Editing Xylem (section)

== Evolution ==
Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the [[Silurian]] (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier [[Ordovician]] rocks.{{citation needed|date=March 2019}} The earliest true and recognizable xylem consists of [[tracheid]]s with a helical-annular reinforcing layer added to the [[cell wall]]. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the ''protoxylem'' (first-formed xylem) of all living groups of vascular plants. Several groups of plants later developed [[pit (botany)|pitted]] tracheid cells independently through [[convergent evolution]]. In living plants, pitted tracheids do not appear in development until the maturation of the ''metaxylem'' (following the ''protoxylem'').{{cn|date=January 2025}}

In most plants, pitted [[tracheid]]s function as the primary transport cells. The other type of vascular element, found in angiosperms, is the [[vessel element]]. Vessel elements are joined end to end to form vessels in which water flows unimpeded, as in a pipe. The presence of '''xylem vessels''' (also called '''trachea'''<ref>{{Cite web|title=Structure of Plants and Fungi{{!}}Digitális Tankönyvtár|url=https://regi.tankonyvtar.hu/hu/tartalom/tamop412A/2011-0073_structure_of_plants_fungi/ch04s04.html|access-date=2021-04-02|website=regi.tankonyvtar.hu|language=hu}}{{Dead link|date=October 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>) is considered to be one of the key innovations that led to the success of the [[angiosperms]].<ref>{{cite journal|author=Carlquist, S.|author2=E.L. Schneider |year=2002|title=The tracheid–vessel element transition in angiosperms involves multiple independent features: cladistic consequences|journal=American Journal of Botany|volume=89|issue=2|pages=185–195|doi= 10.3732/ajb.89.2.185|pmid=21669726|doi-access=}}</ref> However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., [[Amborellaceae]], [[Tetracentraceae]], [[Trochodendraceae]], and [[Winteraceae]]), and their secondary xylem is described by [[Arthur Cronquist]] as "primitively vesselless". Cronquist considered the vessels of ''[[Gnetum]]'' to be convergent with those of angiosperms.<ref>{{cite book|author=Cronquist, A.|date=August 1988|title=The Evolution and Classification of Flowering Plants|location=New York, New York|publisher=New York Botanical Garden Press|isbn=978-0-89327-332-3}}</ref> Whether the absence of vessels in basal angiosperms is a [[cladistics#Definitions|primitive]] condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost.

[[Image:ficusxylem.jpg|thumb|upright=1.3|Photos showing xylem elements in the shoot of a fig tree (''Ficus alba''): crushed in hydrochloric acid, between slides and cover slips]]

To photosynthesize, plants must absorb {{co2}} from the atmosphere. However, this comes at a price: while stomata are open to allow {{co2}} to enter, water can evaporate.<ref name=Sperry2003>{{Cite journal| last1 = Sperry | first1 = J. S.| title = Evolution of Water Transport and Xylem Structure| jstor = 3691719| journal = International Journal of Plant Sciences| volume = 164| issue = 3| pages = S115–S127| year = 2003| doi = 10.1086/368398| s2cid = 15314720}}</ref> Water is lost much faster than {{co2}} is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis.<ref name=Sperry2003/> Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and {{co2}} acquisition) through the use of stomata. Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels.<ref name=Sperry2003/>

The high {{co2}} levels of Silurian-Devonian times, when plants were first colonizing land, meant that the need for water was relatively low. As {{co2}} was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved.<ref name=Sperry2003/> As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water. This transition from [[poikilohydry]] to [[homoiohydry]] opened up new potential for colonization.<ref name=Sperry2003/> Plants then needed a robust internal structure that held long narrow channels for transporting water from the soil to all the different parts of the above-soil plant, especially to the parts where photosynthesis occurred.{{cn|date=January 2025}}

