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Xylem
Xylem
Xylem
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Xylem (blue) transports water and minerals from the roots upwards.

Xylem is one of the two types of transport tissue in vascular plants, the other being phloem; both of these are part of the vascular bundle. The basic function of the xylem is to transport water upward from the roots to parts of the plants such as stems and leaves, but it also transports nutrients.[1][2] The word xylem is derived from the Ancient Greek word ξύλον (xúlon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout a plant.[3] The term was introduced by Carl Nägeli in 1858.[4][5]

Structure

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Diagrammatic structure of xylem cells
Diagrammatic structure of xylem cells

The most distinctive xylem cells are the long tracheary elements that transport water. Tracheids and vessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are called vessels.[6]

Wood also contains two other type of cells: parenchyma and fibers.[7]

Xylem can be found:

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 only.

The branching pattern exhibited by xylem follows Murray's law.[8]

Primary and secondary xylem

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Primary xylem is formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem.[9]

Secondary xylem is formed during secondary growth from vascular cambium. Although secondary xylem is also found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the two main groups in which secondary xylem can be found are:

  1. conifers (Coniferae): there are approximately 600 known species of conifers.[10] All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood.
  2. angiosperms (Angiospermae): there are approximately 250,000[10] known species of angiosperms. Within this group secondary xylem is rare in the monocots.[11] Many non-monocot angiosperms become trees, and the secondary xylem of these is used and marketed as hardwood.

Main function – upwards water transport

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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. The system transports water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well. The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees.[12] 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. The phloem pressure can rise to several MPa,[13] far higher than atmospheric pressure. Selective interconnection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
  • 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. This causes millions of minute menisci to form in the mesophyll cell wall. The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil.[14]
  • 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. This causes a positive pressure that forces sap up the xylem towards the leaves. In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is highest in the morning before the opening of stomata and allow transpiration to begin. Different plant species can have different root pressures even in a similar environment; examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus.[15]

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.[16][17] Capillary action provides the force that establishes an equilibrium configuration, balancing gravity. When transpiration removes water at the top, the flow is needed to return to the equilibrium.[14]

Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. By capillary action, the water forms concave menisci inside the pores. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches.

Transpirational pull requires that the vessels transporting the water be very small in diameter; otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. 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). Even after an embolism has occurred, plants are able to refill the xylem and restore the functionality.[18]

Cohesion-tension theory

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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.[19][20] Despite numerous objections,[21][22] 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),[23][24] and Dixon (1914,1924).[25][26]

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 forces, 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.[14]

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.[14]

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 (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.[27]

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.[28][29]

Measurement of pressure

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A diagram showing the setup of a pressure bomb

Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a pressure bomb to counteract it.[30] When the technology to perform direct measurements with a pressure probe was developed, there was initially some doubt about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination[31] of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.

Evolution

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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.[32] The earliest true and recognizable xylem consists of tracheids 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 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).[9]

In most plants, pitted tracheids 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[33]) is considered to be one of the key innovations that led to the success of the angiosperms.[34] 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.[35] Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost.

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.[36] 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.[36] 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.[36]

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.[36] 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.[36] 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.[32]

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.[36] 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 wicked 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 – 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.[36] 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.[36] This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausting it, plants developed a waterproof cuticle. Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue.[36] 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.[36]

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 – click on the image for a larger view. 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. During the early Silurian, they developed specialized cells, which were lignified (or bore similar chemical compounds)[36] to avoid implosion; this process coincided with cell death, allowing their innards to be emptied and water to be passed through them.[36] 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.[37] The early Devonian pretracheophytes Aglaophyton and Horneophyton have structures very similar to the 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,[38] are an early improvisation to aid the easy flow of water.[39] Banded tubes, as well as tubes with pits in their walls, were lignified[40] 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.[36] Tracheids may have a single evolutionary origin, possibly within the hornworts,[41] uniting all tracheophytes (but they may have evolved more than once).[36]

Water transport requires regulation, and dynamic control is provided by stomata.[42] 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.[36]

