The diameter of these vessel members varies considerably, but a ballpark figure is one fifth of a millimetre. You
can clearly see the slit-like perforations in the so-called perforated end-plates, which are the end walls of the
cell. These plates may disappear completely, leaving a completely open cylinder. The pores in the sides
connect to neighbouring cells. Vessel members may be as wide as half a millimetre (500 micrometres) in
Now imagine stacking several vessel members end-to-end as shown below:
Now we have a vessel! Such vessel carry water up the stem of a plant,
or along a branch, or along a leafstalk, etc. Non-woody plants also
have xylem (but not wood!) - they have small groups of vessels
arranged in a circle around the outer part of the stem. You might think
that large trees would have wide vessels and tracheids, so that water
filters fast through the tree, but actually its the opposite. Trees have
narrow vessels and tracheids so that water filters slowly up the tree.
This indeed is less efficient at carrying water, but wider vessels are
more likely to cavitate.
In a woody plant, the trunk is almost entirely composed of vessels
and tracheids. Tracheids differ from vessels in being much narrower
and they have simple pores rather than perforated end-plates
connecting them together. If you imagine sucking through a long straw,
then the narrower the straw the harder it is to suck the water up. This
is why the narrower vessels and tracheids conduct water at lower
velocities, indeed they are less efficient at carrying water, but wider
vessels cavitate more easily.
Cavitation is the process whereby a water pipe becomes blocked with
air. It happens more easily in the cold and more easily in larger
vessels. It also happens more easily if water is sucked rather than
blown along a pipe. When you think about how water filters through a
tree, you might realize that this could be a big issue for trees! Since
trees suck their water up, they never contain vessels wider than about
half a millimetre at most (xylem vessels are very thin straws!). If the
vessels were wider then they would cavitate. Conifers, which are better
adapted to cold conditions than deciduous broad-leaved trees, don't
even have these classic vessels, but only have the much narrower
tracheids. This enables them to transport water in the cold without
many air blockages. Vessels or tracheids that cavitate are said to be
Notice that the vessel on the left only contains 5 elements (cells) and
would only be about 2 millimetres long in real life. How are we going to
get all the way to the top of a tree? Several hundred elements stack
end-to-end to form a vessel one metre or more long, but eventually
water leaves the vessel through the side pores and enters another
vessel. Thus, a tree contains thousands of such vessels connected
side-by-side and end-to-end. Each vessel may be as long as 10 m in
vines and ring-porous trees. Tracheids extend for several millimetres.
Wood is made up of a tissue called xylem. Xylem has two main functions - the conduction of water and ions
(xylem sap) from the roots to the leaves, and mechanical support. Xylem contains several cell types,
including parenchyma, sclerenchyma and water conducting cells. The water conducting cells are of two
types: 1) narrow tracheids and 2) wider vessel members that form vessels. Xylem vessels develop from
special parenchyma cells that are modified in several ways: 1) They are elongated cells, 2) Another
secondary wall is deposited inside of the primary cellulose cell wall, this secondary wall is made of a
substance called lignin, 3) The cells are stacked end-to-end in a cylinder, and large perforations through
the joining end walls connect the cells together, and 4) The cytoplasm and cell membrane dies away, so that
all that is left is a hollow box.
Below we see a model of a vessel member (click thumbnails to enlarge), seen whole on the left, with a piece
cut-away to show its hollow nature in the middle, and end-on on the right:
Xylem tissue also contains parenchyma in the form of rays (click here to learn more about the overall structure
of wood and tree trunks, including rays). The rows of pores on the sides of the vessel on the left join the vessel
to neighbouring parenchyma ray cells, whilst the long column of pores down the right-hand side, connects to a
Each of these pores that join two neighbouring vessel together and allow them to exchange sap, is protected by
an ingenious and minute valve. These valves automatically close if air enters one of the vessels, for example if
the vessel is damaged or cavitates in cold weather. This prevents air seeping into neighbouring vessels and
blocking them too.
To summarise: xylem conducts water from the roots to other parts of the plant, in both woody and non-woody
plants, but in woody plants it forms the bulk of the stem and is the wood.
