FUNGUS

plural Fungi, any of about 50,000 species of organisms of the kingdom Fungi, or Mycota, including yeasts, rusts, smuts, molds, mushrooms, and mildewsthat lack chlorophyll and the organized plant structures of stems, roots, and leaves. The study of fungi (Latin: mushrooms) is called mycology (from the Greek mykes, mushrooms) because mushrooms are the most conspicuous members of Fungi. Fungi contribute to the disintegration of organic matter that results in the release of carbon, oxygen, nitrogen, and phosphorus from dead plants and animals into the soil or the atmosphere. Fungi also form symbiotic and parasitic relationships with living organisms. Fifty thousand species of fungi have been described, but it has been estimated that the total number may be as high as 100,000250,000. They can be found in the water, soil, air, plants, and animals of all regions of the world having sufficient moisture to enable them to grow. Essential to many household and industrial processes, fungi are also used in the production of enzymes, organic acids, vitamins, and antibiotics. Penicillin, a green mold whose abilities to inhibit the growth of bacteria were first discovered by Alexander Fleming in 1928, is just one of many fungi with beneficial effects on the human environment. Fungi also can destroy crops, cause such diseases as athlete's foot and ringworm, and ruin clothing and food with mildew and rot. In suitable environments, fungi can live for hundreds of years, and specimens of one genus, Armillaria (q.v.), are among the oldest and largest of living organisms. The thallus, or body, of a typical fungus consists of a mycelium, which is a mass of branched, tubular filaments called hyphae (singular: hypha) through which cytoplasm flows. The cell walls of the hyphae are structurally complex and vary in different fungal groups. Most contain either cellulose or a tough carbohydrate substance (chitosan, chitin, or both) similar to cellulose. The cell walls serve a regulatory function in the interchange of materials between the hyphae and their external environment. The mycelium generally reproduces by forming spores, either directly or in special fruiting bodies that are generally the visible part of the fungus. The simplest method of asexual reproduction is a fragmentation of the thallus, either by breaking off a portion of the network of hyphae or by single-cell fission that occurs in some yeasts. Another method common in yeasts is budding, a process by which the nucleus of the parent cell divides and one of the daughter nuclei migrates into a bud on the surface of the cell and the other remains in the parent cell. The majority of fungi reproduce asexually by the formation of bodies called mitospores either directly on the hyphae or on special sporiferous (spore-producing) hyphae, which may be grouped into intricate structures called fruiting bodies. More primitive fungi produce spores endogenously in sporangia, which are saclike fruiting bodies whose entire cytoplasmic content divides into spores. These can be either naked and flagellating (zoospores) or walled and nonmotile (aplanospores). Zoospores swim through moisture in aquatic or terrestrial environments, eventually loosing their flagella and forming walls, inside which each zoospore germinates and grows into a system of hyphae. Spores can also be produced sexually. The type of sporophore that is produced is characteristic of the group to which a fungus belongs. Even when both mitospores (spores formed by asexual means) and meiospores (spores formed by sexual means) are produced by the same fungus as it changes from an asexual to a sexual reproductive phase, they are very different in form and can be easily distinguished. The distribution of fungi is related to the availability of food and to moisture and temperature. The soil provides an ideal habitat for a large number of species. Most aquatic fungi prefer clean, cool waters. The optimum growth temperature is usually between 68 and 86 F (20 and 30 C). Since fungi possess no chlorophyll, they are unable to photosynthesize and must obtain their carbohydrates by secreting enzymes into the surface on which they are growing. The enzymes digest the food, which is then absorbed directly through the hyphal walls. Saprophytic fungi live off dead organisms and are partly responsible for the decomposition of organic matter. Parasitic fungi invade living