Xylem (blue) transports water and minerals from the roots upwards.
Xylem is one of the two types of transporttissue invascular plants, the other beingphloem; both of these are part of thevascular bundle. The basic function of the xylem is to transportwater upward from the roots to parts of the plants such as stems and leaves, but it also transportsnutrients.[1][2] The wordxylem is derived from theAncient Greek wordξύλον (xúlon), meaning "wood"; the best-known xylem tissue is wood, though it is found throughout a plant.[3] The term was introduced byCarl Nägeli in 1858.[4][5]
The most distinctive xylemcells are the long tracheary elements that transport water.Tracheids andvessel elements are distinguished by their shape; vessel elements are shorter, and are connected together into long tubes that are calledvessels.[6]
In transitional stages of plants withsecondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will containprimary xylem only.
The branching pattern exhibited by xylem followsMurray's law.[8]
Primary xylem is formed during primary growth fromprocambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem.[citation needed]
Secondary xylem is formed during secondary growth fromvascular cambium. Although secondary xylem is also found in members of thegymnosperm groupsGnetophyta andGinkgophyta and to a lesser extent in members of theCycadophyta, the two main groups in which secondary xylem can be found are:
conifers (Coniferae): there are approximately 600 known species of conifers.[9] 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 assoftwood.
angiosperms (Angiospermae): there are approximately 250,000[9] known species of angiosperms. Within this group secondary xylem is rare in themonocots.[10] Many non-monocot angiosperms become trees, and the secondary xylem of these is used and marketed ashardwood.
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 duringtranspiration and photosynthesis. Xylemsap 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 thetracheary 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.[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 asolute pressure differential versus the xylem system carrying a far lower load ofsolutes—water and minerals. The phloem pressure can rise to several MPa,[12] far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
Transpirational pull: Similarly, theevaporation ofwater from the surfaces ofmesophyll cells to the atmosphere also creates a negative pressure at the top of a plant. This causes millions of minutemenisci to form in the mesophyll cell wall. The resultingsurface tension causes a negative pressure ortension in the xylem that pulls the water from the roots and soil.[citation needed]
Root pressure: If the water potential of the root cells is more negative than that of thesoil, usually due to high concentrations of solute, water can move byosmosis 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 ahydathode in a phenomenon known asguttation. 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 inVitis riparia but around zero inCelastrus orbiculatus.[13]
The primary force that creates thecapillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits.[14][15] 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.[citation needed]
Transpirational pull results from the evaporation of water from the surfaces ofcells in theleaves. This evaporation causes the surface of the water to recess into thepores of thecell wall. Bycapillary action, the water forms concave menisci inside the pores. The high surface tension of water pulls theconcavity outwards, generating enoughforce to lift water as high as a hundred meters from ground level to atree'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 waterevaporates 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 – anembolism forms, which will spread quickly to other adjacent cells, unlessbordered 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.[16]
Thecohesion-tension theory is a theory ofintermolecular attraction that explains the process of water flow upwards (against the force ofgravity) through the xylem of plants. It was proposed in 1894 byJohn Joly andHenry Horatio Dixon.[17][18] Despite numerous objections,[19][20] 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),[21][22] and Dixon (1914,1924).[23][24]
Water is apolar molecule. When two water molecules approach one another, the slightly negatively chargedoxygen atom of one forms ahydrogen bond with a slightly positively chargedhydrogen atom in the other. This attractive force, along with otherintermolecular forces, is one of the principal factors responsible for the occurrence ofsurface tension in liquid water. It also allows plants to draw water from the root through the xylem to the leaf.[citation needed]
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, utilizingcapillary 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.[citation needed]
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 bycohesion (the pull between individual water molecules, due to hydrogen bonds) andadhesion (the stickiness between water molecules and thehydrophilic cell walls of plants). This mechanism of water flow works because ofwater potential (water flows from high to low potential), and the rules of simplediffusion.[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 thecohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylemosmotic pressuregradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients.[26][27]
Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with apressure bomb to counteract it.[28] 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[29] of transpirational pull from above androot pressure from below, which makes the interpretation of measurements more complicated.
Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from theSilurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlierOrdovician rocks.[citation needed] The earliest true and recognizable xylem consists oftracheids with a helical-annular reinforcing layer added to thecell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in theprotoxylem (first-formed xylem) of all living groups of vascular plants. Several groups of plants later developedpitted tracheid cells independently throughconvergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of themetaxylem (following theprotoxylem).[citation needed]
In most plants, pittedtracheids function as the primary transport cells. The other type of vascular element, found in angiosperms, is thevessel element. Vessel elements are joined end to end to form vessels in which water flows unimpeded, as in a pipe. The presence ofxylem vessels (also calledtrachea[30]) is considered to be one of the key innovations that led to the success of theangiosperms.[31] 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, andWinteraceae), and their secondary xylem is described byArthur Cronquist as "primitively vesselless". Cronquist considered the vessels ofGnetum to be convergent with those of angiosperms.[32] Whether the absence of vessels in basal angiosperms is aprimitive 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.[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] 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.[33]
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.[33] As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water. This transition frompoikilohydry tohomoiohydry opened up new potential for colonization.[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.[citation needed]
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.[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 avoiddesiccation. This early water transport took advantage of thecohesion-tension mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can bewicked 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.[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.[33] This process demands a steady supply of water from one end, to maintain the chains; to avoid exhausting it, plants developed a waterproofcuticle. Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue.[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.[33]
Abanded 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 theearly Silurian, they developed specialized cells, which werelignified (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.[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.
