THE ANATOMY OF A TREE
The major parts of a tree
are leaves, flowers and fruit, trunk and branches, and
Leaves are basically sheets (or sticks) of spongy living
cells connected by tubular conducting cells to the
"plumbing system" of the tree. They are connected to the
air around them by openings called stomates, and protected
from dehydration by external wax layers. They frequently
have hairs, bristles, scales, and other modifications that
help adapt them to their environment.
While branches and
trunks may seem to be "just made of wood," this material
(and the bark around it) consists of many types of cells
adapted for strength, resistance to injury and decay,
transport of liquids, and storage of starch and other
consists mostly of two zones: The inner bark or phloem
actively contributes to the tree's life processes: its
tubular cells form the "plumbing system" through which
sugar and growth regulators, dissolved in water, are
distributed to other parts of the tree from the leaves and
buds where they are made. The outer bark consists of
layers of inner bark cells that have died and cracked as
they have been pushed outward by the tree's growth; outer
bark forms the tree's first line of defense against damage
by insects, people, heat and cold, and other enemies.
normally has three meristematic zones -- that is, cells
that can divide and reproduce themselves. Two of these,
the root tips and the buds at the tips of twigs, allow the
tree to grow lengthwise. The third, located between the
bark and the wood, is the vascular cambium zone, often
referred to merely as "the cambium." Its cells divide
inward and outward, laying down new wood cells on those
already in place and new inner bark cells inside those
cambium is one important key to trees' success. Its growth
from the outside outward allows the tree to cover over
minor wounds and (as we will see later) to wall off and
abandon entire columns of rot-infected wood. This is the
strongest of all a tree's defenses against decay.
tangetial section through wood it is cut longuitudinally
but not throught the centre of the stem. Growth rings tend
to be arranged as irregular patterns of concentric Vs
(Not shown in image)
section through wood is a longitudinal secton which passes
through the centre of the stem. The tracheids, vessels and
fibers are cut along their shortest axes. The growth rings
appear as parallel lines.
transerverse section through the wood the tracheids,
vessels and fibres in the wood are cut across their
shortest axes. The growth rings of the entire section
through the stem well appear as concentric circles.
cambium region is the xylem or wood. Xylem includes
three principal types of cells. There are numerous
tubular conductive cells running parallel to the trunk
or branch that they form, and adapted to carry water
and minerals upward from the roots. There are also
sheets of ray cells -- tubes running from the inner
bark inward toward the center of the tree. The third
type is parenchyma (pa-REN-ki-ma); these cells, while
they are still alive, warehouse starch crystals made
by the action of enzymes on sugar. This starch,
reconverted to sugar by enzymes, is the principal raw
material for the natural fungicides made by trees in
response to injury, and the principal source of energy
for growth after injury. Parenchyma can also become
meristematic under some conditions, taking on a role
similar to the vascular cambium when a tree has
species of trees also have other types of cells, such
as resin ducts, fibers, and tracheids. However,
conductive cells, ray cells, and parenchyma, make up
the bulk of wood, and perform most of its functions.
heartwood-forming species such as red cedar, walnut,
and oak, there may be a distinction between the
newest, living layers of sapwood and heartwood. True
strong heartwood is the result of aging in normal
compartments of a tree. As sapwood becomes less
involved in transporting and storing energy for the
use of the tree, its cells become "toxic waste dumps."
This gives the wood a distinctive color and sometimes
gives it natural resistance to decay. Unlike sapwood,
heartwood can not respond to injury by forming
substances; however, it can discolor.
Heartwood" is a central column of discoloration that
occurs as a young tree matures, when many branches die
and are shed even though there may be no decay.
Discoloration that takes place in the early stages of
decay after a tree is injured is not true heartwood,
but it can keep heartwood from forming.
earliest wood in a trunk or branch is a column of
spongy, styrofoam-like material called pith.
Eventually covered over by layers of wood, pith is the
remains of a primary tissue formed as a twig
elongates. In some species it disintegrates or is
crushed; however it remains in other species, such as
black walnut, whose twigs can be easily recognized by
their chambered pith.
