Series editors' preface - Wood Tools

Figure 2.12 A representative portion of a molecule of
cellulose, the major chemical constituent of wood. The
cellulose polymer is composed of glucose anhydride
monomers
encrusts the cell wall structure after the polysaccharides
are in place. Although lignin contributes
to the compressive strength of wood,
tensile strength is provided principally by its
cellulose.
Extractives are typically low-molecular
weight compounds which are principally associated
with heartwood formation and are
located as much outside the cell wall as within.
Extractives represented among the various
species of wood fall within classifications such
as tannins, terpenes, polyphenols, lignans,
resin acids, fats, waxes and carbohydrates. In
addition to changing appearance of the wood
– mainly as colour – extractives may contribute
to other properties of the wood, such as the
Wood and wooden structures 75
significant decay resistance of cedars that
results from the presence of tropolones.
2.3.2 The cellulose structure within cell
walls
As the most important constituent of wood, the
nature and orientation of cellulose determine
the architecture of the cells. Insight into the
configuration of the cellulose within cell walls
provides an important key to understanding
and anticipating many of the properties and the
behaviour of wood. As a basis for discussion,
Figure 2.13 depicts a conceptual model representing
a typical longitudinal wood cell, such as
a hardwood fibre or a softwood tracheid. The
cell wall is layered. The outer layer, the primary
wall, was the functional cell wall during cell
division in the cambium and during subsequent
enlargement/elongation of the developing
daughter cell. Next, the secondary wall formed
within, giving permanence to the cell dimensions
and shape. The primary wall is very thin
and lacks any apparent structural orientation;
by contrast, the secondary wall occupies the
dominant portion of the cell wall and has three
layers, designated as S1, S2 and S3. When examined
with an electron microscope, the substance
comprising the secondary wall appears
to have oriented striations. These striations indicate
the general direction of cellulose mole-
Figure 2.13 Diagrammatic
representation of the structure
of a wood fibre. Fibres give
wood its strength, have closed
ends and thicker walls than
vessels, which are used to
conduct fluid and nutrients.
The lumen is the void or hole
in the centre of the cell. The
cell wall has four layers and is
composed of microfibrils. The
microfibrils are arranged
randomly in the primary wall.
The direction of the
microfibrils in the three
secondary walls alternates.
There are both amorphous
and crystalline regions within
the microfibrils, which are
themselves composed largely
of cellulose