The earliest macrofossils to bear water-transport tubes are Silurian plants placed in the genusCooksonia.[34] The early Devonian pretracheophytesAglaophyton andHorneophyton have structures very similar to thehydroids 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,[35] are an early improvisation to aid the easy flow of water.[36] Banded tubes, as well as tubes with pits in their walls, were lignified[37] and, when they form single celled conduits, are considered to betracheids. 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] Tracheids may have a single evolutionary origin, possibly within the hornworts,[38] uniting all tracheophytes (but they may have evolved more than once).[33]
Water transport requires regulation, and dynamic control is provided bystomata.[39]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.[33]
Anendodermis probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous.[33] 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.[33] As a result of their independence from their surroundings, they lost their ability to survive desiccation – a costly trait to retain.[33]
During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant.[36] By the middle Devonian, the tracheid diameter of some plant lineages (Zosterophyllophytes) had plateaued.[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.[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.[33]
While wider tracheids with robust walls make it possible to achieve higher water transport tensions, this increases the likelihood of cavitation.[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. 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 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.[33]
Once cavitation has occurred, plants have a range of mechanisms to contain the damage.[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] These pitted surfaces further reduce the flow of water through the xylem by as much as 30%.[33] 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 theDevonian radiation.[42] Conifers, by the Jurassic, developedbordered pits had valve-like structures to isolate cavitated elements. Thesetorus-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.[33] 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 toughsclerenchyma on the outer rim of the stems.[33] 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;[36] vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel.[36] The function of end walls, which were the default state in the Devonian, was probably to avoidembolisms. 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.[36]
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.[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 andSelaginellales independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes.[33]Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids![33] This allowed plants to fill more of their stems with structural fibers, and also opened a new niche tovines, which could transport water without being as thick as the tree they grew on.[33] 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.[33]
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 andmesarch. As it develops in young plants, its nature changes fromprotoxylem tometaxylem (i.e. fromfirst xylem toafter xylem). The patterns in which protoxylem and metaxylem are arranged are essential in studying plant morphology.[citation needed]
As a youngvascular 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.[43][44]
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.[45] 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 ofvascular plants are generally considered to have exarch development.[43]
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 ofseed plants typically have endarch development.[43]
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 manyferns have mesarch development.[43]
In his bookDe plantis libri XVI (On Plants, in 16 books) (1583), the Italian physician and botanistAndrea 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.[46] The Italian biologistMarcello Malpighi was the first person to describe and illustrate xylem vessels, which he did in his bookAnatome plantarum ... (1675).[47][note 1] Although Malpighi believed that xylem contained only air, the British physician and botanistNehemiah Grew, who was Malpighi's contemporary, believed that sap ascended both through the bark and through the xylem.[48] 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.[49] In 1727, English clergyman and botanistStephen Hales showed that transpiration by a plant's leaves causes water to move through its xylem.[50][note 2] By 1891, the Polish-German botanistEduard Strasburger had shown that the transport of water in plants did not require the xylem cells to be alive.[51]
^Malpighi first described xylem vessels and named tracheid cells. From p. 8 of (Malpighi, 1675):" ... haec tubulosa sunt & subrotunda, identidem tamen angustantur, & perpetuo patent, nullumque, ut observare potui, effundunt humorem: Argentea lamina L, 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 sheetsL, 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 filamentsM, which being extended in length straddle – for greater strength and hardness – lines of transverse cellsN, so that it is constructed like a mat.)
^Hales explained that although capillary action might help raise water within the xylem, transpiration caused water to actually move through the xylem.From (Hales, 1727), p. 100: "And by the same [capillary] principle it is, that we see in the preceding Experiments plants imbibe moisture so vigorously up their fine capillary vessels; which moisture, as it is carried off in perspiration [i.e., transpiration], (by the action of warmth), thereby gives the sap vessels liberty to be almost continually attracting fresh supplies, which they couldnot do, if they were fully saturate with moisture: For without perspiration the sap must necessarily stagnate, not withstanding the sap vessels are so curiously adapted by their exceeding fineness, to raise the sap to great heights, in reciprocal proportion to their very minute diameters."
^Nägeli, Carl (1858)."Das Wachstum des Stammes und der Wurzel bei den Gefäßpflanzen und die Anordnung der Gefäßstränge im Stengel" [The growth of the stem and of the root among vascular plants and the arrangement of the vascular strands in the stalk].Beiträge zur Wissenschaftlichen Botanik (Contributions to Scientific Botany) (in German).1:1–156. From p. 9:"Ich will die beiden welche von dem Cambium nach aussen und nach innen gebildet werden, Phloëm und Xylem nennen." (I will call the two parts of the permanent tissue, which are formed by the cambium outwardly and inwardly, "phloëm" and "xylem".)