Until you understand the
differences, all branches look pretty much alike. But
the differences between forks, true branches, and
epicormic sprouts (sometimes called "suckers") are
profound and important. Knowing them could save your
tree, your house, or your life.
a tree is young, its goal is to get its head above the
competition and catch as much sunlight as possible. In
the forest where trees compete for light and space,
the most efficient way to do this is with an "excurrent"
growth habit -- that is, a single, undivided stem and
lateral branches. As it reaches its mature height, the
branching habit becomes "codominant" -- that is, its
stem and branches often subdivide with forks instead
of true lateral branches.
when we domesticate trees, we encourage them to make
this transition much earlier in life... and closer to
the ground. In fact, a standard nursery practice has
been to force trees into a codominant branching habit.
A fork near the tip of a branch has little effect on
the tree's strength; but the lower the fork occurs,
the worse the problem if it fails.
A true branch is
the result of a process that starts with the growth of
a bud into a twig. Normally this begins from the
axillary buds found where each leaf joins the twig.
The meristem (reproducing cells) at the tip of the bud
divide, and the newly formed cells become a twig. The
meristem just under the bark -- the vascular cambium
-- continues to divide so that the twig grows in
diameter, forming a branch.
the base of this twig is a swollen area called the
branch collar. In this area the wood fibers of the
trunk (or parent branch) veer around the twig on each
side and continue toward the trunk or the base of the
tree; the "plumbing system" in the branch also turns
groundward -- none turns upward or goes around the
trunk or parent branch. Since growth occurs at
different times in various parts of the tree, the twig
and branch fibers tend to form interwoven layers, a
little like the laminations in plywood. Together, they
create the extra wood thickness of the branch collar,
which continues to grow as the twig matures. If the
fibers in the crotch at the base of the twig knit well
with those of the trunk or parent branch, a bark ridge
emerges to some extent across the crotch.
Also, natural fungicidal materials saturate the fibers
in the base of the growing twig, forming a protection
zone; this does not happen in the fibers of the trunk
or parent branch. This has important implications for
pruning: if you cut only the protected branch fibers
outside the collar, you protect the tree from decay.
A fork is
a place where a stem grew in two or more directions,
instead of one. Although one side may be larger than
the other, neither side has any natural chemical
protection. Most U-shaped forks, with all bark
visible, are dependable. The problem is with V-shaped
forks, particularly when bark disappears down into the
fork from each side, and much of the branch junction
consists of two bark faces pressed against each
Such forks, though graceful, have many potential
First, there is no bond or strength
between the two bark faces.
Second, as the two sides of the fork
grow, the pressure between them tends to
spread the fork, increasing the splitting
force on its the base
Third, this pressure also crushes the
living tissues under the bark, starving this
area and destroying its defenses.
Finally, rainwater, fungus spores and
other materials seep down into the fork,
rotting bark and wood so that the weaker side
is likely to split from its own weight or
under wind stress
is a common problem to look out for in trees
that have not been maintained carefully -- a
weak fork with what is known as "included bark.
"This makes a tree very likely to split during
high wind, ice load, etc. Many people like the
graceful appearance of a forked or multi-stemmed
tree. It seems insignificant when a tree is
young; but this is when a little preventive
pruning can make a lot of difference. If you
ignore it, you may find out too late that this
is one of the most serious structural problems a
tree can have.
click to enlarge
no way to know how much bark is included in a fork, or
how soon it may fail. When rods and cables are used to
reinforce weak forks, they must be installed properly,
and inspected periodically. Too often, such hardware
merely postpones the failure of the tree until it is
bigger, heavier, and more dangerous. The only good way
to deal with such forks is to prune out the weaker
side while the tree is still young. Such preventive
pruning pays for itself many times over.
("SPROUTS" or "SUCKERS")
branch or trunk leader grows, its tip bud manufactures
a growth regulator that suppresses buds nearer the
base of the tree. But when the tip bud dies or is
removed, many axillary buds (in the the angle between
each leaf and its parent twig) and dormant buds (in
the living wood under the bark) are activated, and the
cambium may be stimulated to produce new adventitious
buds (usually in response to wounding).
Sometimes, if they are carefully managed, suckers can
sometimes be trained to become healthy and useful
parts of a tree. As a rule, however, sucker growth
that occurs as a distress response when a tree is in
trouble is likely to cause further trouble. This is
because suckers are superficially attached to
the surface layers of wood, and because most rapidly
formed wood (such as that typically found in suckers)
is weak. Under ice load or strong wind the tall
spear-like suckers formed around topping cuts are
especially likely to bend over and break; or they may
tear out where they are attached to the surface layer
of the stub, which has been opened up to serious rot.
