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propane units. It is a recalcitrant molecule that is degraded slowly in nature
and along with the hemicelluloses, is a major component of the matrix mater-
ial in wood cell walls. Lignin is a complex molecule and in addition to the
phenyl propane monomers has up to 15 different types of chemical bond to
hold the polymer together. Its distribution within the wood cell wall is not
uniform and lignin is found in greatest concentration in the middle lamella
region and the cell corners. Hence, the progressive enzymic destruction of
wood cell walls leaves these more highly lignified regions as the final rem-
nants of the structure.
Clearly the physical and chemical composition of woody materials has a
direct bearing on the ecology of the organisms that destroy them. The lignin
matrix is, in some measure, a barrier to the activities of microbial cellulolytic
and hemi-cellulolytic enzymes except in the case of basidiomycete white rot
attack where the three wood cell wall components are broken down. However,
in the thick cellulose-rich S 2 , where lignin is more or less absent, cellulase
enzymes are able to function freely. All wood-destroying micro-organisms
are capable of degrading cellulose, but for some their status as lignin degraders
is unclear. This is especially so for lignolytic bacteria, although using
TEM, tunnelling bacteria have been visualised traversing lignin-rich middle
lamellae.
3.1 Archaeological Wood Recovered from the
Marine Environment
The marine environment presents a hostile and seemingly unlikely situation
for the survival of archaeological wood, yet it does survive. Normally, wood
does not survive long enough in marine environments to enter the archaeo-
logical record because of the activities of wood-boring animals and aerobic
microbes. However, studies have shown that rapid burial in the anoxic sedi-
ments of the seabed will protect ships’ timbers and wooden artefacts from the
physical, chemical and biological processes that influence the deterioration
of exposed wood.
Waterlogged wood recovered from the marine environment is usually fra-
gile due to the degradation of the cell wall components and is supported
mainly by seawater. Some heavily degraded waterlogged archaeological wood
may be unable to support its own weight in air. In a wet condition, preserved
archaeological wood can retain its dimension and shape. However, despite
the well-preserved appearance at excavation, recently excavated archaeo-
logical waterlogged wood does not dry as sound wood. When dried, the wood
cells weakened by degradation are often unable to resist drying stresses and
collapse, and surface cracking and splitting may occur resulting in the destruc-
tion of an entire historic artefact.