^Wang, Z.; Chang, C.-C.; Hong, S.-J.; Sheng, Y.-J.; Tsao, H.-K. (2012). "Capillary Rise in a Microchannel of Arbitrary Shape and Wettability: Hysteresis Loop".Langmuir.28 (49):16917–16926.doi:10.1021/la3036242.PMID23171321.
^Askenasy, E. (1895)."Ueber das Saftsteigen" [On the ascent of sap].Botanisches Centralblatt (in German).62:237–238.
^Askenasy, E. (1895)."Ueber das Saftsteigen" [On the ascent of sap].Verhandlungen des Naturhistorisch-medizinischen Vereins zu Heidelberg (Proceedings of the Natural History-Medical Society at Heidelberg). 2nd series (in German).5:325–345.
^Carlquist, S.; E.L. Schneider (2002). "The tracheid–vessel element transition in angiosperms involves multiple independent features: cladistic consequences".American Journal of Botany.89 (2):185–195.doi:10.3732/ajb.89.2.185.PMID21669726.
^Cronquist, A. (August 1988).The Evolution and Classification of Flowering Plants. New York, New York: New York Botanical Garden Press.ISBN978-0-89327-332-3.
^abcdefghNiklas, K. J. (1985). "The Evolution of Tracheid Diameter in Early Vascular Plants and Its Implications on the Hydraulic Conductance of the Primary Xylem Strand".Evolution.39 (5):1110–1122.doi:10.2307/2408738.JSTOR2408738.PMID28561493.
^Taylor, T.N.; Taylor, E.L.; Krings, M. (2009).Paleobotany, the Biology and Evolution of Fossil Plants (2nd ed.). Amsterdam; Boston: Academic Press. pp. 207ff., 212ff.ISBN978-0-12-373972-8.
^White, A. Toby; Kazlev, M. Alan."Glossary".palaeos.com. Archived fromthe original on December 20, 2010.
Cesalpino, Andrea (1583).De Plantis libri XVI [On Plants, in 16 books] (in Latin). Florence, Italy: Giorgio Marescotti. p. 4. From p. 4:"An quædam sicca secundum naturam humorem trahunt? ut lintea, spongiæ, pulveres: ... " (Or [as] dry things attract [i.e., absorb] according to the liquid's nature? [such] as linen, sponges, powders: ... )
Lazenby, Elizabeth Mary (1995) "TheHistoria Plantarum Generalis of John Ray: Book I – a translation and commentary.", doctoral thesis, University of Newcastle upon Tyne, England, UK, vol. 1, p. 160. Available at:University of Newcastle upon Tyne, UK.Archived 2018-08-14 at theWayback Machine
^Grew, Nehemiah (1682).The Anatomy of Plants ... London, England: W. Rawlins. pp. 124–125. From pp. 124–125: "For the great part of the year, it [i.e., the sap] riseth in theBarque [i.e., bark], sc. in the innerMargin adjacent to theWood, and in thespring, in or through theWood it self, and there only."
(Grew, 1682), p. 126. Grew recognized the limits of capillary action (from p. 126): " ... smallGlass-Pipes [i.e., capillary tubes] immersed in Water, will give it [i.e., the water] an ascent for some inches; yet there is a certainperiod, according to thebore of thePipe, beyond which it will not rise." Grew proposed the following mechanism for the ascent of sap in plants (from p. 126): "But theBladders [i.e., parenchymal cells] DP, which surround it [i.e., the column of tracheids], being swelled up and turgid withSap, do hereby press upon it; and so not only a little contract its bore, but also transfuse or strain somePortion of theirSap thereinto: by both which means, theSap will be forced to rise higher therein."
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 Press. p. 58.
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]].
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.Bibcode:2001PCEnv..24..549W.doi:10.1046/j.1365-3040.2001.00697.x.S2CID5039439. is the main source used for the paragraph on recent research.
N. Michele Holbrook; Michael J. Burns; Christopher B. Field (November 1995). "Negative Xylem Pressures in Plants: A Test of the Balancing Pressure Technique".Science.270 (5239):1193–4.Bibcode:1995Sci...270.1193H.doi:10.1126/science.270.5239.1193.S2CID97217181. is the first published independent test showing the Scholander bomb actually does measure the tension in the xylem.
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.S2CID31357329. is the second published independent test showing the Scholander bomb actually does measure the tension in the xylem.
Kenrick, Paul; Crane, Peter R. (1997).The Origin and Early Diversification of Land Plants: A Cladistic Study. Washington, D. C.: Smithsonian Institution Press.ISBN978-1-56098-730-7.
Muhammad, A. F.; R. Sattler (1982). "Vessel Structure ofGnetum and the Origin of Angiosperms".American Journal of Botany.69 (6):1004–21.doi:10.2307/2442898.JSTOR2442898.
Melvin T. Tyree; Martin H. Zimmermann (2003).Xylem Structure and the Ascent of Sap (2nd ed.). Springer.ISBN978-3-540-43354-5. recent update of the classic book on xylem transport by the late Martin Zimmermann
