Outer bark. A close-up view of the
outer bark (periderm).
Click here for a
detailed explanation of bark structure.
The cortex of the primary stem, which
has become part of the outer bark,
consisting of collenchyma and
parenchyma just beneath the cork
cells of the outer bark will eventually
be shed and more cork cells will form
inside of it, so mature bark will lack
these cells.
Pericycle. A close-up view of the
pericycle - a more-or-less complete
cylinder of thick-walled sclerenchyma
cells (stained in pink).
Above: a low-power view of a cross-section through a Tilia (small-leaved lime) stem in its
first-year of growth (actual diameter of about 3 mm). The microscope has been focused on the
rows of large pink cells in the centre - these are the xylem cells that make-up the wood. Outside
the xylem is the bark (stem cortex) and inside the xylem is the pith (stem medulla). The xylem is
traversed by rays of parenchyma cells, like radial spokes (stained green). A schematic tissue
plan of this section is shown below:
A woody stem at the end of its first-year of growth is in transition
between the structure of the green or primary
non-woody stem,
characteristic of non-woody herbaceous plants, and the mature
woody stem with its annual rings of wood. The rings are formed
by the secondary xylem and a first-year stem naturally has only
one of these rings.
Primary growth is the production of the initial
green stem, and
secondary growth is the process by which
the stem thickens by wood formation.

Left: a modified tissue plan showing the rays in the top-right
quadrant only; click diagram to enlarge.

The xylem consists of vessels which conduct sap from the roots
to other parts of the plant, supplying them with water and
minerals absorbed from the soil. The phloem conducts sugary
sap from the leaves and other green parts, where sugars have
been made from carbon dioxide gas, water and sunlight by
photosynthesis. As the stem matures, the primary xylem ceases
to function and the primary phloem becomes crushed by the
expanding tissues inside of it and so also ceases to function. As
secondary xylem expands, the central, older portion becomes
non-conducting and forms the heartwood of a mature stem.
Only the outer sapwood is conducting. The rays transport
certain materials across the stem, including waste chemicals
which are dumped into the heartwood, giving it its characteristic