An endodermis probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous.[36] 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.[36] As a result of their independence from their surroundings, they lost their ability to survive desiccation – a costly trait to retain.[36]

During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant.[39] By the middle Devonian, the tracheid diameter of some plant lineages (Zosterophyllophytes) had plateaued.[39] 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.[39] 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[39] and increased stomatal density, both of which would increase the demand for water.[36]

While wider tracheids with robust walls make it possible to achieve higher water transport tensions, this increases the likelihood of cavitation.[36] 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[43][44] which have developed a mechanism of doing so). Therefore, it is well worth plants' while to avoid cavitation occurring. For this reason, 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.[36]

Once cavitation has occurred, plants have a range of mechanisms to contain the damage.[36] 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.[36] These pitted surfaces further reduce the flow of water through the xylem by as much as 30%.[36] 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 radiation.[45] Conifers, by the Jurassic, developed bordered pits had valve-like structures to isolate cavitated elements. These 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.[36] 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] Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles.[citation needed]

Growing to height also employed another trait of tracheids – 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.[36] 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;[39] vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel.[39] The function of end walls, which were the default state in the Devonian, was probably to avoid embolisms. 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.[39]

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 μm.[36] 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.[36]

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.[36] Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids![36] 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.[36] 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.[36]

Development

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Patterns of xylem development: xylem in brown; arrows show direction of development from protoxylem to metaxylem.

Xylem development can be described by four terms: centrarch, exarch, endarch and mesarch. As it develops in young plants, its nature changes from protoxylem to metaxylem (i.e. from first xylem to after xylem). The patterns in which protoxylem and metaxylem are arranged are essential in studying plant morphology.[citation needed]

Protoxylem and metaxylem

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As a young vascular plant grows, one or more strands of primary xylem form in its stems and roots. The first xylem to develop is called 'protoxylem'. In appearance, protoxylem is usually distinguished by narrower vessels formed of smaller cells. Some of these cells have walls that contain thickenings in the form of rings or helices. Functionally, protoxylem can extend: the cells can grow in size and develop while a stem or root is elongating. Later, 'metaxylem' develops in the strands of xylem. Metaxylem vessels and cells are usually larger; the cells have thickenings typically either in the form of ladderlike transverse bars (scalariform) or continuous sheets except for holes or pits (pitted). Functionally, metaxylem completes its development after elongation ceases when the cells no longer need to grow in size.[46][47]

Patterns of protoxylem and metaxylem

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There are four primary patterns to the arrangement of protoxylem and metaxylem in stems and roots.

  • Centrarch refers to the case in which the primary xylem forms a single cylinder in the center of the stem and develops from the center outwards. The protoxylem is thus found in the central core, and the metaxylem is in a cylinder around it.[48] This pattern was common in early land plants, such as "rhyniophytes", but is not present in any living plants.[citation needed]

The other three terms are used where there is more than one strand of primary xylem.

  • Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside inwards towards the center, i.e., centripetally. The metaxylem is thus closest to the center of the stem or root, and the protoxylem is closest to the periphery. The roots of vascular plants are generally considered to have exarch development.[46]
  • Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside outwards towards the periphery, i.e., centrifugally. The protoxylem is thus closest to the center of the stem or root, and the metaxylem is closest to the periphery. The stems of seed plants typically have endarch development.[46]
  • Mesarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the middle of a strand in both directions. The metaxylem is thus on both the peripheral and central sides of the strand, with the protoxylem between the metaxylem (possibly surrounded by it). The leaves and stems of many ferns have mesarch development.[46]