During the Silurian, {{co2}} was readily available, so little water needed expending to acquire it. By the end of the Carboniferous, when {{co2}} levels had lowered to something approaching today's, around 17 times more water was lost per unit of {{co2}} uptake.<ref name=Sperry2003/> However, even in these "easy" early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid [[desiccation]]. This early water transport took advantage of the ''cohesion-tension'' mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can be [[capillary action|wick]]ed along a fabric with small spaces. In small passages, such as that between the plant cell walls (or in tracheids), a column of water behaves like rubber&nbsp;– when molecules evaporate from one end, they pull the molecules behind them along the channels. Therefore, transpiration alone provided the driving force for water transport in early plants.<ref name=Sperry2003/> However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2&nbsp;cm, severely limiting the size of the earliest plants.<ref name=Sperry2003/> This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausting it, plants developed a waterproof [[plant cuticle|cuticle]]. Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue.<ref name=Sperry2003/> However, dehydration at times was inevitable; early plants cope with this by having a lot of water stored between their cell walls, and when it comes to it sticking out the tough times by putting life "on hold" until more water is supplied.<ref name=Sperry2003/>

[[Image:banded tube.jpg|thumb|A [[banded tube]] from the late Silurian/early Devonian. The bands are difficult to see on this specimen, as an opaque carbonaceous coating conceals much of the tube. Bands are just visible in places on the left half of the image&nbsp;– click on the image for a larger view. Scale bar: 20&nbsp;μm]]

To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. During the [[Llandovery epoch|early Silurian]], they developed specialized cells, which were [[lignin|lignified]] (or bore similar chemical compounds)<ref name=Sperry2003/> to avoid implosion; this process coincided with cell death, allowing their innards to be emptied and water to be passed through them.<ref name=Sperry2003/> These wider, dead, empty cells were a million times more conductive than the inter-cell method, giving the potential for transport over longer distances, and higher {{co2}} diffusion rates.

The earliest macrofossils to bear water-transport tubes are Silurian plants placed in the genus ''[[Cooksonia (plant)|Cooksonia]]''.<ref name=Edwards1992>{{Cite journal |last1=Edwards |first1=D. |last2=Davies |first2=K.L. |last3=Axe |first3=L. |year=1992 |title=A vascular conducting strand in the early land plant ''Cooksonia'' |journal=Nature |volume=357 |issue=6380 |pages=683–685 |doi=10.1038/357683a0 |bibcode=1992Natur.357..683E |s2cid=4264332 }}</ref> The early Devonian pretracheophytes ''[[Aglaophyton]]'' and ''[[Horneophyton]]'' have structures very similar to the [[Hydroid (botany)|hydroids]] of modern mosses.
Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards,<ref name=Niklas1983>{{Cite journal| last1 = Niklas | first1 = K. J.| last2 = Smocovitis | first2 = V.| title = Evidence for a Conducting Strand in Early Silurian (Llandoverian) Plants: Implications for the Evolution of the Land Plants| jstor = 2400461| journal = Paleobiology| volume = 9| issue = 2| pages = 126–137| year = 1983| doi = 10.1017/S009483730000751X| bibcode = 1983Pbio....9..126N| s2cid = 35550235}}</ref> are an early improvisation to aid the easy flow of water.<ref name=Niklas1985/> Banded tubes, as well as tubes with pits in their walls, were lignified<ref name=Niklas1980>{{Cite journal
| last1 = Niklas | first1 = K.
| last2 = Pratt | first2 = L.
| title = Evidence for lignin-like constituents in Early Silurian (Llandoverian) plant fossils
| journal = Science
| volume = 209
| issue = 4454
| pages = 396–397
| year = 1980
| pmid = 17747811
| doi = 10.1126/science.209.4454.396
|bibcode = 1980Sci...209..396N | s2cid = 46073056
}}</ref> and, when they form single celled conduits, are considered to be ''tracheids''. These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure.<ref name=Sperry2003/> Tracheids may have a single evolutionary origin, possibly within the hornworts,<ref name="Qiu2006">{{cite journal|author=Qiu, Y.L. |author2=Li, L. |author3=Wang, B. |author4=Chen, Z. |author5=Knoop, V. |author6=Groth-malonek, M. |author7=Dombrovska, O. |author8=Lee, J. |author9=Kent, L. |author10=Rest, J.|year=2006|title= The deepest divergences in land plants inferred from phylogenomic evidence|journal=Proceedings of the National Academy of Sciences |pmid=17030812|volume=103|issue=42|pmc=1622854|pages=15511–6|doi=10.1073/pnas.0603335103|bibcode = 2006PNAS..10315511Q |display-authors=etal|doi-access=free }}</ref> uniting all tracheophytes (but they may have evolved more than once).<ref name=Sperry2003/>