In the sequence of images below we shall look at the structure
of this stem in more detail, moving from the outside in to the
Phloem. The phloem is sometimes
classed as the inner bark, since it
comes away with bark when this is
peeled from a tree. However, it is a
vital tissue and so does not serve a
protective function. It consists of
thick-walled sclerenchyma fibres
(stained pink) which make it tough and
fibrous and phloem vessels (larger
cells with open lumens) and associated
Xylem. This is the wood of the stem
and is comprised of xylem vessels
(stained pink) and parenchyma cells
(stained green) that are arranged in
radial plates that extend across the
stem as rays and also extend part-way
vertically up-and-down the stem.
Peeling a complete circle of bark from around a tree will kill it, since then the leaves will be unable to deliver
sugars to the roots through the phloem. The rays of parenchyma cells that extend across the xylem also
pass through the phloem, where some of them fan outward to form divergent rays. Small parenchyma cells
are also associated with the large phloem vessels - they can be seen to surround groups of phloem vessels
in the photomicrograph above. These cells transport materials, such as sugars, from the phloem to other
cells and also provide the source of energy needed to 'pump' the sap along the phloem. The phloem
conveys sugary-sap from leaves and other photosynthesising parts (sources) to parts of the plant that
absorb these sugars (such as growing fruit and roots). The roots will turn some of this sugar into starch for
Conduction of non-sugary sap from the roots to the rest of the plant (up the stem) in the xylem does not
require the plant to expend its own energy, as does transport of sugary-sap in the phloem. (Although the
roots expend some energy in loading the mineral salts that they absorb from the soil into the phloem).
Instead it relies on the Sun's energy. As the Sun heats the leaves it causes them to lose water by
evaporation, creating a suction pressure that sucks the sap up along the xylem. The xylem vessel walls need
to be strong so that they do not collapse under the large suction pressures that develop. In contrast, the
plant uses its own energy to push sugary sap along the phloem.
Tilia xylem and rays
Tilia starch sheath
Tilia starch sheath low power
Starch sheath. The starch sheath is
a cylinder of cells one layer thick (not
to be confused with the endodermis of
roots which has quite a different
function). These cells are filled with
starch (stained black) which acts as a
fuel reserve and they separate the
primary xylem (the small green vessels
on the right in the photomicrograph,
just left of the large secondary xylem
vessels) and the pith (visible as the
large cells in the left of the photo).
Some of the rays end at the
protoxylem and some of them extend
right to the starch sheath. Starch is a
polymer (a polymer is a long chain
molecule formed by joining smaller
molecules, or monomers, together
end-on-end) of glucose. When the
plant's leaves manufacture more sugar
than the plant needs, then it will store
that sugar as starch until it is needed,
then the starch will be broken down
into sugar and used for energy as a
chemical building block.
Vascular cambium. The vascular cambium is a layer of cells
(stained green) called initial cells, that form a cylinder between the
secondary xylem (on the left in the photo) and the phloem (on the
right). These cells divide (by mitosis) to produce xylem cells on the
inside and phloem cells on the outside. Some will also divide to
form new cambium initial cells. The columns (radial files) of xylem
and phloem cells produced by the cambium can be clearly seen in
the photo. This cambium is therefore responsible for the
thickening of the stem that occurs in secondary growth. Other
cambial cells, called ray initials, give rise to the rays and are
different in shape. Cambium is an example of a
meristem or region
of growth in a plant where cell division is occurring (that is where
new cells are made). The xylem, phloem and vascular cambium
together constitute the vascular cylinder or stele.
Rays. The rays are made-up of parenchyma cells that retain their
living protoplasm. In contrast, the cells (vessel elements) that
make up xylem vessels have lost their protoplasts and are said to
be non-living, though they are function and indeed the loss of the
protoplast is part of their normal development - clearing the cell to
conduct sap. Thus xylem cells appear empty - only the
strengthened cell walls that form the vessel tube remain. The
parenchyma ray cells are living and store starch and oils, tannins
and other materials of unknown function. The walls of these cells
may become thickened and lignified. Other parenchyma cells form
vertical columns inside the xylem (axial parenchyma) and some
form cross-connections between adjacent rays. Rays transport
and deposit waste materials, toxic to living cells, near to the bark
and in the heartwood and the ray cells of heartwood eventually die.
Tilia tissue plan highlighting pith
Tilia pith
The pith is the central core-cylinder of
the stem and is a light spongy tissue
made-up of large
parenchyma cells.
These cells have thin cellulose walls. In
the young green stem, these cells act
like pressurised foam or cellular solid -
the cells swell with water (they become
turgid) making the stem rigid. Should
they lose water then the stem will sag
as the plant wilts.
Mechanically and structurally, the parenchyma cells are the least specialised of plant cells, though are
responsible for most of the plant's chemical manufacture. As chemically active cells they require oxygen and
parenchyma is typically permeated by air spaces (visible as the small triangular gaps between adjacent cells
in the photo above). As the stem matures, the pith loses its supporting function, as the wood takes over this
role. The pith is typically destroyed as the stem continues to grow, though it may persist for some time as
horizontal plates, especially at the stem nodes (forming nodal diaphragms).

The xylem vessels produced in Spring are larger in diameter than those produced in early Autumn. The
larger diameter of early vessels allows them to transport large amounts of sap to the developing buds,
speeding-up the development of the leaves. They also maintain a stream of water to the leaves to replace
that lost from the leaves by evaporation.  However, wide vessels are problematic in cold weather as they are
more prone to cavitate. Cavitation occurs when a bubble of air forms in the xylem sap, which is being
sucked-up under great pressure, and is more likely to happen in the cold since the column of water becomes
more brittle in the cold and may well snap under the immense pressures needed to deliver water to the top of
a tall tree. Thus, the late wood contains narrower vessels - it has a finer grain and so is denser and visibly
different. Production of wood may stop altogether in winter months in deciduous trees that shed their leaves
in Autumn. This annual progression from wide to narrow vessels, creates the annual tree rings - there is one
ring formed per year of growth.
Structure of the stem in a young sapling
Above: a 3D model of a slice of a more mature woody stem with three annual rings of sap wood
and several rings of central heartwood. Some rays have been diagrammatically added, along
with the sheath of phloem (inner bark) and the outer bark.
Click here to learn more about the cellular
structure of wood.