History

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In his book De plantis libri XVI (On Plants, in 16 books) (1583), the Italian physician and botanist Andrea Cesalpino proposed that plants draw water from soil not by magnetism (ut magnes ferrum trahit, as magnetic iron attracts) nor by suction (vacuum), but by absorption, as occurs in the case of linen, sponges, or powders.[49] The Italian biologist Marcello Malpighi was the first person to describe and illustrate xylem vessels, which he did in his book Anatome plantarum ... (1675).[50][note 1] Although Malpighi believed that xylem contained only air, the British physician and botanist Nehemiah Grew, who was Malpighi's contemporary, believed that sap ascended both through the bark and through the xylem.[51] However, according to Grew, capillary action in the xylem would raise the sap by only a few inches; 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.[52] In 1727, English clergyman and botanist Stephen Hales showed that transpiration by a plant's leaves causes water to move through its xylem.[53][note 2] By 1891, the Polish-German botanist Eduard Strasburger had shown that the transport of water in plants did not require the xylem cells to be alive.[54]

See also

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Explanatory notes

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Xylem

View on Grokipedia
from Grokipedia
Xylem is a complex vascular tissue found in vascular plants, responsible for the unidirectional transport of water and dissolved minerals from roots to shoots and leaves, while also providing mechanical support to enable upright growth. This tissue forms a continuous network throughout the plant body, including roots, stems, and leaves, and is essential for maintaining hydration and structural integrity against gravity and environmental stresses. Structurally, xylem comprises several specialized cell types, primarily tracheids and vessel elements, both of which are elongated, tubular cells that die at maturity to form hollow conduits reinforced with for rigidity and resistance to collapse under tension. Tracheids, present in all vascular plants, connect end-to-end via pits in their walls to allow lateral movement, whereas vessel elements, found in angiosperms and gnetophytes, stack into longer vessels with perforated end walls for more efficient axial flow. Accompanying these conducting cells are xylem parenchyma for short-distance transport and storage of nutrients, and xylem fibers or sclerenchyma cells that enhance mechanical strength. The primary function of xylem relies on the cohesion-tension theory, where transpiration from leaves creates negative pressure that pulls water upward through the conduits, facilitated by the cohesive properties of water molecules and adhesive forces to cell walls. This passive process not only delivers essential minerals like nitrogen and potassium but also contributes to cooling the plant and powering photosynthesis by maintaining turgor pressure. In addition to transport, xylem's lignified structure imparts compressive and tensile strength, allowing plants to grow tall and compete for light without collapsing.

Anatomy

Cellular Composition

Xylem is a complex primarily composed of dead cells at maturity, including tracheids and vessel elements for conduction, fibers for mechanical support, and for storage and short-distance transport. These cell types form a supportive and conductive network in vascular plants, with the conducting elements lacking protoplasts and functioning as hollow conduits. Tracheids are elongated, spindle-shaped cells with tapered ends, typically measuring several times longer than they are wide, and featuring bordered pits on their lateral s that allow lateral water movement between adjacent cells. They predominate in gymnosperms and ferns, providing both water conduction and structural reinforcement due to their thick, lignified secondary walls. Secondary wall thickenings in tracheids vary by developmental stage, including annular (ring-like) patterns for extensibility in early-formed cells and more rigid helical (spiral) or scalariform (ladder-like) arrangements in later ones. Vessel elements, in contrast, are shorter and wider than tracheids, stacking end-to-end to form continuous vessels in angiosperms, connected via perforation plates—openings at the ends that enhance efficient axial water flow. Like tracheids, they have lignified secondary walls with pits for lateral connections, but their morphology allows for greater compared to tracheids alone. Wall thickening patterns in vessel elements include helical and scalariform types, contributing to their structural integrity while permitting conduction. Xylem fibers are elongated sclerenchyma cells that are dead at maturity, with thick, lignified walls and pointed ends, providing significant mechanical support to the tissue. They often occur interspersed among conducting elements, enhancing the overall rigidity of the xylem. Xylem consists of living cells with thin walls, arranged axially or in rays for storage of nutrients and facilitation of radial ; ray parenchyma, in particular, forms horizontal bands in secondary xylem for lateral exchange. Bordered pits occur between various cell types, such as tracheids and parenchyma or vessels and fibers, enabling selective and solute passage while preventing air emboli spread. The cells are organized into vascular bundles in primary growth or continuous cylinders with radial rays in , optimizing both longitudinal conduction and structural stability.