Xylem has other functions apart from conducting water from the roots to other parts of the plant, for one it must
support the weight of the tree. Although the vessels and tracheids support much of the weight, they are mainly
for water conduction, so xylem also contains what are called sclerenchyma fibres. Sclerenchyma is made up of
cells that have very thick lignified walls. Sclerenchyma fibres are sclerenchyma cells that are narrow and
elongated and resemble xylem tracheids, except that they often retain their living cytoplasm. These cells have a
minor role in water transport and are there mainly to give further mechanical strength to the wood and they may
also store certain materials that the plant may need later. The parenchyma rays, found in wood, transport
materials across the tree trunk, rather than along it, and dump certain materials into the heartwood, giving it its
different colour to the outer sapwood.
The central heartwood of a tree trunk or branch is easily seen as it is often a different colour to the outer
sapwood. Only the sapwood conducts water up the plant. The inner heartwood consists of old vessels that
have long since cavitated and then been filled by materials (transported by the parenchyma rays from the
phloem and sapwood into the heartwood) which help prevent infection. In old hollow trees, the heartwood has
rotted away, but the tree lives on quite normally so long as it has new sapwood.
If you haven't already done so, then click here to see how the xylem vessels and parenchyma rays fit together
to make up the wood in the trunk of the tree.
Structure of the valves
The pictures below show close-up views of one of the pores connecting two neighbouring tracheids. These are
similar in size to the pores shown on the vessels in the pictures above. The pores connecting adjacent vessels
tend to be simple pores covered by a porous membrane, but those connecting neighbouring tracheids,
especially in conifers and trees adapted to cold climates, tend to have the more complicated valve arrangement
shown below. These are called bordered pits.
Coming soon: putting it all together - how do we build a tree exactly? Why do trees branch the way that they do?
Above: each bordered pit consists of a dome (which is only about 0.01 millimetres in diameter) with a central
pore. The pore penetrates the tracheid wall and aligns perfectly with a pore in the wall of the neighbouring
tracheid. This allows fluid (sap) to move from tracheid to tracheid. Right: removing the top of the dome, we see a
hollow chamber with a central lens-like mass (the torus) suspended by radial fibres in the centre of the chamber.
This is shown in side-view below. Click the images to enlarge.
Air may enter a vessel or tracheid either if the plant is damaged or if cavitation occurs, blocking the vessel.
When a vessel cavitates it is important not to let the air enter neighbouring vessels and blocking them too!
Since cavitation occurs more frequently in cold conditions, that explains why conifers and other cold-adapted
plants have more of these elaborate valves to protect their sap-conducting xylem. Also, since narrower vessels,
like tracheids, are less likely to cavitate in the cold than wider vessels, this explains why tracheids are more
likely to have these valves than the wider vessels, since the tracheids function best in cold conditions. Conifers
have only narrow tracheids and no wider vessels and the pores connecting their tracheids together are
well-equipped with such valves, adapting conifer xylem for cold conditions.
Air also enters the old heartwood as it dries out and stops conducting sap. In heartwood, the valves are tightly
closed as air has entered the xylem here.
Above: A longitudinal section through conifer wood (Pinus sylvestris, the Scots Pine). The xylem of conifers
does not contain vessels made-up of vessel members. Instead the xylem cells, called tracheids, lack
perforated end-plates and instead the sap flows from cell to cell via pits. The bordered pits are clearly seen
along the tracheids. The parenchyma cells making up the rays are stained blue-green.
Above, right: xylem vessels of Tilia seen in cross-section. The vessels vary considerably in diameter with the
largest vessels about 0.25 mm in diameter. The rows of parenchyma cells (stained green) are the rays.
Click here to learn about the structure of green
herbaceous (primary) stems.
How do the large vessels of broad-leaved trees protect themselves from cavitation?
Conifers, as we have seen, rely on tracheids with a complex valve arrangement so that if air enters one tracheid
conduit then that conduit may be closed off to prevent air spreading throughout the system. In addition, resin
secreted by the tree can aid further blockage of tracheids. Broad-leaved or deciduous trees do often have
tracheids but tend to rely much more on wider xylem vessels with more porous end-plates. This arrangement
allows rapid transport of xylem in the Spring when growth resumes. As the cooler autumn approaches the
diameter of new vessels tends to reduce. Water columns are more brittle in the cold and more likely to 'snap'
and cavitate. Narrower water columns are less brittle. This gradation from wide vessels in early Spring to
smaller vessels in late Summer/Autumn gives the wood a characteristic appearance in the form of annual
The open porous end-plates of xylem elements require a different plugging mechanism should a vessel
cavitate. In these trees there are more living parenchyma cells in the sapwood and these are able to block
adjacent vessels that cavitate by either secreting gum into the vessel or by balloon-like outgrowths called
tyloses which burst into the neighbouring vessel to seal it. The non-conducting heartwood contains vessels
that are typically blocked by gums and/or tyloses. One exception is the red oak, Quercus rubra, which somehow
manages without sealing off its old vessels. Indeed it is possible to blow air through a piece of red oak
heartwood, which is completely porous.
