from that branch. Thus, all the sugars manufactured by leaves on that branch have no [[Carbon dioxide sink|sinks]] to go to but the one fruit/vegetable, which thus expands to many times its normal size.{{cn|date=January 2025}}

===Origin===
When the plant is an embryo, vascular tissue emerges from procambium tissue, which is at the center of the embryo. Protophloem itself appears in the mid-vein extending into the cotyledonary node, which constitutes the first appearance of a leaf in angiosperms, where it forms continuous strands. The hormone [[auxin]], transported by the protein PIN1 is responsible for the growth of those protophloem strands, signaling the final identity of those tissues. [[SHORTROOT]] (SHR), and [[microRNA165]]/[[mir-166 microRNA precursor|166]] also participate in that process, while [[Callose Synthase&nbsp;3]] inhibits the locations where SHR, and microRNA165 can go. Additionally, the expression of NAC45/86 genes during phloem differentiation functions to enucleate specific cells in the plants to produce the sieve elements.<ref>{{Cite journal |last1=Notaguchi |first1=Michitaka |last2=Okamoto |first2=Satoru |date=2015 |title=Dynamics of long-distance signaling via plant vascular tissues |journal=Frontiers in Plant Science |volume=6 |page=161 |doi=10.3389/fpls.2015.00161 |pmid=25852714 |pmc=4364159 |issn=1664-462X|doi-access=free |bibcode=2015FrPS....6..161N }}</ref>

In the embryo, root phloem develops independently in the upper hypocotyl, which lies between the embryonic root, and the cotyledon.<ref>{{cite journal | last1 = Lucas | first1 = William | display-authors = etal | year = 2013 | title = "The Plant Vascular System " Evolution, Development and Functions | journal = Journal of Integrative Plant Biology | volume = 55 | issue = 4| pages = 294–388 | pmid = 23462277 | doi = 10.1111/jipb.12041 | hdl = 10261/76903 | hdl-access = free }}</ref>

In an adult, the phloem originates, and grows outwards from, [[meristem]]atic cells in the [[vascular cambium]]. Phloem is produced in phases. Primary phloem is laid down by the [[apical meristem]] and develops from the [[procambium]]. ''Secondary'' phloem is laid down by the vascular cambium to the inside of the established layer(s) of phloem. The molecular control of phloem development from stem cell to mature sieve element is best understood for the primary root of the model plant ''[[Arabidopsis thaliana]]''.<ref>{{cite journal | doi=10.1111/nph.19003 | title=Phloem development | date=2023 | last1=Hardtke | first1=Christian S. | journal=New Phytologist | volume=239 | issue=3 | pages=852–867 | pmid=37243530 | s2cid=258953259 | doi-access=free | bibcode=2023NewPh.239..852H }}</ref>

In some eudicot families ([[Apocynaceae]], [[Convolvulaceae]], [[Cucurbitaceae]], [[Solanaceae]], [[Myrtaceae]], [[Asteraceae]], [[Thymelaeaceae]]), phloem also develops on the inner side of the vascular cambium; in this case, a distinction between ''external'' and ''internal'' or ''intraxylary'' phloem is made. Internal phloem is mostly primary, and begins differentiation later than the external phloem and protoxylem, though it is not without exceptions. In some other families ([[Amaranthaceae]], [[Nyctaginaceae]], [[Salvadoraceae]]), the cambium also periodically forms inward strands or layers of phloem, embedded in the xylem: Such phloem strands are called ''included'' or ''interxylary'' phloem.<ref>Evert, Ray F. ''Esau's Plant Anatomy''. John Wiley & Sons, Inc, 2006, pp. 357–358, {{ISBN|0-470-04737-2}}.</ref>
'''Transportation:'''
They can be taken up through two processes, either an active process or a passive process of absorption, the water is that the absorption of the movement of water without a lot of energy, and do not require any energy within the process, but in active transporting the water and mineral are only taken up by the use of ATP energy, through a more active process.

==Nutritional use==
[[File:Detaching inner bark of pine.jpg|thumb|Stripping the inner bark from a pine branch]]

Phloem of [[pine]] trees has been used in [[Finland]] and [[Scandinavia]] as a substitute food in times of [[famine]] and even in good years in the northeast. Supplies of phloem from previous years helped stave off starvation in the great famine of the 1860s which hit both [[Finnish famine of 1866–1868|Finland]] and [[Swedish famine of 1867–1869|Sweden]]. Phloem is dried and milled to flour (''pettu'' in [[Finnish language|Finnish]]) and mixed with [[rye]] to form a hard dark bread, [[bark bread]]. The least appreciated was ''silkko'', a bread made only from [[buttermilk]] and ''pettu'' without any real rye or cereal flour. Recently, ''pettu'' has again become available as a curiosity, and some have made claims of health benefits.<ref>{{Cite journal |last=Magnani |first=Natalia |date=2016 |title=Reconstructing Food Ways: Role of Skolt Sami Cultural Revitalization Programs in Local Plant Use |url=http://www.bioone.org/doi/10.2993/0278-0771-36.1.85 |journal=Journal of Ethnobiology |language=en |volume=36 |issue=1 |pages=85–104 |doi=10.2993/0278-0771-36.1.85 |s2cid=147665504 |issn=0278-0771|url-access=subscription }}</ref>

Phloem from [[silver birch]] has been also used to make flour in the past.<ref>{{Cite web |last=Sigrithur |first=Anna |date=2016-09-29 |title=Using Tree Bark Flours in Cooking |url=https://waldenlabs.com/tree-bark/ |access-date=2023-02-09 |website=Walden Labs}}</ref>

==See also==
* [[Apical dominance]]

==References==
{{Reflist}}

==External links==
{{Commons category}}

{{Biological tissue}}
{{Authority control}}

[[Category:Plant anatomy]]
[[Category:Plant physiology]]
[[Category:Tissues (biology)]]