coalesce and eventually break through the S 3 layer into the cell lumen. In
severely decayed wood, the high-lignin-containing compound middle lamella
is the only remnant of the cell wall.
Although soft rot cavities are commonly visualised in the axial tracheids of
softwoods and the fibres of hardwoods, the walls of other cells such as ves-
sels and parenchyma also show cavity attack.
Erosion of wood cell walls by soft rot fungi is less commonly reported.
Hyphae growing in the cell lumen release enzymes that produce erosion pat-
terns on the cell wall surface. These can take several forms including V-shaped
nicks or smooth erosion troughs around the hypha, but in general, this type of
attack is more prevalent in susceptible hardwoods than softwoods.
2.8 Basidiomycete Fungi
Decay of wood by lignicolous basidiomycete fungi in terrestrial situations is
generally described as either brown rot or white rot, although occasionally there
are reports of basidiomycetes producing soft rot cavities. The terms brown rot
and white rot reflect the appearance of badly decayed wood. This distinguishes
between those fungi that destroy polysaccharides rapidly, while lignin deple-
tion is slow – brown rot fungi, and those that are able to decompose all of the
structural wood components (cellulose, hemicelluloses and lignin) – white rot
fungi. The brown rot type of decay has so far not been recorded for marine fungi.
Of the 10 species of marine basidiomycetes recorded, three have been tested
for their decaying ability. These are Digitatispora marina,Halocyphina
villosa andNia vibrissa – all white rot fungi.Using three wood species – balsa
(Ochroma lagopus), Scots pine (Pinus sylvestris) and beech (Fagus sylvatica),
wood blocks were inoculated with the three basidiomycetes as well as the soft
rot fungus Monodictys pelagica. This work is of particular interest because
three of the fungi – D. marina, N. vibrissa andM. pelagica were isolated from
the 450-year-old timbers of the Tudor ship Mary Rose. While balsa is a very low
density, highly perishable timber species and the high weight losses were not
too surprising, it is interesting that significant weight loss values were recorded
in beech after 24 weeks incubation. Although beech is a perishable timber,
the ability of these fungi to cause significant weight loss in so short a time, in
archaeological terms, is a good indicator of their long-term decay ability. For
the record, oak is listed as durable in the Durability Classification.
White rot attack occurs at the lumen surface of the wood cell wall where
hyphae adhere to the wall. Enzymes released along the hyphae create discrete
erosion troughs around the hyphae with mucilaginous material extending into
the troughs; a further feature is the presence of bore-holes resulting from hyphal
penetration through the cell walls. As with the soft rot fungi, decay by basid-
iomycetes is dependent on the availability of sufficient levels of oxygen to
280 Chapter 11