Water transport requires regulation, and dynamic control is provided by [[stoma]]ta.<ref name=gk>{{cite book|author1=Stewart, W.N. |author2=Rothwell, G.W. |title=Paleobiology and the evolution of plants|year=1993|publisher = Cambridge University Press}}</ref>
By adjusting the amount of gas exchange, they can restrict the amount of water lost through transpiration. This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the non-vascular hornworts.<ref name=Sperry2003/>

An [[endodermis]] probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous.<ref name=Sperry2003/> This structure in the roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc. from entering the water transport system). The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.

Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size.<ref name=Sperry2003/> As a result of their independence from their surroundings, they lost their ability to survive desiccation&nbsp;– a costly trait to retain.<ref name=Sperry2003/>

During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant.<ref name=Niklas1985>{{Cite journal| last1 = Niklas | first1 = K. J.| title = The Evolution of Tracheid Diameter in Early Vascular Plants and Its Implications on the Hydraulic Conductance of the Primary Xylem Strand| jstor = 2408738| journal = Evolution| volume = 39| issue = 5| pages = 1110–1122| year = 1985| doi = 10.2307/2408738| pmid = 28561493}}</ref> By the middle Devonian, the tracheid diameter of some plant lineages ([[Zosterophyllophytes]]) had plateaued.<ref name=Niklas1985/> Wider tracheids allow water to be transported faster, but the overall transport rate depends also on the overall cross-sectional area of the xylem bundle itself.<ref name=Niklas1985/> The increase in vascular bundle thickness further seems to correlate with the width of plant axes, and plant height; it is also closely related to the appearance of leaves<ref name=Niklas1985/> and increased stomatal density, both of which would increase the demand for water.<ref name=Sperry2003/>

{{Anchor|XylemCavitation}}
While wider tracheids with robust walls make it possible to achieve higher water transport tensions, this increases the likelihood of cavitation.<ref name=Sperry2003/> Cavitation occurs when a bubble of air forms within a vessel, breaking the bonds between chains of water molecules and preventing them from pulling more water up with their cohesive tension. A tracheid, once cavitated, cannot have its embolism removed and return to service (except in a few advanced angiosperms<ref>{{Cite web|url=http://www.biologydiscussion.com/plants/cavitation-and-embolism-in-vascular-plants-with-diagram/22732|title=Cavitation and Embolism in Vascular Plants (With Diagram)|last=Koratkar|first=Sanjay|date=2016-02-24|website=Biology Discussion}}</ref><ref>{{Cite journal|url=https://www.fs.fed.us/pnw/pubs/journals/pnw_2012_johnson002.pdf|title=Hydraulic safety margins and embolism reversal in stems and leaves: Why are conifers and angiosperms so different? |first1=Daniel M. |last1=Johnson |first2=Katherine A. |last2=McCulloh |first3=David R. |last3=Woodruff |first4=Frederick C. |last4=Meinzerc|date=June 2012|journal=Plant Science  |volume=195 |pages=48–53 |doi=10.1016/j.plantsci.2012.06.010 |pmid=22920998 |bibcode=2012PlnSc.195...48J |url-status=dead |archive-url=https://web.archive.org/web/20210313190658/https://www.fs.fed.us/pnw/pubs/journals/pnw_2012_johnson002.pdf |archive-date=Mar 13, 2021 }}</ref> which have developed a mechanism of doing so). Therefore, it is well worth plants' while to avoid cavitation occurring. For this reason, [[pit (botany)|pits]] in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate. Freeze-thaw cycles are a major cause of cavitation. Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel.<ref name=Sperry2003/>

