Plants use photosynthesis to convert light from the sun into energy used to create the substances they need for growth.
The leaves of plants make glucose and starch from carbon dioxide and water.
They take in water from the ground via their roots and carbon dioxide from the air through their leaves.
Leaves contain chlorophyll (green) that absorbs the sun's light energy to enable photosynthesis to occur.
The process releases oxygen into the atmosphere and reduces carbon dioxide which is why trees
are so important in tackling climate change.
Bark is the trunk's outer skin: the first line of defence against disease and animals.
It also provides protection against extreme weather.
It has two layers. Beneath the bark lies a thin layer of living sapwood.
This consists of three parts:
All plants grow at the tips of their roots and shoots to get longer and taller. They also increase the thickness of their woody framework year-on-year thanks to a layer just under the bark called the cambium. The cambium forms a continuous sheath over the whole tree, producing new phloem and xylem. This new growth is necessary to support the tree's growing weight and to move water and sugars around the tree.
This thin layer in the inner bark is only about 1mm thick and is responsible for transporting sugars around the tree to fuel growth,
reproduction and other essential functions.
Like other plants, trees combine carbon dioxide and water to make sugar (glucose) during photosynthesis.
This glucose is then converted into sucrose for transportation around the tree.
A fresh layer of phloem is grown each year as the old phloem is gradually incorporated into the outer bark.
Xylem is responsible for the movement of water.
Water is drawn up trees through xylem tubes, many of which are no thicker than a human hair.
Trees only use a fraction (typically less than 5%) of the water they take up.
The rest is released into the atmosphere from their leaves in a process known as transpiration.
It is this process that drives the movement of water through the tree.
The addition of new xylem annually creates visible rings in cut trunks and branches.
Each ring consists of two bands, light and dark.
Wood produced in the spring is often lighter in colour because it contains larger vessels to aid
water movement and is therefore less dense.
In the summer, many trees produce smaller, thicker walled fibres to add strength, making it darker in colour.
Scientists think that water transportation limits the ultimate height of trees.
Trees depend on water. If their xylem gets blocked, trees die.
Research suggests that transporting water may be impossible at heights greater than about 450 feet.
Water moves up to 20 times faster through the xylem of flowering trees than it does through the
xylem of conifers. This has a profound effect on where they grow.
At the centre of most trunks lies heartwood.
It may account for about 90% of a tree's overall woody framework but it consists almost entirely of dead wood.
However, heartwood is not at all redundant since it provides strength and support for the tree
and offers some resistance to decay.
As trees add wood each year, the older sapwood ceases to function and turns into heartwood.