Primary and Secondary Xylem

Primary xylem originates from the procambium tissue derived from apical meristems during the primary growth phase of , enabling elongation of , stems, and leaves. This tissue is the first vascular element to form in developing organs and is organized within vascular bundles. It comprises two main components: protoxylem, the early-forming portion with narrow cells featuring annular or helical secondary wall thickenings, and metaxylem, the later-forming portion with wider cells exhibiting scalariform or pitted secondary walls. In contrast, secondary xylem forms through the activity of the , a cylindrical lateral that arises from the fascicular and interfascicular in stems and of woody . This produces secondary xylem cells inward via periclinal divisions, leading to radial thickening of the axis over time. In temperate woody , seasonal fluctuations in environmental conditions cause the to produce distinct annual rings, with earlywood cells larger and thinner-walled than the denser latewood cells formed later in the season. Structurally, secondary xylem differs from primary xylem in being shorter-celled, denser, and more heavily lignified, with extensive secondary wall impregnation providing rigidity. Primary xylem, associated with elongating tissues, often experiences mechanical stress where protoxylem elements are crushed or stretched during organ expansion, whereas secondary xylem layers accumulate durably without such disruption. Herbaceous typically feature only primary xylem, limiting growth to elongation, while woody develop substantial secondary xylem that dominates the stem's cross-section.

Development

Protoxylem and Metaxylem

The primary xylem, formed during early growth, differentiates into two sequential components: protoxylem and metaxylem, based on their maturation timing relative to organ elongation. Protoxylem develops first from procambial cells near the apices of shoots and , where active growth occurs, enabling initial water transport in elongating tissues. Its tracheary elements, including tracheids and vessels, feature thin secondary walls reinforced by annular or helical thickenings, which provide flexibility for stretching during longitudinal expansion. These adaptations allow protoxylem to function temporarily, but the cells are often crushed, stretched, or functionally compromised as the organ elongates further. Metaxylem matures subsequently from remaining procambial cells, typically after primary elongation has subsided, forming a more robust conducting network for mature organs. In contrast to protoxylem, metaxylem elements possess thicker secondary walls with reticulate or pitted thickenings and specialized pit membranes that enhance lateral movement and overall hydraulic . Metaxylem conduits generally exhibit wider lumens, supporting higher flow rates suited to the reduced mechanical stress in non-elongating regions. This sequential maturation is regulated by hormonal signals, particularly , which establishes gradients that induce procambial differentiation into xylem precursors in both and shoot systems. In , for instance, promotes protoxylem formation at the tip, followed by metaxylem development in the elongation zone, ensuring continuous vascular continuity. Similarly, in shoots, directs the patterned differentiation of procambium, coordinating protoxylem and metaxylem to accommodate apical growth phases.

Developmental Patterns

Xylem development exhibits distinct spatial s of maturation, primarily characterized by the relative positions of protoxylem and metaxylem during . In , the pattern predominates, where protoxylem matures first at the periphery of the xylem strand, with metaxylem developing centripetally toward the center. This arrangement facilitates early extension growth at the tip. In contrast, stems typically display an endarch pattern, with protoxylem maturing internally and metaxylem expanding outward centrifugally. Leaves often feature a mesarch pattern, in which protoxylem develops centrally within the strand, and metaxylem matures bidirectionally toward both the interior and exterior. Variations in xylem organization are evident across plant organs and taxa, particularly in the number of protoxylem poles in . Dicotyledonous commonly exhibit diarch (two poles), triarch (three), or tetrarch (four) arrangements, reflecting a more constrained vascular . Monocotyledonous , however, typically show a polyarch condition with six or more poles, enabling greater radial expansion and resource distribution. In secondary xylem, produced by the , cells align in radial files originating from initials, while tangential divisions contribute to ray tissues, establishing a layered, cylindrical . Environmental factors modulate these developmental patterns, particularly in where basipetal xylem maturation—from apex to base—occurs during elongation from the . For instance, hypergravity conditions accelerate metaxylem differentiation and alter properties in stems, demonstrating mechanosensory influences on vascular patterning.