The heartwood of trees consists of cells which are either dead or dying. Parenchyma rays may persist for a
while as they transport waste materials, gums and toxins into the heartwood. These include toxins such as
polyphenols which help to inhibit the growth of micro-organisms. The various chemicals deposited in
heartwood deter wood-eating fungi and insects. Think of the camphor tree, Cinnamomum camphora, which
produces insect-repelling camphor oil in its leaves and wood. The woods of many conifers and broad-leaved
trees are aromatic because of the oils, gums and resins deposited in them. Outside the heartwood is the
conducting wood or sapwood.
Most conifers, such as pines, produce resin which is stored in resin canals in both the bark and wood. Other
conifers may only have resin canals in the bark but can produce traumatic resin canals in response to injury.
(Yew trees do not produce resin but somehow their wood is extremely durable and rot-resistant). The resin
inside the canals is held under slight positive pressure and so it seeps out when the tree is damaged, coating
wounds in insect and microbe-repellent resin. If infection spreads to the wood then it is naturally contained by
three barriers. Tyloses and gums bar spread up and down the tree by blocking xylem conduits. Annual growth
rings act as a barrier to infection spreading inwards (radially) and the parenchyma rays act as living barriers to
infection spreading sideways. In addition, the cambium (the layer of cell-generating or cambial cells between the
outer phloem (inner bark layer) and the inner wood can receive signals from other cells and produces a barrier
zone of tissue laden with defensive materials, such as resins, arranged in an arc or ring around the trunk, in an
attempt to contain the infection. Should this barrier be breached then another barrier is erected behind it.
Even though the heartwood is laden with anti-microbials to slow the rot of this 'dead' tissue, the tree is designed
to especially protect the living sapwood. Some fungi will only attack the dead heartwood and so pose little
problem, whilst others may also attack the living wood. For many trees it is only a matter of time before the
heartwood decays, leaving a hollow trunk. If a region of living sapwood died around the site of the wound then
the hollow bough is open for all to see, otherwise you never know what an old tree contains until it is cut.
Hollowness is much less of a problem for old trees than was once believed. As long as enough sapwood
remains intact then the tree will be structurally sound. Like the hollow bones of a typical mammal, the best use
of supporting material is to invest the same volume of material in a hollow cylinder rather than a solid cylinder of
narrower diameter. For supporting vertical weight it makes no difference, but trees (and bones) are not simply
under compression due to their own weight (self-loading) they are also subject to bending and twisting forces. It
is harder to bend a hollow rod of the same length and mass as it is to bend a narrower solid rod. There is a
limit, of course, because if the hollow cylinder is too wide and the walls too thin then it becomes subject to
buckling. Nevertheless, the central region of such a rod or column contributes very little to structural strength. A
hollow structure also has less weight of its own to support.
Old trees with wide and hollow trunks are statistically more resilient to windfall then smaller saplings with
narrower and solid stems. Of course, if the rot has infiltrated the outer sapwood then the hollow shell and
branches may be prone to breaking, so old trees often have their branches pruned for safety (though in many
cases the pruning is probably over-zealous). On the whole, being hollow when a tree reaches a certain girth is
not a bad thing - the tree has less weight to carry whilst maintaining its structural strength. The wind may also
simply pass through a hollow tree. Additionally, trees will reabsorb and recycle nutrients from the decomposing
heartwood and they have been known to grow down roots inside the trunk for this purpose.
Growth Rings, Knots and Other Features - Hydraulic Architecture
Annual growth rings are a familiar feature in cut logs. Both conifers (softwoods) and broad-leaved trees
(hardwoods) produce annual growth rings. These result from the fact that xylem conduits (tracheids or vessels)
produced in the sapwood in early spring and summer are wider (and thinner walled) when the tree is growing
most rapidly and so needs a lot of water, but later in the year they produce narrower and thicker-walled vessels,
as narrower vessels are less likely to cavitate and the emphasis seems to be on producing stronger wood. Each
ring is thus made-up of early wood, with larger diameter conduits, and late wood with narrower conduits.
