Editing Xylem (section)

=== Patterns of protoxylem and metaxylem ===
There are four primary 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 center of the stem and develops from the center outwards. The protoxylem is thus found in the central core, and the metaxylem is in a cylinder around it.<ref>{{Cite web|last1=White |first1=A. Toby |last2=Kazlev |first2=M. Alan |title=Glossary |url=http://www.palaeos.com/Plants/Lists/Glossary/GlossaryA.html |work=palaeos.com |url-status=dead |archive-url=https://web.archive.org/web/20101220181617/http://palaeos.com/Plants/Lists/Glossary/GlossaryA.html |archive-date=December 20, 2010 }}</ref> This pattern was common in early land plants, such as "[[rhyniophyte]]s", but is not present in any living plants.{{citation needed|date=February 2015}}

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 center, i.e., centripetally. The metaxylem is thus closest to the center of the stem or root, and the protoxylem is closest to the periphery. The roots of [[vascular plant]]s are generally considered to have exarch development.<ref name="FosterGifford1974"/>
* ''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 center of the stem or root, and the metaxylem is closest to the periphery. The stems of [[seed plant]]s typically have endarch development.<ref name="FosterGifford1974"/>
* ''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 many [[fern]]s have mesarch development.<ref name="FosterGifford1974"/>