Function

Water and Mineral

The xylem serves as the primary conduit for unidirectional of water and dissolved minerals from the to the aerial parts of vascular , ensuring hydration and nutrient delivery essential for and growth. This flow is predominantly upward, driven by transpiration pull from leaf evaporation and, to a lesser extent, generated by active uptake in . Water enters the plant through root hairs in the , moving via apoplastic and symplastic pathways across the , cortex, and before reaching the xylem vessels or tracheids in the . The , with its , regulates this entry by forcing water and solutes through selective symplastic routes, preventing unregulated backflow and maintaining the unidirectional ascent. Once in the xylem, the experiences negative pressure from pull, facilitating continuous upward movement against gravity. Minerals, primarily inorganic ions such as (K⁺) and calcium (Ca²⁺), are absorbed from the solution by root epidermal and cortical cells via mechanisms involving proton pumps and channels. These s are then loaded into the xylem passively, carried along with the bulk flow of water driven by , without requiring additional energy expenditure in the . This process distributes essential nutrients like K⁺ for activation and Ca²⁺ for stability throughout the . Xylem sap consists predominantly of , comprising approximately 99% of its volume, with less than 1% solutes including ions, organic compounds, and trace hormones. In large trees, such as those in tropical rainforests, daily xylem flow rates can reach up to 100 liters or more, scaling with canopy size and environmental demand to support high volumes. Flow rates in the xylem are influenced by environmental factors including , which accelerates ; , which modulates transpiration gradients; and availability, which limits uptake during . Low or high evaporative demand can induce — the formation of vapor bubbles in xylem conduits—leading to embolisms that block water transport and reduce . These embolisms pose a of hydraulic , particularly in with vulnerable xylem, prompting adaptations like pit membrane structures to mitigate spread.

Mechanical Support

The xylem provides mechanical support to plants through the lignification of cell walls in its key components—tracheids, vessels, and fibers—which imparts rigidity capable of resisting compressive and tensile forces acting on the plant body. deposition in these walls creates a that withstands under self-weight and external loads, such as wind, allowing plants to maintain structural integrity. In particular, fibers, with their elongated shape and thick secondary walls, contribute disproportionately to load-bearing by distributing stress across the tissue. Secondary xylem, formed through cambial activity in woody , achieves high that supports extreme statures, enabling trees like coast redwoods to reach heights exceeding 100 m while countering gravitational compression at the base. This arises from the accumulation of lignified tracheary elements and fibers, forming a solid matrix that prevents stem collapse and facilitates vertical growth in canopies. Without such reinforcement, the biomechanical demands of height would limit arboreal forms to much shorter profiles. In non-woody or herbaceous , mechanical support integrates the hydroskeleton principle, where generated in living cells interacts with the rigid, dead xylem elements to sustain upright posture without extensive secondary thickening. This hydrostatic framework relies on -filled cells pressing against lignified primary xylem for stability, as seen in stems of grasses and forbs that remain erect under moderate loads. However, a key trade-off exists: thicker lignified walls in xylem cells enhance support by increasing resistance to deformation but diminish by narrowing lumens and increasing path resistance to water flow. Herbaceous thus prioritize thinner-walled primary xylem for balanced support and transport, contrasting with woody where builds denser tissues for superior stability at the of .

