The four classes of fungi

Phycomycetes grow in water, on rotting wood in damp spots, or as obligate parasites of plants. Their mycelium has no septa and is coenocytic.

Phycomycetes reproduce asexually by motile zoospores or non-motile aplanospores formed inside a sporangium (endogenous). A zygospore forms when two gametes fuse; the gametes may be alike (isogamous) or unlike (anisogamous or oogamous). Examples: Mucor, Rhizopus (bread mould) and Albugo (parasite on mustard).

Ascomycetes, the sac fungi, are mostly multicellular (Penicillium) and rarely unicellular (yeast, Saccharomyces). They are saprophytes, decomposers, parasites or coprophilous (growing on dung), with branched, septate mycelium.

Ascomycete asexual spores are conidia, formed outside (exogenously) on special hyphae called conidiophores. Sexual spores are ascospores formed inside sac-like asci, which sit in fruiting bodies called ascocarps.

Examples of ascomycetes are Aspergillus, Claviceps and Neurospora; Neurospora is widely used in biochemical and genetic work, and morels and truffles are prized edible members.

Basidiomycetes include mushrooms, bracket fungi and puffballs, growing in soil, on logs and stumps, or as parasites in living plants (rusts and smuts). Mycelium is branched and septate.

Basidiomycetes usually lack asexual spores but often fragment. They have no sex organs; plasmogamy is by fusion of two somatic cells of different strains, giving a dikaryotic structure that forms the basidium.

Karyogamy and meiosis happen in the basidium, producing four basidiospores on its outside (exogenous). Basidia sit in fruiting bodies called basidiocarps. Examples: Agaricus (mushroom), Ustilago (smut) and Puccinia (rust).

Deuteromycetes are the imperfect fungi, so called because only their asexual or vegetative phases are known. When a member's sexual stage is found, it is moved to its proper class, often ascomycetes or basidiomycetes.

Deuteromycetes reproduce only by conidia and have septate, branched mycelium. Some are saprophytes or parasites, and many decompose litter and help mineral cycling. Examples: Alternaria, Colletotrichum and Trichoderma.