Section through Tilia bark
Above: a section through a stem of a young, one-year old Tilia sapling, showing the outer
bark.
Tilia is the lime tree, not lime of the Citrus variety, but the small-leaved lime, a common
large tree in mixed-oak woodlands, that would have formed whole woods of its own some two
thousand years ago when it was a dominant tree in prehistoric England.
Tilia is sometimes
colloquially called the 'dragon tree'. The outer bark, or
periderm, on the left of the image,
consists of layers of thick-walled
cork (phellem) cells with yellow-brown walls impreganted
with waterproof materials like
suberin and sometimes also lignin, a polymer present in wood.
Beneath this is a layer of flattened living cells called the
phellogen. These cells divide to
produce the brown cork cells on the outside and the
phelloderm cells on the inside (in
addition to sometimes producing new phellogen cells). The phelloderm consists of the
single-layer of small, squarish and thick-walled cells, just inside the flattened phellogen cell
layer, that contain living protoplasts (you can see the nucleus in many of these cells). You
can see which cells have been produced by the phellogen as they are aligned in columns.
The original layer of cells covering of the young green stem is still present as a layer of
modified epidermal cells with the cork cells beneath it. The bark layer would have thickened
greatly as the plant matured. Beneath the phelloderm are several layers of compact roundish
cells, making up the bulk of the image, these are collenchyma cells. Collenchyma have
thickened cellulose cell walls and help support the young sapling stem. Inside the
collenchyma are large cortical parenchyma cells at the right of the image.

Cork used for wine bottle corks usually comes from the cork oak, which produces exceptional
quantities of thin-walled cork cells filled with air.
Rowan bark, lenticels and scars
Left: A labelled version of a section
through the
Tilia outer bark (periderm)
in a first year stem.
Co: collenchyma; CP: cortex
parenchyma; Cu: cuticle; E epidermis;
P: phellem cells; Pg: phellogen; Pd:
phelloderm.

Scale: the diameter of circular the
field-of-view is 1 mm.
lenticel section
Left: a lenticel in the bark of a first year
Tilia stem. lenticels are large pores or
slits in the outer bark where the
epidermis has been disrupted. In this
type of lenticel (found in
Tilia, Fraxinus
(
ash), Quercus (oak) and Sambucus
(elder)) consists of a loose mass of
nonsuberised (non-corky) cells
comprising a filling tissue. At the end of
the season a more compact layer of
suberised cells forms (presumably to
protect against water loss during
winter). The filling tissue has air
channels in-between its cells, which
are continuous with air channels
running throughout much of the plant.
These air spaces allow the living cells
to respire - taking up oxygen from the
air and releasing carbon dioxide.
In trees like Salix (willow), Malus (apple), Magnolia, Pyrus (pear), Populus (poplar, aspen and related trees)
and
Liriodendron (tulip tree) the filling tissue is corky and suberised, though their are air spaces between the
cells, and the tissue becomes more compact with thicker cell-walls at the end of the season, resulting in
definite annual growth layers.

A third type of lenticel is found in
Betula (birch), Fagus (beech), Prunus (plum, cherry and related trees with a
stoned-fruit) and
Robinia (locust trees) in which loose nonsuberised tissue alternates with compact suberised
tissue with the compact tissue forming closing layers and a very definite annual layering.
Above: part of a rowan stem (lying horizontal after part of the parent-tree collapsed). In
addition to the scars where some small branches were pruned (which are in the process of
closing over as wound periderm forms over them) horizontal lenticels are visible. This type of
papery, thin outer bark with horizontal lenticels is also characteristic of birch trees and young
Prunus trees.
The Structure of Tree Bark
Some trees seldom have thick bark, though older parts generally have thicker bark than younger parts,
whereas others produce extremely thick bark that thickens over many years growth. To some extent this
depends on the bark growth rate, but it also depends on how easily the bark sloughs off. Trees like the
London plane (
Platanus x hispanica) have relatively thin bark that sloughs off readily in attractively
vari-coloured layers. These trees are able to tolerate city pollution well and so are often planted in
cities. The regular sloughing-off of pollution-laden bark, with its blocked lenticels, probably contributes
to this tree's pollution tolerance. Oak trees characteristically have thick and deeply-furrowed bark, which
gives the tree considerable protection. Redwoods and
Sequoias have extremely thick reddish and
fibrous bark, up to 2 feet thick or so in places, which is quite spongy in texture. This thick bark gives
these trees the ability to survive the frequent forest fires that are a normal part of their ecosystem's
cycle.

Rhytidome

As a tree grows older, periderms may arise in successively deeper layers within the expanding tree.
These periderms cut-off outer tissues, causing them to die, and layers of dead tissue surround the tree.
These alternating layers of periderm and dead tissues are called
rhytidome. This forms the outer bark
of older stems and roots in trees (in shrubs, these layers usually slough away quickly and tend not to
accumulate to much thickness). The deeper periderms also have lenticels.

The outer bark of monocotyledons is usually very different to the periderm of dicotyledons. In palms, for
example, parenchyma cells at successively deeper layers divide and produce suberised cork cells,
resulting in layers of cork, called storied cork.
beech scar
Left: beech (Fagus sylvatica) bark is of the
'dippled-scaly' type, being fairly smooth and
typically grey. When trees are wounded it is
important that exposed wood can be covered
over by bark before it rots. Rotting wood is
often not a problem, and in old trees the
heartwood rots away sooner or later, but the
fungi which decay dead wood are sometimes
also capable of infecting living wood. The
bark grows over exposed wounds fairly
rapidly. Note how the bark has sealed this
large wound in a beech tree. The wound was
caused by the removal of a large branch.
Characteristic of trees, the bark grows back
faster at the sides that at the top or bottom,
since these are regions prone to splitting
when a tree bends in the wind. The result is a
characteristic vertical fissure where the
growing margins of the bark finally meet to
seal the wound.

Note also the thickening of wood in the trunk,
forming a ring around the base of the (now
missing) branch. This is part of a
strengthening branch collar which reduces
the chance of wood tearing apart due to the
weight of the branch. Stress lines in the bark
also show how the bark has wrinkled in
response to the loading placed upon the
trunk, just above the branch.
Article last updated: 13/4/2014
beech bark
Left: a close-up view of a beech trunk. Click to
enlarge. Notice the wart-like structures on the
trunk - these are dormant buds, called
epicormic buds. These either grow with the
trunk, as it increases in diameter, staying near
the surface, or die and become submerged
within the wood. Some of these buds will
activate if the tree becomes damaged, and put
out new shoots. Epicormic buds are especially
visible on a smooth-barked tree like the
beech. Each bud has its own vascular supply.