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Like all living things, including
ourselves, the tree is entirely made up by a huge community of
individual cells.
The trunk and branches, what we
popularly call 'wood', are made up of longish tubular cells. The closer
these fibrous cells are crammed together, the harder the wood. Trees
with cells that are not so densely packed together are known as
'softwoods'.
Many of these cells connect with each other and form bundles of very thin 'pipelines', with similar
functions as our own blood vessels have, which is to carry fluid and
nutrient around the body of the tree. There is one set of these
pipelines or vessels which carry water and minerals up from the roots
throughout the tree into the leaves. There is another set, which carry
the sugars manufactured in the leaves with the help of sunlight (a
process called photo-synthesis) all the way down to the roots.
The tubular structure of the cells also contributes to the
strength of the wood, as does their tough and fibrous nature.
When we cut a log
from a trunk we notice that there are several different layers,
some of which have been given scientific names:
1 - is the bark or covering skin of the tree, which includes a
thin layer of Cork Cambium (not illustrated).
2 - is the Phloem.
3 - is a thin layer called the Cambium.
4 - is the Xylem or sapwood
5 - is the heartwood (aged Xylem). |
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Let's start with the Cambium (marked 3
on the illustration). This is the thinnest layer of all and you can find
it just under the bark. It is often a bit greener than the other tissues
of the tree. If you have ever seen a damaged twig, which still had only
a thin layer of bark, you may have spotted this greenish layer between
bark and wood.
The Cambium is the place where all the cells that make up the trunk and
branches, as well as the bark, originally come from. Each Cambium cell continuously
divides itself in two when the tree is growing. Each growing season a
new layer is added to make the tree grow wider. At the same time,
the cambium cells at the tip of the twigs divide themselves also to make
the twigs longer.
This means that a side branch, that grew out of the main stem at 5 feet,
will always stay at that height, rather than being 'lifted up' a bit
each year. It will just grow wider and longer.
The sugar molecules, manufactured by the leaves, are joined
together in the walls of the growing cells to form cellulose and lignin,
the main 'ingredients' of wood. Both cellulose and lignin molecules are
chains of hundreds of sugar molecules.
Young stems and branches are usually greenish, because, just like the
leaves, the cambium contains chlorophyll and can make sugars by
photosynthesis. When the stem or twig is young, the cambium is only
covered by a layer of relatively transparent protective cells. In one or
two seasons the stem becomes covered by a layer of phloem and bark and
will become dependant on the chlorophyll in the leaves to supply its food.
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The cambium cells on the outside of the
tree or branch become the fibrous Phloem layer (marked 2 on the
illustration) or inner bark. The Phloem cells form the downwards
pipelines or vessels, which carry the sugars, made by the leaves, all
the way down to the roots, and in this process provide the Cambium with
nutrition.
It must be noted however that the flow in the phloem vessels is reversed
in the spring. After the tree wakes up from a winter of hibernation, the
phloem carries energy and nutrition, stored in the roots and trunk,
upwards to feed the budding tree, until the leaves are ready to begin
photosynthesising a new supply of sugars.
As the Phloem is produced, an other tissue is added to it (not
illustrated above), which is a special layer of Cambium cells, called
the 'Cork Cambium'.
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As the tree grows in width
and girth, the Cork Cambium produces a 'corky-textured layer, which we
know as bark. Like the outer layer of our own human skin, the outer bark
cells are no longer living cells and are there 'only' to protect the
tree. It usually becomes cracked and ridged with age and may peel or
slough off as time goes by. Each species of tree has its own
characteristic bark, which is a great aid to identifying trees,
especially in the winter when there are no leaves to be seen.
The bark protects the tree in many
ways. It stops the trunk and branches from drying out too much, because
it is relatively waterproof. It helps to insulate the more sensitive
cells on the inside to some extent from heat and cold. The
corks, which we stopper our bottles with, come from the bark of the Cork
Oak and is a good example of a super-insulating bark.
Some trees have thick, fire-resistant bark. This makes it possible for
some trees to survive the milder forms of flash forest fires, which burn
all the debris and brush in a forest, but does not damage the larger
trees too badly.
The hard tough skin of bark also protects the tree from pest, fungi,
diseases and parasitic plants trying to get a foothold on other growing
tissues.
Most barks contain also chemicals which defend the tree. There are the
tannins, the same astringent chemicals which make your mouth go dry when
you drink a cup of very strongly brewed tea and which we use for
preserving leather. The taste of tannins makes us grimace and for the
same reason they prevent many creatures from regarding the bark as a
tasty nibble.
