• When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution
    and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism
    from spreading).

  • [33] 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.

  • 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.

  • [33] 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.

  • Main function – upwards water transport The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water-conducting
    channels reaching all parts of the plants.

  • [11] Three phenomena cause xylem sap to flow: • Pressure flow hypothesis: Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute
    pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals.

  • [33] This allowed plants to fill more of their stems with structural fibers, and also opened a new niche to vines, which could transport water without being as thick as the
    tree they grew on.

  • • Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant.

  • 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.

  • [33] However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2 cm, severely limiting the size of the earliest plants.

  • [25] 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 gradients, axial potential gradients
    in the vessels, and gel- and gas-bubble-supported interfacial gradients.

  • [4][5] Structure The most distinctive xylem cells are the long tracheary elements that transport water.

  • In small passages, such as that between the plant cell walls (or in tracheids), a column of water behaves like rubber – when molecules evaporate from one end, they pull the
    molecules behind them along the channels.

  • [33] 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 μm, and lengths of up to 10 m.[33] 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.

  • [33] Small pits link adjacent conduits to allow fluid to flow between them, but not air – although these pits, which prevent the spread of embolism, are also a major cause
    of them.

  • [33] 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.

  • [33] As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water.

  • Scale bar: 20 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.

  • Because of this tension, water is being pulled up from the roots into the leaves, helped by 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).

  • [33] 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.

  • 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.

  • Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue.

  • Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants.

  • [33] This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausting it, plants developed a waterproof cuticle.

  • As CO2 was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved.

  • Once an embolism is formed, it usually cannot be removed (but see later); the affected cell cannot pull water up, and is rendered useless.

  • [33] 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.

  • In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem

  • This mechanism of water flow works because of water potential (water flows from high to low potential), and the rules of simple diffusion.

  • When one water molecule is lost another is pulled along by the processes of cohesion and tension.

  • • Root pressure: If the water potential of the root cells is more negative than that of the soil, usually due to high concentrations of solute, water can move by osmosis into
    the root from the soil.

  • 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.

  • During the early Silurian, they developed specialized cells, which were lignified (or bore similar chemical compounds)[33] to avoid implosion; this process coincided with
    cell death, allowing their innards to be emptied and water to be passed through them.

  • 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.

  • Therefore, transpiration alone provided the driving force for water transport in early plants.

  • Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport.

  • Damage to a tracheid’s wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel.

  • Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column.

  • [13] The primary force that creates the capillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits.

  • [33] 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.

  • [33] 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.

  • [36] 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[36]
    and increased stomatal density, both of which would increase the demand for water.

  • It also allows plants to draw water from the root through the xylem to the leaf.

  • Xylem transport is driven by a combination[29] of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.

  • Any use of water in leaves forces water to move into them.

  • 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.

  • Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels.

  • This attractive force, along with other intermolecular forces, is one of the principal factors responsible for the occurrence of surface tension in liquid water.

  • [33] The high CO2 levels of Silurian-Devonian times, when plants were first colonizing land, meant that the need for water was relatively low.

  • [48] However, according to Grew, capillary action in the xylem would raise the sap by only a few inches; in order to raise the sap to the top of a tree, Grew proposed that
    the parenchymal cells become turgid and thereby not only squeeze the sap in the tracheids but force some sap from the parenchyma into the tracheids.

  • Tracheids end with walls, which impose a great deal of resistance on flow;[36] vessel members have perforated end walls, and are arranged in series to operate as if they were
    one continuous vessel.

  • A tracheid, once cavitated, cannot have its embolism removed and return to service (except in a few advanced angiosperms[40][41] which have developed a mechanism of doing

  • The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.

  • The presence of xylem vessels (also called trachea[30]) is considered to be one of the key innovations that led to the success of the angiosperms.

  • [36] 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.

  • Metaxylem vessels and cells are usually larger; the cells have thickenings which are typically either in the form of ladderlike transverse bars (scalariform) or continuous
    sheets except for holes or pits (pitted).

  • Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well.

  • [45] This pattern was common in early land plants, such as “rhyniophytes”, but is not present in any living plants.