Transport Mechanisms

Cohesion-Tension Theory

The cohesion-tension theory explains the ascent of in xylem as a passive process driven by from mesophyll cells, which creates negative hydrostatic pressure (tension) in the leaf xylem, pulling a continuous column of upward from the roots against and frictional losses. Proposed by Henry H. Dixon and John Joly in their 1894 paper, the theory relies on the cohesive forces between molecules—arising from hydrogen bonding—and adhesive forces between and hydrophilic xylem cell walls, forming an unbroken filament capable of spanning tall plants. This mechanism operates without active cellular energy input in the xylem, contrasting with earlier root-pressure hypotheses, and accounts for rates up to hundreds of liters per day in large trees. Key biophysical properties underpin the theory's feasibility. Water exhibits exceptional tensile strength under metastable conditions, reaching approximately 30 MPa in degassed, pure samples, far exceeding the typical tensions required for in most and enabling the to resist rupture. Bordered pits between adjacent xylem conduits, featuring semi-permeable membranes, limit the spread of embolisms by restricting air seeding across pores under tension, with pore diameters typically 20–200 nm that maintain hydraulic isolation while permitting water flow. The magnitude of tension generated is thermodynamically linked to the relative (RH) in intercellular spaces via the Kelvin-derived for liquid-vapor equilibrium: P=−RTVmln⁡(RH)P = -\frac{RT}{V_m} \ln(RH) where PP is the xylem pressure (negative under tension), RR is the universal gas constant (8.314 J mol⁻¹ K⁻¹), TT is absolute temperature (K), VmV_m is the partial molar volume of water (≈1.8 × 10⁻⁵ m³ mol⁻¹), and RH is the relative humidity (0–1); this relation connects evaporative demand during transpiration to the resulting pull on the xylem sap. Empirical evidence supports the theory's predictions. Direct and indirect measurements, such as those using the Scholander pressure chamber on excised shoots, have demonstrated xylem sap tensions ranging from -1 to -20 MPa in transpiring leaves and stems across diverse species, with higher values in tall conifers like redwoods under peak evaporative conditions. Following cavitation-induced embolisms, which introduce air and reduce conductivity, root pressure—generated by active ion uptake in roots—can drive refilling of vessels at night or in wet soils, restoring up to 50–100% of lost hydraulic function in herbaceous and woody plants within hours to days. These observations confirm the dynamic balance between tension-driven transport and embolism repair in maintaining xylem functionality.

Xylem Pressure Measurement

Xylem pressure measurement is essential for understanding dynamics in , relying on empirical techniques that quantify negative s generated by transpiration pull under the cohesion-tension mechanism. One of the most widely adopted methods is the Scholander pressure bomb, which measures equilibrium tension in or stem xylem by enclosing the excised tissue in a sealed chamber and gradually increasing external gas until appears at the cut surface, indicating the balancing of internal tension. This technique, introduced in , provides indirect estimates of xylem and has been validated against direct methods in various species, though it assumes a continuous liquid column from the measurement point to the cut end. Direct measurement of sap is achieved using the xylem pressure probe, which involves inserting a fine oil-filled microcapillary into an intact xylem vessel to sense via a , allowing real-time monitoring without excision. Developed as an adaptation of the cell pressure probe in the late , this method captures transient pressures but is limited to accessible vessels in herbaceous or thin-stemmed due to insertion challenges in woody tissues. For quantifying —air-filled conduits that disrupt flow— techniques apply controlled negative pressures via spinning excised stem segments in a custom rotor, measuring the percentage loss of as a function of applied tension to generate vulnerability curves. This approach, refined in the , enables rapid assessment of thresholds across . Key findings from these methods reveal typical xylem tensions ranging from -0.5 MPa in roots to -2.5 MPa in leaves of mesic trees, escalating to -10 MPa or more in tall conifers like redwoods to overcome gravitational and resistive forces. Diurnal variations show pressures becoming more negative during midday transpiration peaks (e.g., dropping 1-2 MPa from predawn values) before recovering nocturnally, with pronounced cycles in arid-adapted species. Species differences are evident, as conifers often sustain higher peak tensions than co-occurring angiosperms due to their tracheid-based xylem, which resists at greater negatives despite lower conductivity. Challenges in these measurements include probe clogging from viscous components in the xylem pressure probe, which can artifactually elevate readings, and inadvertent introduction of air bubbles during insertion or that trigger premature . Recent advances since the 2000s, such as , address these by non-invasively imaging formation in intact stems using or lab-based scanners to visualize air-water interfaces at micrometer resolution without pressure artifacts. This technique has confirmed spread patterns and refilling dynamics in living plants, enhancing accuracy over traditional hydraulic methods.