organisms to obtain their food, often causing disease and death. Plants are the most common hosts, but humans and lower animals also serve as hosts. Symbiotic relationships include those between fungi and algae (lichen), plants (mycorrhizae), and certain insects. Fungi were formerly classified in the plant kingdom and are still considered plants in some classification systems. The chitin in their structures and their ability to obtain food from an outside source, however, have caused many taxonomists to propose that they be classified in a separate kingdom, Fungi. plural fungi any of about 50,000 species of organisms of the kingdom Fungi, or Mycotaincluding yeasts, rusts, smuts, mildews, molds, and mushrooms. They are among the most widely distributed organisms on Earth and are of great importance. Many fungi are free-living in soil or water; others form parasitic or symbiotic relationships with plants or animals, respectively. Historically, the fungi were included in the plant kingdom, but because they lack chlorophyll and the organized plant structure of stems, roots, and leaves, they are now considered to constitute a separate kingdom. Fungi are eukaryotic organisms having two common characteristics: anatomically, their principal mode of vegetative growth is through mycelium; physiologically, their nutrition is based on absorption of organic matter. They are the culmination of a major direction in evolution distinctly different from that of plants or animals, an evolutionary line established by organisms whose nutrition was based on absorption of organic matter. The mushrooms, by no means the most numerous or economically significant of the fungi, are the most conspicuous members of the group; thus, the Latin word for mushroom, fungus (plural fungi), has come to stand for the whole group. Similarly, the study of fungi is known as mycologya broad application of the Greek word for mushroom, mykes. Fungi other than mushrooms are sometimes collectively called molds, although this term is better restricted to fungi of the sort represented by bread mold. (Slime molds, straddling the animal and plant worlds, are treated in the article protist.) Additional reading General works D.L. Hawksworth, B.C. Sutton, and G.C. Ainsworth, Ainsworth & Bisby's Dictionary of the Fungi, 7th ed. (1983), remains the standard reference for terminology and definitions; Walter H. Snell and Esther A. Dick, A Glossary of Mycology, rev. ed. (1971), is an excellent dictionary of mycological terms; John Ramsbottom, Mushrooms & Toadstools (1953), offers a beautifully illustrated discussion of the occurrence and activities of one group of fungi; and Constantine J. Alexopoulos and Charles W. Mims, Introductory Mycology, 3rd ed. (1979), is an excellent text for both beginning and advanced students. Elizabeth Moore-Landecker, Fundamentals of the Fungi, 3rd ed. (1990), is a good introduction. Other introductions include Lilian E. Hawker, Fungi, 2nd ed. (1974); John Webster, Introduction to Fungi, 2nd ed. (1980); and J.H. Burnett, Fundamentals of Mycology, 2nd ed. (1976), somewhat difficult for the novice. Harold C. Bold, Constantine J. Alexopoulos, and Theodore Delevoryas, Morphology of Plants and Fungi, 5th ed. (1987); and C.T. Ingold, The Biology of Fungi, 5th ed. (1984), offer surveys of the subject. Frederick A. Wolf and Frederick T. Wolf, The Fungi, 2 vol. (1947, reissued 1969), discusses morphology, taxonomy, physiology, genetics, ecology, and medical and industrial mycology. Ernst Athearn Bessey, Morphology and Taxonomy of Fungi (1950, reprinted 1985), is a reference strong on phylogeny and the bibliography of classification. William D. Gray, The Relation of Fungi to Human Affairs (1959), discusses useful and destructive fungi, with emphasis on the application of mycology to industry; and D.L. Hawksworth and B.E. Kirsop (eds.), Filamentous Fungi (1988), explores applications of these fungi to biotechnology. G.C. Ainsworth and Alfred S. Sussman (eds.), The Fungi, 4 vol. in 5 (196573), an advanced treatise written by specialists in various fields, discusses fungi in terms of cells, the organism, populations, and classification. A large number of topics on the biology of fungi are discussed by specialists in the following collections: John E. Smith, David R. Berry, and Bjorn Kristiansen (eds.), The Filamentous Fungi, 4 vol. (197583); Anthony H. Rose and J. Stuart Harrison (eds.), The Yeasts, 2nd ed., 3 vol. (198789); and Garry T. Cole and Bryce Kendrick (eds.), Biology of Conidial Fungi, 2 vol. (1981). Questions of morphological development are covered in John E. Smith (ed.), Fungal Differentiation: A Contemporary Synthesis (1983); Paul J. Szaniszlo and James L. Harris (eds.), Fungal Dimorphism: With Emphasis on Fungi Pathogenic for Humans (1985); and G. Turian and H.R. Hohl (eds.), The Fungal Spore, Morphogenetic Controls (1981). A.H. Reginald Buller, Researches on Fungi, 7 vol. (190950), is a classic collection of studies on various aspects of fungi, particularly strong on spore dispersal, development, and sexual reproduction; the first six volumes were reprinted in 1958. A useful guide of laboratory procedures for studying and handling fungi is presented in Russell B. Stevens (ed.), Mycology Guidebook (1974, reprinted with corrections and index by Joseph F. Ammirati, 1981). General discussion of physiological topics include Michael O. Garraway and Robert C. Evans, Fungal Nutrition and Physiology (1984); David H. Griffin, Fungal Physiology (1981); and Ian K. Ross, Biology of the Fungi: Their Development, Regulation, and Associations (1979). For an overview of progress in modern genetics of fungi, including genetic engineering, see J.W. Bennett and Linda L. Lasure (eds.), Gene Manipulations in Fungi (1985); William E. Timberlake (ed.), Molecular Genetics of Filamentous Fungi (1985); and John F. Peberdy and Lajos Ferenczy (eds.), Fungal Protoplasts: Application in Biochemistry and Genetics (1985). Special subjects Lucy Kavaler, Mushrooms, Molds and Miracles (1965), discusses various discoveries of fungal products. Valentina P. Wasson and R. Gordon Wasson, Mushrooms, Russia, and History, 2 vol. (1957), is an ethnomycological classic. C.T. Ingold, Dispersal in Fungi (1953, reprinted with additions 1968), and Spore Liberation (1965), discuss the means by which fungi liberate their spores. Karl Esser and Rudolf Kuenen, Genetics of Fungi (1967; originally published in German, 1965), offers a general treatment; and John R. Raper, Genetics of Sexuality in Higher Fungi (1966), explores life cycles and sexual mechanisms in Ascomycetes and Basidiomycetes. D. Parkinson and J.S. Waid (eds.), The Ecology of Soil Fungi (1960), is a series of essays; T.W. Johnson, Jr., and F.K. Sparrow, Jr., Fungi in Oceans and Estuaries (1961), provide important references; and Wm. Bridge Cooke, The Fungi of Our Mouldy Earth, new ed. (1986), studies the fungi of the environment, with emphasis on water. Chester W. Emmons et al., Medical Mycology, 3rd ed. (1977); and Clyde M. Christensen, Molds, Mushrooms, and Mycotoxins (1975), study pathogenic fungi. J. Walter Wilson and Orda A. Plunkett, The Fungous Diseases of Man (1965), is a medical treatise. John Willard Rippon, Medical Mycology: The Pathogenic Fungi and the Pathogenic Actinomycetes, 3rd ed. (1988), is a textbook. E.C. Large, The Advance of the Fungi (1940, reprinted 1962), is a classic book on plant-disease fungi. Special biochemical topics are discussed in J.W. Bennett and Alex Ciegler (eds.), Secondary Metabolism and Differentiation in Fungi (1983); and John D. Weete, Lipid Biochemistry of Fungi and Other Organisms (1980). Special groups of fungi Clyde M. Christensen, Common Fleshy Fungi, 2nd ed. (1955), is an easy-to-use manual for identifying common mushrooms; J. Walton Groves, Edible and Poisonous Mushrooms of Canada, rev. ed. (1979), identifies mushrooms; L.R. Hesler, Mushrooms of the Great Smokies (1960), is a field guide, with black-and-white photographs; Ren Pomerleau and H.A.C. Jackson, Mushrooms of Eastern Canada and the United States, trans. from French (1951), is a good manual; Alexander H. Smith, Mushrooms in Their Natural Habitats, 2 vol. (1949), is useful for the Pacific Northwest region; and Gary H. Lincoff, The Audubon Society Field Guide to North American Mushrooms (1981), identifies many species. Kenneth B. Raper and Charles Thom, A Manual of the Penicillia (1949, reprinted 1968); and Kenneth B. Raper and Dorothy I. Fennell, The Genus Aspergillus (1965, reprinted 1977), are helpful. See also K.J. Scott and A.K. Chakravorty (eds.), The Rust Fungi (1982); and Robert W. Lichtwardt, The Trichomycetes, Fungal Associates of Arthropods (1986). Lichens Mason E. Hale, Jr., The Biology of Lichens, 3rd ed. (1983), is an intermediate-level introduction, and How to Know the Lichens, 