Editing Phloem (section)

== 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|pages=244|doi=10.3389/fpls.2013.00244|issn=1664-462X|pmc=3701861|pmid=23847646|doi-access=free}}</ref><ref>{{Cite book|last=Bhatla|first=Satish C.|url=https://www.worldcat.org/oclc/1077622456|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}}</ref>