Editing Xylem (section)

=== {{anc|Proto}}Protoxylem and metaxylem ===
As a young [[vascular plant]] grows, 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 usually distinguished by narrower vessels formed of smaller cells. Some of these cells have walls that contain thickenings in the form of rings or helices. Functionally, protoxylem can extend: the cells can 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 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.<ref name="FosterGifford1974">{{Cite book|last1=Foster|first1=A.S.|last2=Gifford|first2=E.M.|year=1974|title=Comparative Morphology of Vascular Plants|edition=2nd|location=San Francisco|publisher=W.H. Freeman|isbn=978-0-7167-0712-7|pages=[https://archive.org/details/comparativemorph00fost/page/55 55–56]|url=https://archive.org/details/comparativemorph00fost/page/55}}</ref><ref>{{Cite book|last1=Taylor|first1=T.N.|last2=Taylor|first2=E.L.|last3=Krings|first3=M.|year=2009|title=Paleobotany, the Biology and Evolution of Fossil Plants|edition=2nd|location=Amsterdam; Boston |publisher=Academic Press|isbn=978-0-12-373972-8 |pages=207ff., 212ff}}</ref>