People want their trees to "look like trees" -- that
is, to have a large, shady crown; and we think of
multiple stems as "graceful," not understanding that
this beauty often comes at the expense of strength and
safety. At first blush, this would seem to force us
into a choice between beauty and strength. But the
more we understand about trees, the more we appreciate
healthy structures. It is difficult to remain
enchanted by something that is likely to split open
and crush anything in its path.
Myths About Roots
have ever seen the entire root system of a tree. Since
roots are mostly out of sight, most of our ideas about
them come from glimpses and assumptions. So a great
mythology has grown up -- where they are, what they
look like, how they work, and how we should manage
them. Some of those that come up most often include:
The root system is more or less a mirror image
of the top of the tree.
Most tree species have deep taproots; if a
tree's taproot is cut, the tree dies.
A tree's roots extend to the tips of the
Tree species are "deep-rooted" or
Roots seek water.
understand what a tree's roots looks like, think about
what they do, and how.
Their principal jobs are:
is powered by sugar received from the leaves;
"burning" this sugar requires oxygen, which the roots
must find in the small spaces between soil particles.
all, a large tree may have over 30 miles of roots,
with about 5-million root tips, plus many small
colonies of beneficial fungus. The actual work of
absorbing water and minerals from the soil is done by
one-celled projections ("root hairs") from the
absorbing roots, along with colonies of beneficial
fungi (mycorrhizae, pronounced MY-co-RISE-ee) that
live on or in the feeder roots.
Absorbing Roots feed into long, thin Conducting Roots,
which carry water and minerals back toward the trunk.
In undisturbed forest soils the conducting roots may
extend outward as much as 2 or more times the height
of the tree, mostly in the top 1-2 feet of soil. These
conducting roots gradually converge into thick lateral
Brace Roots, which provide most of the tree's support.
So roots extend in an ever-widening disk, wherever
they find all the things they need -- water, minerals,
and oxygen. There is little oxygen deeper than
about 18" in clay soils; in sandy soils the oxygen may
go deeper, but the water or minerals may be in short
supply; a high water table may also limit oxygen
penetration; or bedrock may prevent roots from
envision a tree's root system, imagine a wine
glass sitting on a tray -- the root system is
the tray, and its thickness is determined by
some limiting factor. Tree species are not
necessarily "deep-rooted" or "shallow-rooted" --
certain species are more or less sensitive to
various levels of moisture, oxygen, and
minerals. Often roots will "bounce" off a soil
hardpan or a high water table, forming surface
roots where other species may not be able to
live at all.
In undisturbed forest soils, roots may extend
well beyond the tips of the branches, but a
heavy clay soil will greatly slow down their
outward growth. Roots are opportunistic: they do
not "seek" either water or minerals, but when
they find them, they prosper.
Trees seldom have deep taproots, especially in clay
soils. When a seed germinates, a rootlet heads
downward into the soil, but it quickly branches to the
sides, extending only wherever it encounters the
essentials for its growth -- water, minerals, oxygen,
and growing space. There is no particular
difference between the taproot (if there is one) and
any other root -- if you cut it, all other roots
attached below the cut are lost, but those attached
above it will still function, provided a fungus or
other disease organism does not enter through the cut.
deep sandy soils with wide seasonal variation in
rainfall, trees have been known to form "striker
roots" -- taproot-like connections between two sets of
conducting and absorbing roots, one just above each
seasonal water table. However, this is fairly
There are more similarities than differences between
roots and structures in the crown. The connections
between roots and parent roots are similar to branch
structures, as are mechanisms for dealing with injury
and decay -- work that roots are extremely effective
in handling. And roots suffer the same loss of stored
energy when they must react to injury and contain
Anatomical differences include: (1) Roots do not have
pith. (2) Roots usually have more parenchyma (living
food-storage cells) and fewer fibers. (3) In roots
there is less distinction between growth rings than in
trunks and branches. (4) Roots do not produce normal
Roots have important functional differences, as well.
They are adapted for uptake of water and minerals, so
(unlike trunks and branches) their bark withstands
moisture and low light conditions.
also require considerable oxygen, which they must
extract from small spaces between soil particles.
the most important distinction is that we rarely see
roots, so we know little about them, and we tend to
ignore them. This is unfortunate, since most problems
with trees reflect mismanagement of roots.
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