but since adults are found in the galleries as well, it is not clear how the wee-
vils survive inundation of the wood, albeit briefly, and how they deal with the
high salt content in the wood consumed.
3 Wood Structure
Throughout history, wood has been a natural, renewable resource with a range
of properties and uses. The characteristics of timber are defined by its cell com-
ponents and the arrangement of these components, forming a three-dimensional
structure that has strength, flexibility and decorative appeal. The first important
distinction to be made when considering the use or value of timber is the source;
i.e.is the material a hardwood, derived from deciduous, broad-leaved trees,
or is it a softwood, derived from evergreen, coniferous species. Although this
rule is not totally universal, for the most part the distinction holds true and it
allows us to recognise anatomical features and cell types that are quite char-
acteristic of hardwoods or softwoods. A second consideration when examin-
ing timber for particular purposes is the distinction between sapwood and
heartwood. These features are often easily seen on the exposed face of a cross-
cut log where commonly the lighter coloured, outer sapwood band can be dis-
tinguished from the often darker inner heartwood. The significance of these
two zones is the difference in durability, since the heartwood has greater nat-
ural resistance to damage by marine animals, insects and microorganisms, by
virtue of the significantly higher levels of chemical extractives in the tissue.
It is also important to recognise that different wood species have different
levels of natural durability based on the performance of heartwood exposed
to ground contact. The time taken to fail in soil contact provides a class index
of durability, ranging from very durable to perishable species.
The terms hardwood and softwood do not necessarily reflect the strength,
engineering or durability properties of a particular wood species, for instance
yew (Taxus baccata) that is a softwood species, is extremely dense, strong and
highly durable. Wood density is therefore an important consideration when
assessing the strength and long-term performance of a species and in the case
of hardwoods is imparted via densely packed axial bundles of fibres. Fibres
are elongate, thick-walled, cylindrical cells with pointed ends having a very
narrow central lumen running the length of the cell. Their vertical orientation
within the trunk of the tree, alongside wide, open, thin-walled vessels that have
a water-conducting function, and axial parenchyma and fibre-tracheid cells,
make up the axial elements of the trunk and the converted timber. In addition
to the axial system, there is also a radial system of cell elements that is com-
posed of ray parenchyma cells, forming uni- or multi-seriate sheets of cells
orientated vertically and extending from the outer cambial zone below the bark,
to the centre of the trunk. It is significant to mention that in addition to the
282 Chapter 11