Editing Xylem (section)

=== Cohesion-tension theory ===
The ''cohesion-tension theory'' is a theory of [[intermolecular attraction]] that explains the process of water flow upwards (against the force of [[gravity]]) through the xylem of plants. It was proposed in 1894 by [[John Joly]] and [[Henry Horatio Dixon]].<ref>{{cite journal |last1=Dixon |first1=Henry H. |last2=Joly |first2=J. |title=On the ascent of sap |journal=Annals of Botany |date=1894 |volume=8 |pages=468–470 |url=https://babel.hathitrust.org/cgi/pt?id=mdp.39015001235830;view=1up;seq=566}}</ref><ref>{{cite journal |last1=Dixon |first1=Henry H. |last2=Joly |first2=J. |title=On the ascent of sap |journal=Philosophical Transactions of the Royal Society of London, Series B |date=1895 |volume=186 |pages=563–576 |url=https://babel.hathitrust.org/cgi/pt?id=uva.x001659730;view=1up;seq=609|doi=10.1098/rstb.1895.0012 |doi-access=free }}</ref> Despite numerous objections,<ref>{{cite journal|author=Tyree, M.T.|year=1997|title=The Cohesion-Tension theory of sap ascent: current controversies|journal=Journal of Experimental Botany|volume=48|issue=10|pages=1753–1765|doi=10.1093/jxb/48.10.1753|doi-access=free}}</ref><ref>{{cite journal|author1=Wang, Z. |author2=Chang, C.-C. |author3=Hong, S.-J. |author4=Sheng, Y.-J. |author5=Tsao, H.-K. |year=2012|title=Capillary Rise in a Microchannel of Arbitrary Shape and Wettability: Hysteresis Loop|journal=Langmuir|volume=28|issue=49|pages=16917–16926|doi=10.1021/la3036242|pmid=23171321}}</ref> this is the most widely accepted theory for the transport of water through a plant's vascular system based on the classical research of Dixon-Joly (1894), Eugen Askenasy (1845–1903) (1895),<ref>{{cite journal|last=Askenasy|first=E.|title=Ueber das Saftsteigen|trans-title=On the ascent of sap|journal=Botanisches Centralblatt|year=1895|volume=62|pages=237–238|language=de|url=https://babel.hathitrust.org/cgi/pt?id=uc1.c079536710;view=1up;seq=269}}</ref><ref>{{cite journal |last1=Askenasy |first1=E. |title=Ueber das Saftsteigen |journal=Verhandlungen des Naturhistorisch-medizinischen Vereins zu Heidelberg (Proceedings of the Natural History-Medical Society at Heidelberg) |date=1895 |volume=5 |pages=325–345 |url=https://www.biodiversitylibrary.org/item/43534#page/349/mode/1up |series=2nd series |trans-title=On the ascent of sap |language=de}}</ref> and Dixon (1914,1924).<ref>{{cite book|last=Dixon|first=H|title=Transpiration and the ascent of sap in plants|year=1914|publisher= Macmillan and Co.|location=London, England, UK|url=https://archive.org/stream/transpirationasc00dixo#page/n5}}</ref><ref>{{cite book|last=Dixon|first=H|title=The transpiration stream|year= 1924|publisher=University of London Press, Ltd|location=London|pages=80}}</ref>

Water is a [[polar molecule]]. When two water molecules approach one another, the slightly negatively charged [[oxygen]] atom of one forms a [[hydrogen bond]] with a slightly positively charged [[hydrogen]] atom in the other. This attractive force, along with other [[intermolecular force]]s, is one of the principal factors responsible for the occurrence of [[surface tension]] in liquid water. It also allows plants to draw water from the root through the xylem to the leaf.{{cn|date=January 2025}}

Water is constantly lost through transpiration from the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and tension. Transpiration pull, utilizing [[capillary action]] and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves forces water to move into them.{{cn|date=January 2025}}

[[Transpiration]] in leaves creates tension (differential pressure) in the cell walls of mesophyll cells. Because of this tension, water is being pulled up from the roots into the leaves, helped by [[Cohesion (chemistry)|cohesion]] (the pull between individual water molecules, due to hydrogen bonds) and [[adhesion]] (the stickiness between water molecules and the [[hydrophilic]] cell walls of plants). This mechanism of water flow works because of [[water potential]] (water flows from high to low potential), and the rules of simple [[diffusion]].<ref>{{cite book|last=Campbell|first=Neil|title=Biology|year=2002|publisher=Pearson Education, Inc.|location=San Francisco, CA|isbn=978-0-8053-6624-2|pages=[https://archive.org/details/biologyc00camp/page/759 759]|url=https://archive.org/details/biologyc00camp/page/759}}</ref>

Over the past century, there has been a great deal of research regarding the mechanism of xylem sap transport; today, most plant scientists continue to agree that the ''cohesion-tension theory'' best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem [[osmotic pressure]] [[gradient]]s, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.<ref>{{cite journal|last=Zimmerman|first=Ulrich|title=What are the driving forces for water lifting in the xylem conduit?|journal=Physiologia Plantarum|year=2002|doi=10.1034/j.1399-3054.2002.1140301.x|pmid=12060254|volume=114|issue=3|pages=327–335|bibcode=2002PPlan.114..327Z }}</ref><ref name=tyree1997>{{cite journal|last=Tyree|first=Melvin T.|title=The cohesion-tension theory of sap ascent: current controversies|journal=Journal of Experimental Botany|volume=48|issue=10|pages=1753–1765|year=1997|doi=10.1093/jxb/48.10.1753|doi-access=free}}</ref>