A eukaryotic organism belonging to the kingdom Fungi. Its cell walls are made from chitin and a fungus is heterotrophic, i.e. it obtains food by absorbing substances rather than synthesizing them. Fungi range in size from being single-celled to being very large; a honey fungus (Armillaria ostoyae) in Oregon occupies 8.9 km2 and probably weighs more than 600 tonnes. A multi-celled fungus consists of a network of fine filaments, hyphae, which form a mycelium. It reproduces by spores, produced in and released from a fruiting body that in many species is the visible ‘mushroom’ or ‘toadstool’. Other species form a feeding body composed of a multi-nucleated mass of protoplasm. Fungi occur worldwide and there may be 5 million species.