Fungi are organisms such as yeasts, moulds, mildews and rusts, smuts or sooty moulds, puff balls, mushrooms, and bracket fungi. They may be microscopic in size or up to more than a foot in diameter. Unlike other plants, fungi possess none of the green pigment chlorophyll, necessary for the utilisation of CO2 from the air to form carbohydrates. Instead, they are dependent upon the substrate on which they grow to provide those food substances so essential for their growth and development. They live either saprophytically on dead organic material or as parasites on living organisms belonging to the plant or animal kingdom. Unlike flowering plants they reproduce themselves by spores, not seeds. These spores are minute, often under a hundredth of a millimetre in diameter, and are borne on elaborate fruiting bodies or sporophores. It is the fruiting body which is usually the most conspicuous part of the fungus plant. It varies in size from something less than a millimetre in diameter to more than 20 cm; it may be an elaborate structure, varying from a more or less undifferentiated mass of mycelium to a small, black, globose body, or a mass of brightly coloured fleshy discs, a puff ball or a mushroom. Identification of fungi is based on the microscopic structure of these fruiting bodies, since the vegetative mycelium (hyphae) which ramifies throughout the growth medium is of a more uniform character. It must be remembered that there are as many, if not more, species of fungi than there are seed plants and that the humid mild climate experienced in New Zealand is ideal for fungal growth.
There are three main groups of fungi:
Phycomycetes (literally algal fungi) which include filamentous fungi resembling colourless algae. They are chiefly aquatic and contain many soil and water moulds, as well as downy mildews of plants and the common bread mould, Rhizopus.
Ascomycetes or sac fungi, where typically eight spores are borne within a saclike hypha called an ascus. Such structures are formed either singly (in a yeast) or are united on an elaborate fruiting body which may be an apothecia (a disc-like fruiting body in which the fertile part is exposed) (2) or a perithecia (a flask-shaped body in which the fertile region is covered, opening to the exterior by a pore) (1)
Basidiomycetes, in which four spores are borne superficially on a structure known as a basidia. They may be formed singly as in rusts (5) or united on a large elaborate fruiting body, such as a mushroom (9) or a polypore (11), a bracket fungus (10), or a puff ball (7).
There is a subsidiary group, known as the fungi imperfecti, which includes forms with asexual spores. In some genera within this group a complete life cycle is unknown or may not exist. It includes many important plant pathogens which are responsible for fruit rots as well as leaf spots, stem or root rots of plants, and includes the common moulds, Aspergillus and Penicillium (4).