Evolution

Origins in Early Plants

The origins of xylem trace back to the transition from non-vascular to vascular during the period, approximately 430 million years ago. Early land , such as Cooksonia-like , represent the first evidence of , marking a pivotal for terrestrial life. These primitive lacked the complex structures of modern vascular systems but possessed rudimentary xylem that enabled efficient water conduction from the . In non-vascular , precursor conducting cells known as hydroids facilitated limited water transport but lacked true lignification, relying instead on thin, non-reinforced cell walls that prevented the development of rigid, supportive structures. The of lignified xylem in early vascular , particularly simple tracheids in rhyniophytes, overcame these limitations by providing both mechanical strength and efficient . These tracheids, characterized by annular or spiral thickenings and the absence of vessels, allowed for the programmed death of cells to form hollow conduits, a key innovation absent in bryophyte hydroids. This development conferred significant adaptive advantages, enabling early plants to achieve greater heights—up to several centimeters in —and resist drought by facilitating long-distance water transport and structural support against gravity and wind. Fossil evidence from the in , dating to the around 410 million years ago, preserves protoxylem-like structures in plants such as and Asteroxylon, revealing central xylem strands with narrow tracheids that supported upright growth in a desiccating environment. The genetic foundations of xylem formation in these early plants involved conserved regulatory genes, such as homologs of ATHB8, a homeodomain-leucine zipper that specifies provascular cell identity and promotes tracheary element differentiation. Studies of ATHB8 homologs across land indicate their ancient origin, predating the diversification of vascular lineages and linking morphology to molecular mechanisms of vascular patterning.

Diversification Across Plant Groups

In most gymnosperms, such as , cycads, and Ginkgo, xylem is composed primarily of tracheids, which serve as the primary water-conducting cells without vessels, allowing for efficient water transport in cold climates where these plants often dominate. However, gnetophytes, another gymnosperm , possess vessels. The small diameter of these tracheids enhances resistance to embolisms induced by freeze-thaw cycles, as narrower conduits minimize the expansion of air bubbles during formation and subsequent thawing, thereby maintaining hydraulic function in temperate and boreal environments. This structural adaptation supports the persistence of gymnosperms in regions with frequent winter freezing, though it limits overall conductivity compared to more advanced vascular systems. Vessels evolved independently in several lineages, including gnetophytes, some ferns, and lycophytes as early as the late Permian, in addition to their definitive development in angiosperms. Angiosperms exhibit a key evolutionary innovation in xylem structure with the development of vessels, which first appeared in the fossil record during the Early to mid-Cretaceous period, approximately 100-140 million years ago, coinciding with a major radiation of flowering plants. These vessels, formed by stacked vessel elements with perforated end walls, enable significantly faster water conduction than tracheids alone, facilitating higher rates of photosynthesis and supporting diverse growth forms from herbs to large trees. Additionally, angiosperm xylem features diversified fibers that provide enhanced mechanical support, allowing for taller statures and broader ecological niches without compromising transport efficiency. In ferns and lycophytes, xylem consists of simple tracheids characterized by scalariform pits—ladder-like arrangements of bordered pits on their walls—that facilitate lateral water movement while restricting air seeding to prevent spread. Unlike seed plants, most ferns and lycophytes lack from a , resulting in primary xylem only, which constrains plant size and height to typically under a few meters and limits their competitive ability in resource-rich habitats. This primitive organization reflects their ancient lineage and to shaded, moist understories where high conductivity is less critical. Comparatively, vessels in angiosperms provide 10- to 100-fold higher than tracheids in gymnosperms or ferns, primarily due to their larger diameters and lack of end walls, which reduce flow resistance and support greater rates per unit of wood area. However, this comes with trade-offs, as tracheid-based systems offer superior resistance to in variable environments, balancing safety and performance across clades. In response to , drought-adapted angiosperm species often exhibit vessel enlargement to maintain hydraulic under water stress, as seen in drought-deciduous trees where wider conduits enhance uptake during brief wet periods while relying on shedding for survival. Such plasticity underscores the adaptive diversification of xylem traits to environmental pressures.

