The outermost layer is the bark. This is protects the tree against mechanical damage, infection and fire. To
assist in these functions, the bark consists of tough outer layers, and
corky layers that absorb impacts, and it
is also rich in tannins, which give it its dark colour and bitter taste. (The barks of many trees are poisonous and
should not be eaten, they can have a vomiting or laxative effect at the very least!) This makes it unattractive to
many predators. In particular the tree wants to deter predators from reaching the valuable fine fibrous layer
which is the innermost layer of bark and is shown as the dark-brown fibrous layer between the outer bark and
the lighter coloured wood. This fibrous layer is the phloem.

The
phloem consists of conductive vessels that carry sugary sap from the leaves, where sugars are
made (or from organs that store starch such as roots) to the rest of the plant body and tough supporting fibres
(called sclerenchyma). When bark is pulled from a tree, the phloem often gets pulled off with it (indeed the
phloem is usually considered to be the innermost layer of bark). This is why stripping a complete circle of bark
from around the tree will kill it, as the phloem can no longer send food to the roots and the roots die. If just a
small strip of phloem remains, however, at least part of the tree can survive.

Beneath the phloem are one or more
rings of wood (xylem tissue) - one ring per year of growth. The tree or
branch shown here has four growth rings and so is about four years old. The wood consist mainly of vessels, a
few rings of which a few are shown in the diagram as pale-coloured cylinders raised above the cut surface, to
enhance their visibility. (In reality the wood is jam-packed with vessels which are polygonal rather than circular
in cross-section). These
conduct water and minerals from the roots to the crown. Notice that the growth
rings occur because the conducting vessels alternate from large to smaller vessels as you move across a ring.
It is the transition between wide and narrow vessels that creates the visible ring. This is characteristic of
deciduous trees, in which growth is rapid in the spring and so the tree produces large vessels to carry plenty of
water, and then growth slows in autumn and ceases in winter. In the cold parts of the year, the tree is growing
only slowly, if at all, and so it needs less water, indeed if it has lost its leaves then it can not grow and will not
lose much water and so it needs very little water. Smaller vessels are also less likely to cavitate (block with air)
in cold weather, so smaller vessels are an advantage in colder weather, whereas in warmer weather larger
vessels are better.
Water moves up the trunk in the vessels at about one millimetre per second.

The dark heartwood in the centre of the tree is not conducting. The old vessels have stopped working and so
have been plugged with dark materials to stop them being used by infectious agents, such as fungi, to invade
the tree.

Finally notice the dark-coloured radial spokes, these are called
rays and are plates of living cells that carry
materials to and from the outer layers and inner layers of the trunk (that is radially as oppose to the phloem
and xylem vessels which carry stuff vertically up and down the trunk). For example, the rays carry the
dark-coloured materials into the heartwood. The rays also store food. Almost all of the tissue between the rays
consists of xylem (wood) vessels.
In-between the wood and the phloem is a very thin cylinder of cells, called the cambium. These cells divide (split
into two) and produce new wood cells on the inside and new phloem cells on the outside. The phloem cells are
much smaller than the wood cells, and so the phloem is always a thin layer surrounding the wood and forming
the innermost layer of bark. Another cylinder of cambium cells occurs inside the bark and adds new cells to the
bark.

In its first months of life, a tree has no wood, it starts as a small green fleshy shoot. The remains of this early
tissue are lost (they tend to get crushed) in the centre of the woody trunk.

Why do trunks taper?

Firstly, trunks taper because the base is older and so has more growth rings which are added as cones as
explained above. However, a tapering trunk is also part of the tree's growth strategy. Firstly, this reduces the
weight of the trunk and reduces what engineers call its self-loading, that is the stress its own weight puts it
under, and the greatest weight is borne by the base which therefore needs to be wider. Tapering also
minimises the bending stress that a trunk is subject to in high winds. Tall narrow columns fail by buckling, whilst
thick short columns crush under their own weight, but a cone is a good compromise, allowing the tree to be tall
without getting too heavy.

Thigmomorphogenesis

'Thigmo' means 'touch', whilst 'morphogenesis' means the 'creation of shape.' Thigmomorphogenesis is the
phenomenon whereby a plant adapts its shape or form according to mechanical stimuli, such as touch or
bending by wind. Buffeting a sapling, either by frequently shaking it, or by placing it in a windy place, causes it
to grow a shorter but thicker trunk. Thicker trunks are stiffer and resist the wind better. In high winds it is the
spindly young trees that topple more often than the old thickset trees. Trees that grow in sheltered forests,
where their neighbours help to shelter them from winds, grow thinner but taller, which is good since they must
grow tall to reach the light ahead of their rivals, otherwise they will get beaten to it and become shaded by taller
trees. When growing in isolated windy places, however, the tree responds to being bent about by the wind, by
growing a thicker and shorter trunk. Most trees have a
safety factor of about 4. This means that they can
withstand some four times the usual loading. This is a large safety margin by engineering standards, but it
helps ensure the survival of the tree in high winds, especially if it has the extra weight of snow upon its
branches.

Why do branches taper?

Branches taper toward their tips. This gives them better resistance against bending at twisting, especially at the
branch base, where these forces are greatest. When a branch fails in high winds, it tends to shear away at the
base, often leaving an elongated scar where it has stripped some bark from the trunk just above and beneath
its attachment point. Willow branches, however, are designed to shear far from their base, which scatters twigs
and small branches into nearby water - these may take root further downstream and grow into new trees!
Branch collars are specially constructed  thickenings of wood at the branch base, with special mechanical
properties that give the wood a
polylaminate construction that helps to stop cracks from enlarging. A
'polylaminate' is a many-layered material and the many layers enable the material to bend more without
cracking apart.

