where stored nutrients are located. Since fungal growth follows the ray
parenchyma cells, wedge-shaped staining patterns are evident when
cross sections of the wood are examined. Stain fungi do not usually
colonize heartwood.
Melanin-like compounds within hyphae or pigmented substances
produced extracellularly cause blue, gray-black, red, brown, green, or
other stains within the wood. Fungi that cause stains do not directly
degr ade wood cell walls, nor do they cause significant reductions in wood
strength. Stains are usually considered detrimental to wood quality but
have also been valued for their unique coloration. Green-stained wood,
created by the fungus Chlorociboria,was selected by numerous artists in
thefifteenth and sixteenth centuries for intarsia panels; the green-colored
wood was used for rendering natural scenery with trees and floral leaves
or for depicting book covers, fabric, or porphyry (Blanchette, Wilmering,
and Baumeister 1992). The stain is not light sensitive and has survived
many centuries without loss of color. Interestingly, remnants of fungal
hyphae are still present in green-stained wood from several intarsia panels
examined during recent restoration and conservation work (Blanchette,
Wilmering, and Baumeister 1992).Bacterial degradation of wood
Bacteria that cause deterioration in wood are most often associated with
waterlogged conditions. Buried wood from wet terrestrial sites or fromA G W D C M I 61cda bFigure 4a–d
Degradation of wood by white-rot fungi:
(a)Tangential section showing fungal hyphae
within tracheids causing a localized degrada-
tion of the cell walls around the hyphae
(arrows); (b) and (c) transverse sections of
eroded cell walls. The fungus degrades all
wall components, resulting in localized ero-
sion troughs and an overall thinning of cell
walls; (d) delignification of birch (Betula)
wood by a different species of white-rot fun-
gus. Preferential degradation of lignin results
in loss of the middle lamella between cells.
The fibers and vessels, consisting of cellulose,
readily detach and separate. Scanning electron
micrographs; bar 5 40 μm.