History

Early Observations

The earliest recorded observations of plant vascular structures appeared in ancient texts, where they were likened to animal anatomy. In the 4th century BCE, described plant "veins" in his Enquiry into Plants as elongated structures resembling muscle tissue, thicker and with lateral branches that contained fluid, though his accounts were based solely on macroscopic examination without magnification. Such descriptions remained rudimentary for centuries, as the lack of microscopic tools prevented detailed analysis of internal tissues until the invention of the compound microscope in the early . The advent of microscopy in the mid-17th century marked a pivotal shift toward systematic plant anatomy. Italian physician Marcello Malpighi, using early microscopes, provided the first detailed accounts of woody tissues in his 1675 work Anatome Plantarum, portraying them as networks of minute ducts or vessels arranged in bundles that facilitated fluid movement, drawing analogies to animal circulatory systems. Concurrently, English botanist Nehemiah Grew independently advanced these ideas in his 1682 publication The Anatomy of Plants, where he coined the term "xylem"—derived from the Greek xylon meaning "wood"—to classify the hard, lignified vascular elements distinct from softer bast tissues, emphasizing their role in structural integrity. In the , improved and techniques enabled finer distinctions within xylem. German botanist Alexander Sanio differentiated tracheids—elongated cells with tapered ends connected by pits—from vessels, which are wider, tube-like structures formed by stacked elements without end walls, as detailed in his 1863 studies. Parallel efforts identified 's chemical nature; Anselme Payen isolated it as a key woody component in 1838 through treatments yielding insoluble residues, while Joseph Wiesner's 1879 phloroglucinol-HCl test specifically detected in xylem walls via a characteristic red coloration from reactions with coniferaldehyde groups, confirming its impregnation in cell walls for rigidity.

Modern Discoveries

In the mid-20th century, the cohesion-tension theory of xylem sap ascent, originally formulated by Dixon and Joly in 1894, underwent significant refinement through technological advances in microscopy. Post-1950s microscopy, particularly (TEM) applied to and fiber analysis starting around 1951, provided ultrastructural details of xylem elements, including bordered pit membranes and their pores, which are critical for preventing while facilitating water flow under negative pressure. These observations confirmed the theory's predictions by visualizing how pit membrane architecture supports metastable water columns, reducing air-seeding risks during tension. Further, cryo-scanning electron microscopy (cryo-SEM) in later decades directly imaged vessel contents and embolisms, validating the theory's emphasis on continuous water columns and highlighting that lower in pit pores to enhance hydraulic stability. A pivotal advancement in the 1980s came from studies on xylem , led by Martin H. , who integrated anatomical and physiological data to elucidate mechanisms. In his 1983 book Xylem Structure and the , Zimmermann detailed how tension-induced disrupts water columns, using innovative pressure probe techniques to measure negative pressures and quantify vulnerability across . These works established as a primary hydraulic limitation, influencing subsequent models of plant water relations and drought vulnerability. Genetic research in the 2000s uncovered key regulators of xylem differentiation, with the discovery of the VASCULAR-RELATED NAC-DOMAIN (VND) transcription factor family in Arabidopsis thaliana. Kubo et al. (2005) identified VND6 and VND7 as master switches that initiate protoxylem and metaxylem vessel formation by activating downstream genes for secondary cell wall biosynthesis and programmed cell death. Post-2010 CRISPR/Cas9 studies have built on this, enabling precise editing of VND-interacting genes to modulate vessel dimensions and density; these genetic tools have accelerated functional genomics of xylem development. Recent advances up to 2025 emphasize xylem plasticity amid climate-driven , revealing adaptive adjustments in anatomy to maintain hydraulic conductance. Studies on species like show organ-specific plasticity, where reduces tracheid diameter and increases pit membrane thickness in and stems, boosting resistance without sacrificing efficiency. This aligns with IPCC assessments of intensifying , where xylem trait variability predicts forest resilience. Bioengineering efforts leverage these findings, using to modify xylem-related genes for drought-tolerant crops; for example, editing metaxylem phenotypes in optimizes hydraulic architecture under stress. Such modifications, targeting VND pathways, enhance overall stability in arid conditions.

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