View source for Tracheid

{{Short description|Component of Xylem}}
[[File:Podocarpoxylon mcgeei radial section with tracheids Torrey 1923 Plate10 fig18.png|thumb|220px|In radial section, two tracheids of a coniferous wood species are shown. A series of bordered [[Pit (botany)|pits]] are also appearing in each tracheid.]]
[[File:Tracheid of oak (from Marshall Ward).png|thumb|150px|A tracheid of [[oak]] shows [[Glossary of botanical terms#pit|pits]] along the walls. It has no [[Glossary of botanical terms#perforation plate|perforation plates]]. Angiosperms have both tracheids and vessel elements.<ref>{{citation |title=Atlas of Plant and Animal Histology |publisher=University of Vigo, Spain |author=Megías M, Molist P, Pombal MA. |url=http://mmegias.webs.uvigo.es/index.html |access-date=11 December 2024}}</ref>]]

A '''tracheid''' is a long and tapered [[Lignin|lignified]] cell in the [[xylem]] of [[Tracheophyta|vascular plants]]. It is a type of conductive cell called a tracheary element. [[Angiosperms]] also use another type of conductive cell, called [[vessel element]]s, to transport water through the xylem. The main functions of tracheid cells are to [[Transpiration|transport water and inorganic salts]], and to provide structural support for trees. There are often [[Pit (botany)|pits]] on the [[cell walls]] of tracheids, which allows for water flow between cells. Tracheids are dead at functional maturity and do not have a [[protoplast]]. The [[wood]] ([[softwood]]) of [[gymnosperm]]s such as pines and other [[conifer]]s is mainly composed of tracheids.<ref>{{Cite journal|last1=Cuny|first1=Henri E.|last2=Rathgeber|first2=Cyrille B. K.|last3=Frank|first3=David|last4=Fonti|first4=Patrick|last5=Fournier|first5=Meriem|date=2014|title=Kinetics of tracheid development explain conifer tree-ring structure|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/nph.12871|journal=New Phytologist|language=en|volume=203|issue=4|pages=1231–1241|doi=10.1111/nph.12871|pmid=24890661|bibcode=2014NewPh.203.1231C |s2cid=22862428 |issn=1469-8137|url-access=subscription}}</ref> Tracheids are also the main conductive cells in the primary xylem of [[fern]]s.<ref name="Pittermann-2011">{{Cite journal|last1=Pittermann|first1=Jarmila|last2=Limm|first2=Emily|last3=Rico|first3=Christopher|last4=Christman|first4=Mairgareth A.|date=2011|title=Structure–function constraints of tracheid-based xylem: a comparison of conifers and ferns|journal=New Phytologist|language=en|volume=192|issue=2|pages=449–461|doi=10.1111/j.1469-8137.2011.03817.x|pmid=21749396|issn=1469-8137|doi-access=free|bibcode=2011NewPh.192..449P }}</ref>

The tracheid was first named by the German botanist Carl Gustav Sanio in 1863, from the German ''Tracheide''.<ref>{{Cite journal|last=Sanio|first=C.|year=1863|title=Vergleichende Untersuchungen über die Elementarorgane des Holzkörpers|journal=Bot. Zeitung|volume=21|pages=85–91; 93–98; 101–111|issn=2509-5420}}</ref>

== 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|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"/>

==References==
{{reflist}}

==Further reading==
*{{cite book |last1=Wilson |first1=K. |first2=D. J. B. |last2=White |year=1986 |title=The Anatomy of Wood: Its Diversity and Variability |publisher=Stobart & Son Ltd |location=London |isbn=0-85442-033-9 }}

==External links==
{{NSRW Poster|Tracheid}}
*Pictures of softwood tracheids [http://botweb.uwsp.edu/anatomy/images/gymnospermwood/pages/Anat0288.htm in cross section] and [http://botweb.uwsp.edu/anatomy/images/xylemdevelopment/pages/Anat0182.htm  in maceration]; both in pine.

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

[[Category:Plant anatomy]]
[[Category:Plant physiology]]