Once cavitation has occurred, plants have a range of mechanisms to contain the damage.<ref name=Sperry2003/> Small pits link adjacent conduits to allow fluid to flow between them, but not air&nbsp;– although these pits, which prevent the spread of embolism, are also a major cause of them.<ref name=Sperry2003/> These pitted surfaces further reduce the flow of water through the xylem by as much as 30%.<ref name=Sperry2003/> The diversification of xylem strand shapes with tracheid network topologies increasingly resistant to the spread of embolism likely facilitated increases in plant size and the colonization of drier habitats during the [[Devonian explosion|Devonian radiation]].<ref>{{Cite journal |last1=Bouda |first1=Martin |last2=Huggett |first2=Brett A. |last3=Prats |first3=Kyra A. |last4=Wason |first4=Jay W. |last5=Wilson |first5=Jonathan P. |last6=Brodersen |first6=Craig R. |date=2022-11-11 |title=Hydraulic failure as a primary driver of xylem network evolution in early vascular plants |url=https://www.science.org/doi/10.1126/science.add2910 |journal=Science |language=en |volume=378 |issue=6620 |pages=642–646 |doi=10.1126/science.add2910 |pmid=36356120 |bibcode=2022Sci...378..642B |s2cid=253458196 |issn=0036-8075}}</ref> Conifers, by the Jurassic, developed [[Pit (botany)#Types of pits|bordered pits]] had valve-like structures to isolate cavitated elements. These [[Pit (botany)#Torus and margo|torus-margo]] structures have an impermeable disc (torus) suspended by a permeable membrane (margo) between two adjacent pores. When a tracheid on one side depressurizes, the disc is sucked into the pore on that side, and blocks further flow.<ref name=Sperry2003/> Other plants simply tolerate cavitation. For instance, oaks grow a ring of wide vessels at the start of each spring, none of which survive the winter frosts.{{citation needed|date=October 2023}} Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles.{{citation needed|date=November 2022}}

Growing to height also employed another trait of tracheids&nbsp;– the support offered by their lignified walls. Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem. However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough [[sclerenchyma]] on the outer rim of the stems.<ref name=Sperry2003/> Even when tracheids do take a structural role, they are supported by sclerenchymatic tissue.

Tracheids end with walls, which impose a great deal of resistance on flow;<ref name=Niklas1985/> vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel.<ref name=Niklas1985/> The function of end walls, which were the default state in the Devonian, was probably to avoid [[embolism]]s. An embolism is where an air bubble is created in a tracheid. This may happen as a result of freezing, or by gases dissolving out of solution. Once an embolism is formed, it usually cannot be removed (but see later); the affected cell cannot pull water up, and is rendered useless.

End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, ''Cooksonia''.<ref name=Niklas1985/>

The size of tracheids is limited as they comprise a single cell; this limits their length, which in turn limits their maximum useful diameter to 80&nbsp;μm.<ref name=Sperry2003/> Conductivity grows with the fourth power of diameter, so increased diameter has huge rewards; ''vessel elements'', consisting of a number of cells, joined at their ends, overcame this limit and allowed larger tubes to form, reaching diameters of up to 500&nbsp;μm, and lengths of up to 10&nbsp;m.<ref name=Sperry2003/>

Vessels first evolved during the dry, low {{co2}} periods of the late Permian, in the horsetails, ferns and [[Selaginellales]] independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes.<ref name=Sperry2003/>
Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids!<ref name=Sperry2003/> This allowed plants to fill more of their stems with structural fibers, and also opened a new niche to [[vine]]s, which could transport water without being as thick as the tree they grew on.<ref name=Sperry2003/> Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation.<ref name=Sperry2003/>
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