Geoscience Reference
In-Depth Information
F
IG. 48
'Fairy ring' of toadstools. The photograph shows the fruiting bodies of three species of mycorrhizal fungi
on roots of birch. The main, outer, ring is formed by
Hebeloma crustuliniforme;
four large whitish toad-
stools,
Lactarius pubescens
form a partial inner ring, while a single brownish toadstool nearest the tree is
Leccinum roseofracta.
(Photograph P.A.Mason.)
From the ecological point of view, fungi, as in the case of bacteria, can be clas-
sified on the basis of their food requirements: for example, 'sugar fungi' require only
sugars or simple carbon compounds. Other types of fungi can assimilate or decom-
pose cellulose or lignin. Basidiomycetes are typical lignin-decomposing fungi, al-
though they grow on wood or lignin materials only slowly and often can assimilate
lignin better when it is associated with some more available nutrient, such as cellu-
lose.
Two main groups of fungal species, known as 'white rots' and 'brown rots',
are recognized in the decay and rotting of wooden structures, especially in wet situ-
ations. Some typical microscopic moulds are wood-attacking fungi and others, like
the bracket fungi, may form large macroscopic growths on the trunks or branches of
trees. Lignin is the characteristic chemical component of woody tissues, and is man-
ufactured in plants by the enzymatic oxidation and polymerization of phenylpropane
derivatives. It is understandable that stable, complex, insoluble substances of this kind
are not easily attacked and decomposed.
Apart from fungi, few microorganisms can digest wood or lignin directly. Some
termite species degrade wood and lignified tissues with the assistance of certain
fungal species which they cultivate in a kind of fungal garden within the termite
mound. Other insoluble materials, for example, chitin which is present in the cuticle