Some trees, like Pines, Firs and Eucalyptus produce gums and resins,
which ooze out when the tree is wounded to try and seal and disinfect
this wound. The anti-septic properties of the tree gums and resins have
of course been utilised since time immemorial by people too in medicine.
Bark has pores and these can get clogged up by dust and air pollution.
The Plane tree, which graces so many of London's streets is famous for
its ability to cope with heavy pollution. Plane bark renews itself
continuously by shedding plaques of old dirty bark, revealing fresh new
bark with clean pores underneath
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The dividing Cambium
cells on the inner side of the trunks and branches form the Xylem, also
known as sapwood, because it is here that the long Xylem cells form the
pipelines or vessels, which carry the water sucked up by the roots, with
dissolved minerals and other nutrients in it, ever upwards towards the
leaves.
In our temperate climate trees are
not able to grow in the winter, because there is not enough strength in
the sunlight to continue the food gathering process in the leaves by
photo-synthesis.
Leaves also have a lot of water in them, which makes them not only lush,
but also vulnerable to freezing. So in the autumn our trees close down.
They prepare for winter by storing their food reserves in the roots and go
to sleep until the warming sun of spring wakes them once more and makes
their sap rise again.
The fact that our temperate trees only grow in the warmest half of the
year is responsible for all the rings we can see in a sliced log. The
large pale bit of a ring ('spring-wood') is the growth the tree has achieved early in the
season when conditions were at its best.
The darker line in the growth
ring ('summer-wood') represents the growth later in the season, which was slower and
the cells are therefore smaller, denser and have thicker walls.
This also means that tropical trees, which can put on growth throughout
the year do not have the same sort of growth rings as temperate trees.
By counting the rings in a log, you can tell how many years old the tree
was from which it came, as well as whether particular years were good
growing years, where the prevailing wind can from, and so on. Please see Tree
Rings and Age for more information.
The amount of water rising through the sapwood is nothing short of
miraculous and it is yet another proof of Natures superior engineering
ability. In a mature tree this can be as much as 1400 litres a day and
that amounts to many tons of water during a growing season. The tree will
only use about 1% of all this water for its photo-synthesis. The rest of
the water is transpired from the leaves into the air. This has at several
major functions.
Transpiration prevents the tree from overheating on a hot day. The
evaporation from a mature Oak, for example, can absorb the equivalent of
the heat given out by ten 1-kilowatt electric fires!
Transpiration also has an important ecological function, because this
air-moistening effect benefits all plants and other creatures in its
environment, and it makes an essential contribution to the formation of
rain clouds. This is particularly important in land-locked areas, which do
not get the benefit of rain blowing in from the sea and in many areas of
the world, forests are the primary source of precious rain.
The mechanism by which a tree achieves this feat is very complicated, but
in order to understand the basic principle you can compare the sapwood to
a bundle of siphoning tubes. The water is sucked up by the roots
through a process called osmosis and aided by the fact that water
molecules stick together. As the water evaporates from the leaves, the
binding power in the water causes a chain reaction all the way down, which
is powerful enough to lift gallons of water to the top of the tree, which
in large trees can be a few hundred feet.
When you next walk in a forest or woodland, you might like to give a
thought to the fact that all these tree trunks around you are great columns full
of water!
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The heartwood consists of the growth
rings in the centre of the trunk. They are aged sapwood and thus form part
of the Xylem. They tend to have a darker colour than the sapwood, because
they no longer transport any water and are therefore much drier, and they
tend to be clogged up with gums, resins and other substances, depending on
tree species. Because
they are no longer taking part in the trees vital processes, they can be
said to be 'dead wood'. Please note that not all trees have a clear
distinction between sapwood and heartwood.
A hollow
tree is a tree where the heartwood, which has less resistance to rot than
the living parts, has gradually decayed away.
Surprisingly, this makes no difference to the vigour of the tree and may
even enhance it ability to stand up to severe gales, due to the immense
strength of a tubular structure. In the great gale of 1987 when so
thousands of wonderful trees were blown over, none of the many ancient
hollow trees in Windsor Park (and other places) succumbed to the storm.
(Having said that the heartwood in the tree may rot, it must be noted that heartwood can be incredibly strong
and hard. I used to live in a 400 year old farmhouse in Wales, in
which the first floor was resting on roughly hewn old oak beams. One day,
I attempted to drill two holes in one of them to hang up an indoor swing
for my small children to enjoy on a rainy day. Drilling into Oak heartwood
was one of the most difficult jobs I've ever done and the power drill was
literally smoking! It was like drilling into high quality steel and it
made me appreciate the expression: "Heart of oak" in patriotic
songs.)
