Kingdom Fungi

Fungi cannot make their own food (they are heterotrophs). They are found everywhere, but like warm, humid places: think of mould on bread or orange peel. Apart from one-celled yeasts, a fungus is made of long threads called hyphae; a network of hyphae is a mycelium. Some hyphae are one continuous tube with many nuclei (coenocytic); others have cross walls (septate). Fungal walls are made of chitin and polysaccharides.

Most fungi live on dead matter (saprophytes). Some are parasites: Puccinia causes wheat rust and Albugo attacks mustard. Some live in partnership: in lichens (with algae) and in mycorrhiza (with plant roots). Useful fungi: yeast makes bread and beer; Penicillium gives antibiotics.

Fungi reproduce in three ways: vegetatively (fragmentation, fission, budding), by asexual spores (conidia, sporangiospores, zoospores), and by sexual spores (oospores, ascospores, basidiospores). The sexual cycle has three steps: plasmogamy (the cell contents of two cells fuse), karyogamy (the two nuclei fuse), then meiosis. In ascomycetes and basidiomycetes there is an in-between stage with two separate nuclei per cell (dikaryon, n + n).

The four classes:

• Phycomycetes: hyphae without cross walls; zoospores or aplanospores; thick-walled zygospores. Mucor, Rhizopus (bread mould), Albugo.
• Ascomycetes (sac fungi): asexual conidia; sexual ascospores formed inside sacs called asci, held in fruiting bodies (ascocarps). Penicillium, yeast, Aspergillus, Neurospora; edible morels and truffles.
• Basidiomycetes: no asexual spores; basidiospores on club-shaped basidia in basidiocarps. Agaricus (the mushroom), Ustilago (smut), Puccinia (rust).
• Deuteromycetes ("imperfect fungi"): only the asexual stage is known (Alternaria, Colletotrichum, Trichoderma). When the sexual stage is found, they move to another class.