vertical and radial orientation of cell types there is the possibility for water
movement through narrow pits in the wood cell walls.
In softwoods, the major difference to the anatomy of the wood is the absence
of vessels and fibres that are replaced by elongate axial tracheids. Tracheids
conduct water and provide strength to the timber and are characterised by
having elaborate bordered pits in the cell wall. Tracheid cell wall thickness
changes through the growing season so that in the early part, the wall is com-
paratively thin with a wide lumen whereas later in the season the walls are
much thicker and the lumina much narrower. This can be seen with the naked
eye as representing annual growth rings in cross-cut logs. Like hardwoods, soft-
woods also have a radial system of cell elements composed of ray parenchyma
and ray tracheids and the bordered pits provide a similar avenue for water
movement in the tangential direction. In effect, wood might be described as a
labyrinthine network of microscopic tubes and galleries connected together
via fine holes in the walls.
Wood cell walls have an outer primary and an inner secondary wall layer. The
thin primary wall is formed first and is flexible to accommodate changes to the
shape of the cell as it grows and develops. The secondary wall is thicker and
more complex and is laid down when the cell shape is established. In addition,
the secondary wall is composed of three layers – the outermost S 1 , adjacent to
the primary wall, the S 2 and the innermost S 3 , which are adjacent to the cell
lumen. Cells are held together by the middle lamella that cements the outer sur-
face of primary wall layers of one cell to the primary walls of adjacent cells.
The three major chemical components of wood cell walls are cellulose,
hemicelluloses and lignin, but of these only cellulose performs a structural func-
tion by providing strength to the material. Cellulose is a long-chain homo-
polymer composed of thousands of glucose monomers bonded together by
regular 1–4 chemical linkages. The molecular organisation of cellulose in
wood cell walls is based on aggregations of cellulose chains into microfibrils
that adopt a helical orientation within the cell wall. The angle of the helix is
different in different cell wall layers, but in the S 2 layer the angle is particularly
steep. In addition, the S 2 layer is thicker than the other layers and is extremely
cellulose rich. Unlike cellulose, hemicelluloses are short-chained, branched
heteropolymers composed of several different monosaccharides as well as
glucose units. The branched nature of the molecules that are about 200 units
in size, requires three different types of bonding and hemicelluloses are con-
sidered to be part of the matrix in which cellulose microfibrils are embedded.
However, the enzymatic breakdown of hemicelluloses bears close similarity
to that of cellulose degradation through the activity of hydrolase andoxidase
enzymes.
In contrast to the two-dimensional nature of the wood polysaccharides,
lignin is described as a three-dimensional polymer composed of phenyl
Conservation of Ancient Timbers from the Sea 283