Hardwoods may be ring-porous, such as ash, elm and oak, which means that the early wood in each ring
contains complete rings of very large diameter vessels around the trunk. Others may be diffuse-porous, such
as birch, maple, beech, poplar, lime and walnut, which means that the gradient from large diameter early
vessels to smaller diameter late vessels across a ring is less dramatic; instead the larger vessels (which are
smaller than the largest vessels in ring-porous trees) are more evenly distributed across the ring. In diffuse
porous trees the rings are still visible as there is a narrow band of small diameter vessels in the latest wood.
Ring-porous trees are much less common in the tropics, where most hardwood trees are diffuse-porous.
Temperate ring-porous trees may have only 1-4 or so rings of sapwood. At the other extreme, tropical diffuse-
porous trees like ebony may have little or no heartwood. Temperate diffuse-porous trees and conifers may
have 100 or more rings of sapwood. Many tropical trees also produce no growth rings, or they may periodically
shed leaves and produce growth rings that are not annual.
The very large early wood vessels of ring-porous trees, such as oak, conduct water very rapidly, but cavitate
easily, whether in the cold or when the suction (negative) pressure generated by transpiration becomes too
great, such as in the midday sun. However, vessels in these trees, which cavitate in the heat of day, can be
actively refilled at night (by an unknown mechanism). Even with many of their vessels cavitated, oaks can
transport as much water per leaf as diffuse-porous trees like maple. Maple has much narrower vessels which do
not cavitate easily, but which conduct less water. The result is that both trees can deliver a similar amount of
water to their leaves - maple by using narrower vessels that conduct more slowly but cavitate less often, and
oak with many cavitated vessels, but with enough large vessels working to rapidly deliver water. Narrower
vessels can tolerate higher transpiration pressures, such as may occur in drought, and the maple has a
shallower root system than oak. Oak, on the other hand, with its vessels less resistant to water shortage, has a
deeper root system and hence a more uniform supply of water. (H. Taneda and J.S. Sperry, 2008, A case-study
of water transport in co-occurring ring- versus diffuse-porous trees: contrasts in water-status, conducting
capacity, cavitation and vessel refilling. Tree Physiology 28, 1641–1651).
Knots can be seen as circular features inside cut wood. They may weaken wood used in construction, but they
are essential to the tree. They represent either xylem which is moving sideways, or obliquely at an angle, to the
main vertical grain in the trunk as it courses towards a branch which it supplies, or a dormant bud enclosed by
wood. If the branch was dead then living wood will not bond tightly to it and the know will be loose and easily
pushed out from a plank of wood. Many trees have epicormic buds on the surface of the trunk. These are
dormant but grow enough each year to stay at the surface of the trunk as it widens. They have their own xylem
supply which again cuts across the main vertical grain. Damage to the main trunk may activate these buds to
produce replacement shoots. Masses of buds may distort the growth of the trunk, forming a bur. Epicormic
buds may also arise de novo, usually in response to damage, and these are called adventitious epicormic
buds (an adventitious plant part is any which grows in an 'unexpected' position). For example, new buds may
develop from a cut tree stump.
Circular vessels are a curious feature found in both hardwoods and softwoods. These may be found around
the base of a branch in an oak. They consist of concentric circles of xylem vessels or tracheids which appear to
be conducting xylem sap nowhere and are probably non-functional as conducting elements (some xylem sap
can move slowly sideways from one vessel to another, but I am not sure if this occurs here). Several of these
'whorls' may be situated at intervals in a ring around the base of a branch. These circular vessels clearly serve
to reduce the flow of sap into a branch. The conductivity of xylem through trunk/branch junctions is often less
than half that through the stem. This ensures that the lower branches, which are nearer to the roots, do not
steal more than their fair share of xylem flow, leaving enough for the branches higher up the tree.
Additionally, these circular features increase mechanical strength. When a branch is overloaded by wind and/or
snow it may fail. If a branch fails at its base, then there is a strong tendency for an elliptical scar to form as
wood and bark peel away below the branch as it tears away from the tree. If you think of the grain of the tree,
this makes sense as the shearing / tearing force pulls along the grain, detaching fibres below the branch/trunk
junction along with the branch. However, where the grain is interrupted by the circular vessels at the
branch/trunk junction, then the tearing tends to stop - the circular vessels act to stop cracks spreading.
