Geoscience Reference
In-Depth Information
In addition to abundant bacterial species, soils contain many other microorganisms
of which brief mention has already been made of fungi and actinomycetes. Micro-
fungi grow as tubular filaments, called hyphae, and interwoven masses of hyphae are
known as mycelium. Fungal mycelia are often visible to the naked eye. There are
thousands of fungal species and some of them are typical soil dwellers. Usually fungi
require preformed organic carbon compounds for their nutrition, so are heterotrophic
organisms. They do not possess chlorophyll and therefore are not photosynthetic or-
ganisms. Although often very diverse, their nutritive requirements are generally fairly
simple. In morphology and mode of reproduction fungi show great variety. They re-
produce by producing spores, some of which are quite small, a few microns in dia-
meter, but others, like the chlamydospores and sclerotia can be much larger, up to 100
microns in size. Spores are formed both asexually and, in many instances, by a sexual
process.
Four major groups of fungi may be distinguished: phycomycetes, ascomycetes,
basidiomycetes and fungi imperfecti. Phycomycetes are typical moulds. They often
grow quite rapidly and form spores in large numbers. In the course of sexual repro-
duction, short lateral hyphae come together and fuse forming zygospores. Phycomy-
cetes commonly found in soils include several
Mucor
and
Mortierella
species; some
aquatic species also may occur in water-logged soils.
Typical ascomycetes produce, in sexual reproduction, eight spores that are con-
tained in a tubular or a spherical structure, known as an ascus. Asci may be grouped
together either in a disc-like structure or in a flask-shaped organ, called a perithecium.
Ascomycetes also produce vegetative type spores.
Chaetomium
is a typical soil asco-
mycete genus and it is an active cellulose decomposer. Frequently, many soil ascomy-
cetes are associated with dung or animal droppings.
The formation of a club-shaped hypha, called a basidium which bears four spores
on the end of short stalks, is a characteristic of the basidiomycetes. Basidia may be
grouped on the surfaces of gills of a stalked cap, as in the toadstools, or in a spheric-
al body, as in the puffballs. Soil basidiomycetes are not well characterized; although
possibly hundreds of species may grow in soil or on plant debris their presence is
only revealed when they form fruiting bodies. There are two main divisions, the gas-
teromycetes which have their fruiting layers concealed internally, and the hymeno-
mycetes, in which the fruiting layers are open to the air. The study and determination
of the many species is a matter for the specialist.
Rhizoctonia
species are common in
soils. Forest soils harbour many hymenomycetes (i.e. the mushroom type fungi) and
gasteromycetes (i.e the puffball type). In addition, mycorrhiza on tree roots are very
