growth and structure
Wood is a complex, natural product. Between
different tree species, its density, stability, durability,
strength, burning properties, electrical resistance, reaction
during drying, impact resistance, bending properties, acoustic
properties, pulping qualities, workability and appearance varies
Hardwoods and softwoods
Trees are classified into two groups: hardwoods
and softwoods. This can be confusing because not all hardwoods
are hard and not all softwoods are soft. Balsa wood, for example,
is a hardwood, while some of Australia's hardest timbersfor
example, Callitris pine, are softwoods. The difference is in
the cellular structure of the wood.
Hardwood timbers are made up of four cell types. Small wood
fibres make up the bulk of the wood. Large cells, called vessels,
function as pipes that move sap up the tree through the mass
of fibres. Other cells are largely used to store food. With
some experience it is possible to distinguish different hardwood
timbers based on the number, size and location of the vessels.
Softwoods do not have vessels. Softwoods have a simpler fibrous
structure based on only two cell types.
The easiest way to distinguish between the two tree groups is
to remember that flowering trees are all hardwoods while cone-bearing
trees are softwoods. Hardwoods include eucalypts, wattles and
oaks while pines and cypresses are softwoods.
Growth rings and the pith
As the tree trunk and branches thicken, a
series of concentric layers of wood cells are laid down around
a central core called the pith. These appear as growth rings.
The pith is the remnant of the growing shoot that gives the
tree its height. Because the pith has a cell structure different
to the rest of the tree, it is often easily seen as a corky
pipe in the logs centre.
Growth rings often, but not always, represent annual growth.
In temperate areas, tree growth during spring is represented
by the presence of large cells with thin walls that appear lighter
in colour. The darkness is due to the presence of smaller, thick-walled
cells that are laid down towards the end of the growing season.
In some hardwoods, the vessels form a ring in the early wood
(ring-porous species) making the rings easier to see.
Sapwood and heartwood
The living trees stem is composed of
different layers, illustrated in the cross-section diagram above.
The cambium is a very thin layer of cells that divide to produce
the bark cells (which protect the tree) and the wood cells.
Sugars produced in the leaves travel down the inner bark, or
phloem, feeding the cambium and, ultimately, the roots.
Only a very small proportion of the wood cells produced by the
cambium are actually alive. Most of the newly formed wood cells
hollow out to form the sapwood through which water and dissolved
minerals travel from the roots to the leaves. Other cells are
used to store food provided by photosynthesis, including starch.
The sapwood is almost always creamy white or yellow in colour
and is usually two to five centimetres thick.
In most tree species the sapwood band can be easily distinguished
from the inner heartwood. The heartwood supports the trees
stem but doesn't otherwise contribute to its growth. Heartwoods
colour comes from resins, minerals and other compounds being
deposited in the cells as they are decommissioned from their
role as sapwood. These deposits, and the lack of food, add durability,
colour and strength to the timber.
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