So it is important to realise that a tree with decaying heartwood is NOT a
decaying tree! It is a natural process in many older trees and makes them
all the more valuable as a habitat for many different forms of wildlife.
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The leaves have four layers:
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A top layer (or skin) of transparent
cells with a thin waterproof coating.
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A layer of tightly packed cells which
contain little bodies called 'chloroplasts', which hold the
green pigment 'chlorophyll'. Chlorophyll does not only colour the
leaves green, but it is also the catalyst in the process of
photosynthesis.
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A spongy layer of loosely arranged
cells through which water and gases can move freely.
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A bottom layer (or skin), which has
tiny pores in it (called 'stomata' from a Greek word meaning 'mouth') through which gasses can move in
an out of the leaf and through which it can transpire water. It was
found that an oak leaf has as many as 35.000 stomata per square inch
of leaf.
The chemical formula of photosynthesis
is: 6CO2 + 6H2O > C6H12O6
+ 6O2
It means that 6 carbon dioxide molecules + 6 water molecules form 1
sugar molecule and 6 oxygen molecules. (Like all living beings trees
breath in oxygen and breath out carbon dioxide. This process must not be
confused of course with the process of photo-synthesis, which apart from
providing the tree with energy and nourishment also has the additional
benefit of turning carbon dioxide into oxygen as illustrated by the above
formula. A mature tree can give off the equivalent oxygen supply as
would be used up by 10 people).
This magical process is transforming air, water and light into a physical
substance is brought through the medium of chlorophyll. Molecules of water
are 'broken open' by the green chlorophyll in the leaves with energy
provided by sunlight, and combined with the carbon dioxide, a gas commonly
present in the air, into sugars to provide the tree with its basic
substance to create its bulk. The green
chlorophyll in plants is not all the same. There are about ten types,
which each fulfill a distinct role in this food-manufacturing process.
Scientist are still trying to work out the finer details and intricacies.
Just as in the trunk the veins in the leaves, which are of course clearly
visible in broadleaves, have xylem and phloem vessels to provide water and
minerals to the leaves and carry away the sugars in dilute form.
Sunlight is an indispensable ingredient in this labour and at night the
pores close down. It can do this because the stomata have two cells on
either side, called 'Guard cells', which resemble lips. The guard cells
act like valves and are able to regulate water evaporation and intake and
output of gases.
Leaves, which do not get enough sunlight, for example
those on lower branches, can no longer function and tend to die back
which also causes the demise of the branch they used to feed. This is a
form of self-pruning and is a constant process in growing trees.
There are huge amounts of leaves on a mature tree, up to quarter of a
million, and the combined leaf-surfaces creates an enormous area for
evaporation.
In autumn the leaves of deciduous trees respond to the colder weather and
reduced sunlight by creating a barrier of special corky cells where the
leaf stem joins the twig. This seals the leaf off from the tree's
circulatory system. The chlorophyll breaks down, which fades the green
colour in the leaves. Other pigments in the leaf, which had been present
all along, but were dominated by the green chlorophyll, may show through
as lovely autumn colours before the leaf falls of the tree.
In some hotter countries without a marked summer and winter, deciduous
trees drop their leaves at the onset of the dry season and re-grow them
again in response to rain.
The majority of deciduous trees (those that loose their leaves in winter)
have wide and flat leaves and the majority of evergreen trees have
needle-like leaves. As always there are exceptions. There are broadleaf
trees which stay evergreen, such as the Holm oak and rhododendrons. And
there are needle-leaved trees, such as larches, dawn redwoods and bald
cypresses, which shed their leaves.
All trees with evergreen leaves tend to have hard shiny surfaces to
prevent the leaves from drying out too much.
Manufacturing a whole set of new leaves every year takes a lot of energy.
Different species have different strategies in solving the problems the
weather and the fluctuating seasons has posed them. The temperate
deciduous trees have decided on the whole that it is worth the effort to
renew themselves every year. The
nordic evergreen trees, who have a relatively short growing period to
perform this huge job, have opted for needles, which possess an anti-freeze
chemical. The compact shape of the needle gives is also better able to
cope with frost and dry weather than the broad leaf.
In Botany the 3 great groups of trees are classed as follows:
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Broadleaves, which have true flowers
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Conifers, which have cones, as
opposed to flowers
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Palms, which are a sort of giant
tree-grasses in the way they grow and flower.