The xylem conduits define the grain of the wood, which is usually more-or-less vertical and parallel to the trunk
axis, except near the roots where the xylem conduits spread out more-or-less horizontally as they connect to the
roots. However, in some trees the xylem conduits may spiral either clockwise or anticlockwise around the stem.
This strengthens wood as it is harder to split the wood along the grain than it would be if the grain was straight.
It also appears to ensure more even distribution of xylem flow around the stem. There are also slower
conducting lateral connections between xylem vessels, so some sap is exchanged between adjacent xylem
vessels, causing water originating in one vessel to fan out around a ring by a degree or so.
The rays of parenchyma cells have an important function in food storage, storing starch and/or oils, especially
those towards the base of the trunk and in the wood of the roots. Trees in colder climates tend to store oils
preferentially to starch. The rays also transport manufactured chemicals towards the centre of the trunk where
they are deposited, forming heartwood in which the ray cells gradually die as they accumulate these toxic 'waste
materials' which are often anti-fungal and may serve to reduce the rate of heartwood rot. These materials
usually give the heartwood a darker colour. Some trees form no heartwood at all, such as alder (Alnus
glutinosa), aspen (Populus tremuloides) and many maples.
Left: a branch stump on a felled beech
tree (Fagus sylvatica) stripped of bark
the grain of the wood is clearly visible.
Spiral vessels can be seen at the
Read more about the vascular
architecture of plants.
Click on the image to enlarge.
Above left: a cut-away side-view of the pore valve structure. A
pore penetrates each dome and a central membrane of fibres
separates the contents from the tracheid on the left from those
of the tracheid on the right. Fluid can flow through the pore on
the left side, into the chamber, across the porous membrane of
fibres, into the chamber of the neighbouring tracheid on the
right side and then through the other pore and into the right
hand tracheid, and likewise fluid can travel in the opposite
direction from the right hand tracheid into the left hand
tracheid. Centre and right: if air enters one of the tracheids,
causing the pressure to drop, then the torus gets forced to one
side or the other and seals the pore. This closing of the valve
stops air spreading from one tracheid to the next. Rightmost: a
labelled diagram showing the anatomical features, serving as a
key (click to enlarge).
Above: a model of a slice through a ring-porous tree, such as a deciduous oak. In ring-porous trees the
early wood formed in spring contains wider vessels than the wood formed later in the year. The later wood is
therefore denser and slightly darker and results in an obvious annual growth ring. In diffuse-porous trees
there is little or no difference in vessel width between the late and early wood, as in horse chestnut, Aesculus
hippocastanum. Some hardwoods are intermediate, for example the earlier vessels may be wider but
dispersed among narrower later vessels. Conifers lack this type of vessel and have narrower tracheids
instead and are neither ring-porous nor diffuse-porous.
The phloem is a narrow layer (a few mm thick) beneath the outer bark. This consists of phloem tubes which
conduct sugary sap from sugar-sources, such as photosynthesising leaves, to sugar-sinks, such as ripening
fruit. Beneath this is the cambium, a thin layer of one or a few cells which generate new phloem and wood as
the tree grows in girth. Beneath this is the wood, or xylem, which makes up the bulk of the trunk. In most
trees only the outermost layer of wood, the sapwood, is alive and conducting water rich in mineral nutrients
from the roots to the rest of the tree in the transpiration stream. The xylem may also sometimes carry sugary
sap, particularly in early spring when sugars are being transported from storage in the roots. In temperate
ring-porous trees such as oaks, the sapwood consists only of the outermost 1-4 rings of wood, the rest of the
wood being heartwood which is non-functional as transport tissue and usually a darker colour when fresh
(in some trees the heartwood and sapwood are the same colour). Fungal root can also colour wood. In
temperate diffuse-porous trees and conifers, there may be more than 100 sapwood rings, whilst in ebony
(diospyros ebenum) a tropical diffuse-porous tree, there may be no or very little heartwood 9making the
heartwood especially valuable). At the very centre of the heartwood a small cylinder of pith may remain as a
remnant of the original young green stem before it thickened by secondary growth. Xylem also contains
radial rays or sheets of parenchyma cells. When wood dries, the cells tend to shrink in width rather than
length, so the length of the log does not change much. The rays prevent radial shrinkage and so the wood
cells shrink more tangentially (around the circumference of the trunk) resulting in radial cracks.