Fungi, although often referred to as plants, are not closely related to this species group. For those amongst us who are fastidious about classification, I should add that in modern taxonomy, life forms are divided into six or more kingdoms, which are the Archaea, bacteria-like organisms living in extreme environments such as hot springs; the Eubacteria, which are the conventional bacteria like Escherichia coli; Fungi, with which this chapter is concerned; Chromista, which used to be called Protista (single-celled organisms such as Amoeba and Paramecium); Plantae (plants); and Animalia (animals). This six-kingdom taxonomy is now regarded as somewhat unsatisfactory, since the Chromista kingdom is clearly polyphyletic (having more than a single ancestor). This has led some recent experts to propose up to six additional kingdoms to replace the single Chromista, so we end up with twelve kingdoms instead of six. For further information on recent taxonomy, see Colin Tudge’s superb book The Variety of Life (2002, Oxford University Press).

Fungi, like plants, have cell walls, but in fungi these are made of chitin and not cellulose, as in plants. Also fungi do not photosynthesise. Fungal species and their relatives are hugely diverse, and include many troublesome plant diseases such as potato blight (Phytophthora infestans), now regarded as belonging to Chromista and not a true fungus, powdery mildews and black spot on our garden roses. The yeasts of brewing and baking are also fungi, as are the moulds we include in blue cheeses and the delicious underground fruiting bodies known as truffles. As almost all people know, the familiar mushrooms and toadstools are simply the fruiting bodies of extensive underground fungal networks called mycelia. The notorious pest fungus and tree killer called honey fungus (Armillaria mellea) has been cited as the largest living organism, a single mycelium occupying many hectares, yet being found by DNA testing to be a single continuous organism.