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 In
order to hold up the trunk and crown, the tree has a large system of
underground branches, which we know of course as 'roots'. I have not many
figures about the extent of tree roots required to fulfill this structural
anchoring function, apart from the fact that tree roots have been known to
penetrate well over 40 feet deep into the earth. It is of course not easy
to dig up a mature tree, roots and all, just to measure it, nor would it
be an attractive task as it would no doubt kill the tree. Other plants
have been measured in this way. A single grass plant was found to have a
staggering 400 miles of root, including its side-strands, which gives some
idea of just how enormous the root system of a much larger tree would be.
The only tree I know of which has been measured was a Finnish Pine tree with a total root length of 50 kilometres and
over 5 million root tips.
Apart from anchoring the tree, and in the process binding the soil
together and protecting it from erosion, the main function of the root is
to suck up water and minerals from the earth.
The vital parts of the roots are its sensitive growing tips, which explore
the earth, and know how to steer around hard objects. A large tree
will have many millions. Each one has a protective cap, which has been
likened to a thimble. Where necessary, an oily solution is secreted
around this cap as a lubricant to help it burrow through the soil.
Just behind its protective cap is the only place where the root grows in
length. As the root grows thousands of miniscule white hairs are send out
from this section, which attach themselves to particles of earth to gather
moisture and minerals. The active life of these hairs may be weeks or
months and they will die off as the root-section to which they are attached
begins to mature. They all die off in the autumn as the tree
prepares for winter.
The larger, more mature roots do not any longer take
part in the mining and suction processes. Just like the branches above
ground, they serve as Phloem and Xylem pipelines, as well as supplying
structural strength.
This means that if you are watering a young tree in dry weather, you
should not only water just around the trunk, but also in a wider circle,
so you don't neglect the thirsty ends of the ever-spreading roots.
Like the crowns of trees, the shape of a root system can vary from tree to
tree. There are root systems that spread wide and far , rather than
going for depth; root systems the shape of a heart and Taproot system,
where one main root penetrates deep into the Earth with a few smaller side
branches. Many trees uses mixtures of these 3 possibilities.
Most trees live in symbiosis (mutual dependency and cooperation) with
fungi in the soil amongst their roots. The association between roots and
fungi is called a mycorrhizal system. The thin filaments of fungi enormously improve the absorption
power of water and nutrients to the tree, so much so that some trees would
not be able to grow without the partnership of the fungi. In return the
fungi benefit, as they receive nutrients made by the tree's
photosynthesis. Fungi cannot make carbohydrates themselves, because they
have no chlorophyll. This means that they are either dependant on dead and
decaying matter to feed on or have to live in symbiosis with another
plant.
An important consequence of the dependence of tree roots on fungi is that
attempts to transplant trees larger than seedlings and saplings is often
doomed to failure, unless the tree can be moved with its whole root-ball
intact, including the earth which contains the fungi.
The widespread filaments of symbiotic fungi have often been mistakenly
thought to be tree roots.
You can read Andrew Cowan's article on 'Fungi',
which explains why fungi
are perhaps the most unappreciated, undervalued and unexplained
organisms on earth.
If you are interested in roots and fungi,
browse through "Trees
and Toadstools", an out of print work by Dr. M.C. Rayner, an
English biologist who was instrumental in many ground-breaking discoveries
in this field. The linked page also contains further useful links. The
excellent website on which her book is published is also very highly
recommended: Journey
To Forever.
A note on Water transport
and the Cohesion theory
If you wonder how a tree can lift water higher
than human-invented mechanical pumps you may like to read this quote from
Roland Ennos' excellent book "Trees", of which interesting
excerpts can be found on http://www.fathom.com/course/21701736/index.html
"Wood structure and water
transport
Images of magnified sections of pieces of wood have shown scientists how
its structure is well suited to transport water up from the roots to the
leaves of a tree. Over 90 percent of the wood cells are arranged along
the axis of the trunk or branch, like thousands of closely packed
drinking straws. Water can flow through them up the tree. But what
drives the water upwards? Over the last two centuries several possible
mechanisms have been suggested, but only one, the cohesion theory, has
stood up to experimental investigations.
The cohesion theory
According to this theory, water is actually lifted up trees from above,
using the power of the sun; it is pulled up under tension as the sun
evaporates water from the leaves. When first suggested in 1894 this
theory was greeted by disbelief, but since then a large amount of
evidence has been found to support it. For a start, it has been shown
that if water is held in a narrow pipe it can actually withstand large
stretching forces without breaking, just like an elastic band. The
water's strength is due to the cohesion between its molecules.
Experiments have shown that the cohesive strength of water can hold up a
column of fluid nearly three kilometres high."
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