Xylem is one of the two types of transport tissue in vascular plants,
phloem being the other. The basic function of xylem is to transport
water from roots to shoots and leaves, but it also transports some
nutrients. The word "xylem" is derived from the Greek word
ξύλον (xylon), meaning "wood"; the best-known xylem tissue is
wood, though it is found throughout the plant. The term was
Nägeli in 1858.
2 Primary and secondary xylem
3 Main function – upwards water transport
3.1 Cohesion-tension theory
3.2 Measurement of pressure
5.1 Protoxylem and metaxylem
5.2 Patterns of protoxylem and metaxylem
6 See also
7.1 General references
Cross section of some xylem cells
The most distinctive xylem cells are the long tracheary elements that
Tracheids and vessel elements are distinguished by
their shape; vessel elements are shorter, and are connected together
into long tubes that are called vessels.
Xylem also contains two other cell types: parenchyma and fibers.
Xylem can be found:
in vascular bundles, present in non-woody plants and non-woody parts
of woody plants
in secondary xylem, laid down by a meristem called the vascular
cambium in woody plants
as part of a stelar arrangement not divided into bundles, as in many
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.
Primary and secondary xylem
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.
Secondary xylem is formed during secondary growth from vascular
cambium. Although secondary xylem is also found in members of the
Ginkgophyta and to a lesser extent in
members of the Cycadophyta, the two main groups in which secondary
xylem can be found are:
conifers (Coniferae): there are some six hundred species of conifers.
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.
angiosperms (Angiospermae): there are some quarter of a million to
four hundred thousand species of angiosperms. Within this group
secondary xylem is rare in the monocots. Many non-monocot
angiosperms become trees, and the secondary xylem of these is used and
marketed as hardwood.
Main function – upwards water transport
The xylem 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 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. 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, 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
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.
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 stomata open 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.
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.
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.
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).
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. Despite numerous objections,
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), Askenasy (1895), and Dixon (1914,1924).
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.
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.
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.
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
Measurement of pressure
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. 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[citation
needed] of transpirational pull from above and root pressure from
below, which makes the interpretation of measurements more
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. 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 plants. Several groups of plants later
developed pitted tracheid cells, it seems, through convergent
evolution. In living plants, pitted tracheids do not appear in
development until the maturation of the metaxylem (following the
In most plants, pitted tracheids function as the primary transport
cells. The other type of tracheary element, besides the tracheid, is
the vessel element.
Vessel elements are joined by perforations into
vessels. In vessels, water travels by bulk flow, as in a pipe, rather
than by diffusion through cell membranes. The presence of vessels in
xylem has been considered to be one of the key innovations that led to
the success of the angiosperms. 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. 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.
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.
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.
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. 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. 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.
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. 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. 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. 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. 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.
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) to avoid implosion; this process coincided
with cell death, allowing their innards to be emptied and water to be
passed through them. 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. The early Devonian
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, are an early improvisation to aid the easy flow of
water. Banded tubes, as well as tubes with pitted ornamentation on
their walls, were lignified 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.
Tracheids may have a single evolutionary origin, possibly within the
hornworts, uniting all tracheophytes (but they may have evolved
more than once).
Water transport requires regulation, and dynamic control is provided
by stomata. 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
An endodermis probably evolved during the Silu-Devonian, but the first
fossil evidence for such a structure is Carboniferous. 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. As a result
of their independence from their surroundings, they lost their ability
to survive desiccation – a costly trait to retain.
During the Devonian, maximum xylem diameter increased with time, with
the minimum diameter remaining pretty constant. By the middle
Devonian, the tracheid diameter of some plant lineages
(Zosterophyllophytes) had plateaued. 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.
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 and increased stomatal
density, both of which would increase the demand for water.
While wider tracheids with robust walls make it possible to achieve
higher water transport pressures, this increases the problem of
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[verification needed] 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
Cavitation is hard to avoid, but once it has occurred plants have a
range of mechanisms to contain the damage. Small pits link
adjacent conduits to allow fluid to flow between them, but not
air – although ironically these pits, which prevent the spread
of embolisms, are also a major cause of them. These pitted
surfaces further reduce the flow of water through the xylem by as much
as 30%. Conifers, by the Jurassic, developed an ingenious
improvement, using valve-like structures to isolate cavitated
elements. These torus-margo structures have a blob floating in the
middle of a donut; when one side depressurizes the blob is sucked into
the torus and blocks further flow. Other plants simply accept
cavitation; for instance, oaks grow a ring of wide vessels at the
start of each spring, none of which survive the winter frosts. Maples
use root pressure each spring to force sap upwards from the roots,
squeezing out any air bubbles.
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. Even when
tracheids do take a structural role, they are supported by
Tracheids end with walls, which impose a great deal of resistance on
flow; vessel members have perforated end walls, and are arranged
in series to operate as if they were one continuous vessel. 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.
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. 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.
Vessels first evolved during the dry, low CO2 periods of the late
Permian, in the horsetails, ferns and
and later appeared in the mid Cretaceous in angiosperms and
gnetophytes. Vessels allow the same cross-sectional area of wood
to transport around a hundred times more water than tracheids!
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. Despite
these advantages, tracheid-based wood is a lot lighter, thus cheaper
to make, as vessels need to be much more reinforced to avoid
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 is
important in the study of plant morphology.
Protoxylem and metaxylem
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 which contain thickenings in the form of rings or helices.
Functionally, protoxylem can extend: the cells are able to 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 which are
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.
Patterns of protoxylem and metaxylem
There are four main 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 in a cylinder around it. This pattern was common in
early land plants, such as "rhyniophytes", but is not present in any
living plants.
The other three terms are used where there is more than one strand of
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 closest to the
periphery. The roots of vascular plants are normally considered to
have exarch development.
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 closest to
the periphery. The stems of seed plants typically have endarch
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
Wikimedia Commons has media related to Xylem.
Soil plant atmosphere continuum
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Dermal tissue: Epidermis
Vascular tissue: Phloem
Ground tissue: Parenchyma
Meristematic tissue: Primary