Editing Tracheid (section)

== Evolution ==
Tracheids were the main conductive cells found in early vascular plants.

In the first 140–150 million years of vascular plant evolution, tracheids were the only type of conductive cells found in fossils of plant xylem tissues.<ref>{{Cite journal|last=Sperry|first=John S.|date=2003-05-01|title=Evolution of Water Transport and Xylem Structure|url=https://www.journals.uchicago.edu/doi/abs/10.1086/368398|journal=International Journal of Plant Sciences|volume=164|issue=S3|pages=S115–S127|doi=10.1086/368398|bibcode=2003IJPlS.164S.115S |s2cid=15314720|issn=1058-5893|url-access=subscription}}</ref>  Ancestral tracheids did not contribute significantly to structural support, as can be seen in extant ferns.<ref>{{Cite journal|last1=Sperry|first1=John S.|last2=Hacke|first2=Uwe G.|last3=Pittermann|first3=Jarmila|date=2006|title=Size and function in conifer tracheids and angiosperm vessels|url=https://onlinelibrary.wiley.com/doi/abs/10.3732/ajb.93.10.1490|journal=American Journal of Botany|language=en|volume=93|issue=10|pages=1490–1500|doi=10.3732/ajb.93.10.1490|pmid=21642096|bibcode=2006AmJB...93.1490S |issn=1537-2197|url-access=subscription}}</ref>

The [[fossil]] record shows three different types of tracheid cells found in early plants, which were classified as S-type, G-type and P-type. The first two of them were lignified and had pores to facilitate the transportation of water between cells. The P-type tracheid cells had pits similar to extant plant tracheids. Later, more complex pits appeared, such as bordered pits on many tracheids, which allowed plants to transport water between cells while reducing the risk of cavitation and embolisms in the xylem.

As tracheids evolved along with secondary xylem tissues, specialized inter-tracheid pits appeared.<ref name="Pittermann-2011"/> Tracheid length and diameter also increased, with tracheid diameter increasing to an average length of 80 μm by the end of the [[Devonian]] period.<ref>{{Cite journal|last=Niklas|first=Karl J.|title=The Evolution of Tracheid Diameter in Early Vascular Plants and ITS Implications on the Hydraulic Conductance of the Primary Xylem Strand|date=September 1985|url=https://pubmed.ncbi.nlm.nih.gov/28561493/|journal=Evolution; International Journal of Organic Evolution|volume=39|issue=5|pages=1110–1122|doi=10.1111/j.1558-5646.1985.tb00451.x|issn=1558-5646|pmid=28561493|s2cid=13045808}}</ref>

Tracheids then evolved into the vessel elements and structural fibers that make up angiosperm wood.<ref name="Pittermann-2011"/>