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Parts of the Trunk
Heartwood: The heartwood surrounds the pith at the centre of
the trunk. Sap no longer flows in the heartwood as the cells
have been blocked by natural substances which have been laid
down to make the timber more durable as the tree grows older.
It is darker in colour and is more resistant to insect and fungal
attack
Sapwood: The sapwood lies towards the outside of the trunk
and surrounds the heartwood. It is the more living part of the
tree and contains cellulose and lignin. It is lighter in colour
and is not as durable as the heartwood
Bark: The bark is a layer of tough fibres which protects
the tree from insects, animals, fungi and the weather. It also
retains moisture and minerals within the tree.
Bast: The bast lies just under the bark. Here phloem cells
carry the sap manufactured in the leaves down to all parts of
the tree. The sap is distributed towards the inside of the tree
through the medullary rays.
Cambium layer: The Cambium Layer lies just under the bast. This
is where most of the growth of the tree takes place. Here xylem
cells grow and carry water and minerals up the trunk to the leaves.
Medullary Rays: Rays are cells which lie across tire trunk. They
radiate out from the medulla or pith of the trunk which lies
at the centre. The rays distribute sap from the phloem cells
in the bast inwards towards the pith.
Pith: The Pith is a small area at the very centre of the
trunk. It is dark in colour and is all that is left of the young
sapling.
Annual rings: The Annual Rings show the plant's yearly growth.
In spring the tree will grow fast and so the cells are thin-walled
and lighter in colour. This is called the early-wood. The late-wood
is darker in colour. It grows during the summer when growth is
slower and the cell-walls are thicker.
Diffuse porous
and Ring
porous trees:
Because the climate in the Tropics is more even all year-round,
the trees there grow steadily during all seasons. These trees
do not then have noticeable annual rings and are called Diffuse
Porous trees.
The cells of trees from northern and southern temperate climates
will grow at different rates during the different seasons. They
have noticeable annual rings and are called ring porous. |
