Kingdom Fungi

Kingdom Fungi comprises eukaryotic, heterotrophic, and (with the notable exception of unicellular yeasts) mostly multicellular organisms. The body of a typical fungus consists of long, slender, branched thread-like structures called hyphae, and the entire network of hyphae together makes up the mycelium. In some fungi, the hyphae are continuous tubes filled with multinucleate cytoplasm -- called coenocytic (aseptate) hyphae -- while in others, the hyphae are divided into cells by cross-walls known as septa, called septate hyphae. Fungal cell walls are made of a tough polysaccharide called chitin and (in many species) fungal cellulose, and the cell membrane is characteristically rich in a distinctive sterol called ergosterol.

Nutrition. Being heterotrophic, fungi obtain nutrition in one of three ways: most are saprophytic, absorbing nutrients from dead organic matter after secreting digestive enzymes onto it; many are parasitic, deriving nutrients from a living host and often causing serious plant diseases; and a smaller number are symbiotic, living in mutually beneficial association with other organisms, as seen in lichens (with algae) and mycorrhizae (with the roots of higher plants).

Reproduction. Fungi reproduce by vegetative means (fragmentation, fission and budding), by asexual means (typically through spores called conidia or sporangiospores), and by sexual means (through oospores, ascospores and basidiospores). The sexual cycle in fungi generally involves three distinct stages: plasmogamy, the fusion of protoplasms between two motile or non-motile gametes; karyogamy, the fusion of the two nuclei brought together by plasmogamy; and meiosis in the zygote, which restores the haploid number of chromosomes and, in many fungi, also results in the formation of spores.

On the basis of the structure of the mycelium, the mode of asexual spore formation, and the fruiting bodies produced during sexual reproduction, Kingdom Fungi is classified into four major classes.

Phycomycetes. These fungi are found predominantly in aquatic habitats and on decaying wood in moist and damp places, or as obligate parasites on plants. Their mycelium is aseptate and coenocytic. Asexual reproduction takes place through zoospores (motile) or aplanospores (non-motile), both produced inside a structure called a sporangium. A thick-walled zygospore is formed by the fusion of two gametes during sexual reproduction. Common examples include Mucor, Rhizopus (the bread mould fungus) and Albugo (the parasitic fungus responsible for the 'white rust' of crucifers).

Ascomycetes. Commonly called sac-fungi, this class is largely multicellular, as in Penicillium, though a few forms are unicellular, as in yeast (Saccharomyces). Ascomycetes may be saprophytic, decomposers, parasitic or coprophilous (living on dung). Their mycelium is branched and septate. Asexual spores called conidia are produced exogenously on specialised structures called conidiophores; on germination, a conidium produces a new mycelium. Sexual spores, called ascospores, are produced endogenously inside sac-like structures called asci (singular: ascus), arranged in various fruiting bodies known as ascocarps. Examples include Aspergillus, Claviceps and Penicillium, along with the cup-shaped morel mushroom Morchella and Neurospora, which is widely used in biochemical and genetic research.

Basidiomycetes. This class includes the familiar mushrooms, bracket fungi, and the puffballs, as well as important plant parasites such as rusts and smuts. They grow in soil, on logs and tree stumps, and as obligate parasites on living plants. Their mycelium is branched and septate. Vegetative reproduction commonly occurs through fragmentation. This class characteristically lacks asexual spores. Sexual reproduction gives rise to specialised club-shaped structures called basidia, on which basidiospores are formed exogenously (typically four spores on each basidium), after processes of plasmogamy and karyogamy have already occurred within the basidium itself. Basidiospores are formed in fruiting bodies known as basidiocarps. Common examples include Agaricus (the edible mushroom), Ustilago (the smut fungus) and Puccinia (the rust fungus). …