hemicellulose are also present. Lignin has complex three-dimensional poly-
meric structure comprising various phenylpropane units. Lignin apparently
infiltrates and encrusts the cell-wall structure after the polysaccharides are
in place. Although lignin contributes to the compressive strength of wood,
cellulose provides the major contribution to tensile strength.
Cellulose, hemicelluloses, and lignin are essentially permanent
products synthesized by the developing wood cells soon after division in
the cambium. Extractives are principally associated with heartwood forma-
tion and are located as much outside the cell wall as within. These extra-
neous materials are called extractives because they can be extracted from
wood with the appropriate solvent with little change to the basic wood
structure. Extractives are typically low-molecular-weight compounds that,
among the various species of wood, fall within classifications such as tan-
nins, terpenes, polyphenols, lignins, resin acids, fats, waxes, and carbohy-
drates. In addition to influencing the appearance of the wood, mainly as
color, extractives may contribute to other properties of the wood, such as
significant decay resistance in some species.Cellulose within the cell wall
The nature and orientation of cellulose determine the architecture of the
cells. Insight into the configuration of the cellulose within cell walls pro-
vides the important key to understanding and anticipating many of the
properties and resulting behavior of wood.
Figure 6 presents a conceptual model representing a typical longi-
tudinal wood cell, such as a hardwood fiber or a softwood tracheid. The
cell wall has layered structure. The outer layer, the primary wall, was
the functional cell wall during cell division in the cambium and during
subsequent enlargement or elongation of the developing daughter cell.
Immediately after enlargement the secondary wall formed within, giving
permanence to the cell’s dimensions and shape. The primary wall is very
thin and lacks any apparent structural orientation; in contrast, the sec-
ondary wall occupies the dominant portion of the cell wall and has three
layers, designated as S 1 , S 2 , and S 3 , each with orientation revealed by
striations visible under the electron microscope. The direction of these
striations, as diagrammed in Figure 6, indicates the general orientation of
aligned cellulose. The apparent groupings, as suggested by ridges seen in
micrographs, are referred to as fibrils(subgroupings are sometimes termed
microfibrils).
Within the thinner S 1 and S 3 layers, the fibril orientation is nearly
perpendicular to the cell axis, whereas fibrils within the dominant S 2 layer
are oriented more nearly parallel with the cell axis. Experimental evidence
provides a theoretical explanation for the arrangement of cellulose within
fibrils. In random areas, called crystallites, cellulose molecules (or, more
likely, portions of cellulose molecules) are aligned into a compact crys-
talline arrangement. Adjacent areas in which cellulose is nonparallel are
called amorphous regions. The hemicelluloses and lignin are also dis-
persed between crystallites and through the amorphous regions. Within
the fibrils, water molecules cannot penetrate or disarrange the crystallites.
Water molecules can, however, be absorbed by hydrogen bonding, in one
or more layers, to the exposed surfaces of crystallites and components of
amorphous regions—namely, at the sites of available hydroxyl groups.
Such polar groups of the polysaccharide fractions on exposed wall surfaces10 Hoadley