Works Cited

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in spiram contorta, componuntur, ut facile laceratione, (velut in bombycinis tracheis expertus sum,) in hanc oblongam & continuatam fasciam resolvantur. Lamina haec, si ulterius microscopio lustretur, particulis squamatim componitur; quod etiam in
tracheis insectorum deprehenditur. Spiralibus hisce vasculis, seu ut verius loquar, tracheis, ligneae fibrae M adstant, quae secundum longitudinem productae, ad majorem firmitudinem & robur, transversalium utriculorum ordines N superequitant, ita
ut fiat veluti storea.” ( … these [vessels] are tubular and somewhat round, yet often become narrow, and they are always open, and none, as [far as] I could perceive, exude a liquid: they are composed of silvery sheets L, twisted into a helix, although
they can easily be unbound, by tearing, into this somewhat long and connected strip (just as I have done in silkworm treacheas). This sheet, if it be examined further with a microscope, is composed of scale-like particles; which likewise is observed
in the tracheas of insects. On these helical vessels, or as I will more rightly say, “tracheas”, there stand woody filaments M, which being extended in length straddle – for greater strength and hardness – lines of transverse cells N, so that it is
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p. 8.
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the spring, in or through the Wood it self, and there only.”
50. ^ See:
1. (Grew, 1682), p. 126. Grew recognized the limits of capillary action (from p. 126): ” … small Glass-Pipes [i.e., capillary tubes] immersed in Water, will give it [i.e.,
the water] an ascent for some inches; yet there is a certain period, according to the bore of the Pipe, beyond which it will not rise.” Grew proposed the following mechanism for the ascent of sap in plants (from p. 126): “But the Bladders [i.e., parenchymal
cells] DP, which surround it [i.e., the column of tracheids], being swelled up and turgid with Sap, do hereby press upon it; and so not only a little contract its bore, but also transfuse or strain some Portion of their Sap thereinto: by both which
means, the Sap will be forced to rise higher therein.”
2. Arber, Agnes (1913). “Nehemiah Grew 1641–1712”. In Oliver, Francis Wall (ed.). Makers of British Botany: A Collection of Biographies by Living Botanists. Cambridge, England: Cambridge University
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52. ^ See:
1. Strasburger, Eduard
(1891). Histologische Beiträge [Histological Contributions] (in German). Vol. 3: Ueber den Bau und die Verrichtungen der Leitungsbahnen in den Pflanzen [On the structure and the function of vascular bundles in plants]. Jena, Germany: Gustav Fischer.
pp. 607–625: Aufsteigen giftiger Flüssigkeiten bis zu bedeutender Höhe in der Pflanze [Ascent of poisonous liquids to considerable heights in plants], pp. 645–671: Die Leitungsfähigkeit getödteter Pflanzentheile [The ability of the killed parts of
plants to conduct [water]].
2. (Jansen & Schenck, 2015), p. 1561.
2. C. Wei; E. Steudle; M. T. Tyree; P. M. Lintilhac (May 2001). “The essentials of direct xylem pressure measurement”. Plant, Cell and Environment. 24 (5): 549–555. doi:10.1046/j.1365-3040.2001.00697.x.
S2CID 5039439. is the main source used for the paragraph on recent research.
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270 (5239): 1193–4. Bibcode:1995Sci…270.1193H. doi:10.1126/science.270.5239.1193. S2CID 97217181. is the first published independent test showing the Scholander bomb actually does measure the tension in the xylem.
4. Pockman, W.T.; J.S. Sperry;
J.W. O’Leary (December 1995). “Sustained and significant negative water pressure in xylem”. Nature. 378 (6558): 715–6. Bibcode:1995Natur.378..715P. doi:10.1038/378715a0. S2CID 31357329. is the second published independent test showing the Scholander
bomb actually does measure the tension in the xylem.
5. Campbell, Neil A.; Jane B. Reece (2002). Biology (6th ed.). Benjamin Cummings. ISBN 978-0-8053-6624-2.
6. Kenrick, Paul; Crane, Peter R. (1997). The Origin and Early Diversification of Land
Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press. ISBN 978-1-56098-730-7.
7. Muhammad, A. F.; R. Sattler (1982). “Vessel Structure of Gnetum and the Origin of Angiosperms”. American Journal of Botany. 69 (6): 1004–21.
doi:10.2307/2442898. JSTOR 2442898.
8. Melvin T. Tyree; Martin H. Zimmermann (2003). Xylem Structure and the Ascent of Sap (2nd ed.). Springer. ISBN 978-3-540-43354-5. recent update of the classic book on xylem transport by the late Martin Zimmermann

Photo credit: https://www.flickr.com/photos/31064702@N05/5697471591/’]