Xylem Conduits

The water-carrying tubes that make up the bulk of wood tissue consist of two possible types: tracheids and
xylem vessels.
Tracheids are narrower and made-up of elongated cells connected to one-another by pit pores
which allow the sap to flow from tracheid cell to tracheid cell.
Xylem vessels are wider, and made up of shorter
cells, called
vessel elements, more regularly stacked on top of one-another to form a distinct vessel. The
end-walls between cells in the vessel are perforated by porous-plates, called
perforation plates which contain
large, open often slit-like pores to allow sap to pass from cell to cell. Individual vessels may span a few
centimetres to a metre or so of trunk height. To scale the height of the tree, the sap must flow from one xylem
vessel to another.

See also: the detailed
structure of wood xylem and vascular architecture.

Rays

Xylem conduits (vessels and tracheids) are often described as 'dead' cells because at maturity the living
protoplast (cytoplasm, cell nucleus and cell membrane) disappears, leaving a specially modified cell wall as the
functional conduit. These conduits form a system of tubes running more-or-less vertically up the trunk. Wood
does contain living cells, however,in the form of radial sheets of
parenchyma cells, which look like spokes in a
cross-section of a trunk or branch. An individual ray consists of a vertical plate of cells, which may be only one
cell wide (
uniseriate rays) or they may be several cells wide (multiseriate rays). In conifers most rays are
uniseriate (some are two cells wide -
biseriate rays) though resin canals may run along their length, making
them appear multiseriate. Some tracheids may also run along a ray (radial tracehids). An individual ray does
not span the entire height of the trunk, indeed they span only a few millimetres at most. Conifer rays are 1 to 50
cells tall and account for 6-10% of wood volume. In hardwoods, rays account for 6.5 to 30% of the wood volume
and may be several millimetres wide. rays also extend into the phloem (inner layer of the bark). Some
hardwoods, e.g. willow and poplars also have uniseriate rays. The rays of oak are multiseriate.
Tree Growth: Wood, Tree Trunks and Branches
Above is a diagrammatic slice through a deciduous hardwood tree trunk or branch, for example oak (there are
several types of hardwood structure and they all differ in colour and grain size, but this serves as a general
illustration). Click on the diagram for a larger version.
Link to the cellular structure of wood...
Link to the structure of bark...
Left: a diagrammatic representation of tree growth
- each year a cone of wood is added on top of the
old wood - with the tree increasing in girth.
Plant stems                Stem growth                 Woody stems               Wood
Trunk structure
Ray parenchyma cells function in the storage of fuels (energy reserves) such as starch or fats and also
transport materials slowly across the trunk. In particular, rays transport toxic antimicrobial and waste materials
to the
heartwood. The heartwood at the centre of the trunk is  truly dead, since the xylem is no longer
functional in conducting sap and the ray cells are also either dead or dying.

Burs and Buttresses

The exposed root bases of some trees flare out at the base, forming flanges called buttresses or buttress
roots
. These buttresses serve to anchor trees that grow on shallow soils and are found for example on Fagus
(beech) and more dramatically on fig trees (
Ficus). These roots resist pulling forces that would otherwise tip the
shallowly rooted tree.

Burs are bulbous outgrowths on the side of the trunk. They may consist of clumps of dormant epicormic
buds
. These buds may sprout if the main shoot system gets damaged, such as when a tree is cut down to a
stump, and so produce replacement shoots. Epicormic buds may be dispersed more-or-less evenly along the
trunk, as in oaks, or they may cluster towards the base of the tree, as in birches.

Other burs may actually be
galls, which may contain buds. These are pathological growths triggered by
infection. The abnormal tissue growth provides a home and nourishment for the infectious agent, e.g. crown
gall is caused by the bacterium
Agrobacterium which alters the normal growth pattern of the plant. These galls
often reach unnatural looking dimensions.

Scars

Another distinctive feature of any tree of significant age, are scars. Scars may form where lightning or an
impact damages part of the trunk, or when a branch is lost, either by natural shedding, breakage or removal by
a tree surgeon or wood-cutter. If left open these wounds could become infected by harmful pathogens. They
are also areas of weakness and the tree forms thick callus tissue which bulges as it slowly grows over the
wound.
oak scar
If you examine scars on trees then you will notice that the callous growth is greater on the sides than the top
and bottom, such that a circular wound may eventually close to form a vertical slit which can be closed
completely if the lips of the callous meet. If the scar is very large then it may be unable to seal the wound
completely, as the large wound on the oak above. The reason why callus growth is more rapid at the sides is
because the sides of the scar are structurally the weakest points. When the trunk bends in the wind, the
stresses become focused at the sides of open wounds, which might begin to crack and widen. Thus the tree
focuses resources on rapidly reinforcing these weak points.

Xylem and wood

Transport in plants
tree cones
tree trunk slice
How do trees grow?

Trees add new wood to the outside of the existing wood, forming a new growth ring over one year. The tree
gets taller with each layer of wood added, since the tapering trunk of the tree makes it kind of cone-shaped and
every new layer of wood is like adding a new cone on top of the existing cones (rather like a series of Russian
dolls) as shown below:
Cut surface of an oak log