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True fungi, a group of heterotrophic organisms with absorptive nutrition, capable of utilizing insoluble food from outside the cell by secretion of digestive enzymes and absorption. Glycogen is the primary storage product of fungi. Most fungi have a well-defined cell wall that is composed of chitin and glucans. Spindle pole bodies, rather than centrioles, are associated with the nuclear envelope during cell division in most species. Typically, the fungal body (thallus) is haploid and consists of microscopic, branched, threadlike hyphae (collectively called the mycelium), which develop into radiating, macroscopic colonies within a substrate or host. The filamentous hypha may be divided by cross walls (septa) into compartments. Hyphal growth is apical. Some species are coenocytic (without cross walls); others, including yeasts, are unicellular. Reproductive bodies are highly variable in morphology and size, and may be asexual or sexual.
Great changes in understanding the phylogenetic relationships of fungi have been brought about by the use of characters derived from deoxyribonucleic acid (DNA) sequences and the use of computer-assisted phylogenetic analysis; these changes are reflected in current classification schemes. A modern classification follows:
- Phylum: Chytridiomycota
- Class: Zygomycetes
- Phylum: Ascomycota
- Class: Archiascomycetes
- Phylum: Basidiomycota
- Class: Hymenomycetes
Fungi are found in practically every type of habitat. Most are strictly aerobic, although a few are anaerobes that live in the gut of herbivores. Some species are thermophilic. Many fungi form saprobic (including parasitic) relationships with animals and plants; the majority are saprobes. As now recognized, the Eumycota are a monophyletic group of the crown eukaryotes, presumed to have been present in the fossil record 900–570 million years ago. See Fungi, Myxomycota, Oomycetes