2nd ed. (1979), is an authoritative guide. Annie L. Smith, Lichens (1921, reprinted with additions 1975), a classic work on lichenology, is still a useful reference source. See also David L. Hawksworth and David J. Hill, The Lichen-Forming Fungi (1984). Yasuhiko Asahina and Shoji Shibata, Chemistry of Lichen Substances (1954, reprinted 1971; originally published in Japanese, 1949), is a classic reference book. Bruce Fink, The Lichen Flora of the United States (1935, reissued 1971), is an advanced source. G.G. Nearing, The Lichen Book (1947, reissued 1962), is a good popular guide with drawings, popular names, and descriptions. Constantine John Alexopoulos Vernon Ahmadjian The Editors of the Encyclopdia Britannica Evolution and phylogeny The origin of the fungi is obscure, the fossil record being scanty and virtually meaningless. The older theory supposed the fungi to have originated by loss of chlorophyll from one or two groups of algae, one school favouring the development of the fungi monophyletically (i.e., from a single ancestor) from the green algae, the other postulating that the lower fungi originated from the green algae but that the Ascomycetes came from the class Florideae of the red algae and later gave rise to the Basidiomycetes. Most present-day mycologists derive the fungi from ancestral flagellates (algal or protozoan organisms bearing flagella, whiplike swimming organs) but yield that the Oomycetes may belong to a different evolutionary line because of their unique biochemical and cytological features: they synthesize the amino acid lysine, they have cellulosic walls and a special organization of the tryptophan-synthesizing enzymes; their thallus is diploid; and they reproduce through oogamy (production of differentiated egg cells). As for the interrelationships among fungal groups, there is much controversy. The modern tendency is to emphasize flagellation as an important phylogenetic criterion in the lower fungi. Thus, all the posteriorly uniflagellate fungi (i.e., those with a single flagellum located at the rear end of the organism) are brought together in one class, Chytridiomycetes, all the anteriorly uniflagellate in the class Hyphochytridiomycetes, and so on, as may be seen in the section on classification below. Whether the Plasmodiophoromycetes should be grouped with the Myxomycetes (slime molds) and removed from the fungi because of their plasmodial phase and their swarm cells, which bear two anterior whiplash flagella, is a moot question. Biochemical characters have been useful markers to map the probable evolutionary relationships of fungi. Because of common biochemical attributes, such as similarity in wall composition (presence of both chitin and b-1,3-1,6-glucan), the same pattern of organization of tryptophan enzymes, and synthesis of lysine by the same unique pathway (aminoadipic acid), the Chytridiomycetes, Ascomycetes, and Basidiomycetes are believed to constitute the main axis of fungal evolution, with the flagellated Chytridiomycetes representing the most primitive or ancestral forms. Other major groups of fungi, Zygomycetes (mucorales), Hemiascomycetes (ascomycetous yeasts), and Heterobasidiomycetes (basidiomycetous yeasts), are also in the same evolutionary camp; they have chitinous walls and make lysine by the aminoadipic acid pathway but show significant differences in cell-wall composition and organization of tryptophan biosynthetic enzymes. Hence, these fungal groups are believed to be side branches from the main evolutionary axis. Virtually all mycologists agree that the Basidiomycetes have been derived from the Ascomycetes. This opinion is based on the similarity of the nuclear cycle of the ascus and basidium, the supposed homology of the clamp connection (a structure joining two adjacent cells in the Basidiomycetes) with the crozier (a hook-shaped terminal cell found in Ascomycetes), and the similarity of the binucleate mycelium of Basidomycetes with the ascogenous (ascus-producing) hyphae of Ascomycetes. Not all, however, are agreed as to which group of Ascomycetes gave rise to the Basidiomycetes, nor indeed as to which Basidiomycetes are primitive and which are advanced. Whether the holobasidium (simple, club-shaped basidium) or the heterobasidium (septate or deeply divided basidium) came first is a highly controversial question that has a great bearing on the origin of the Basidiomycetes. Also, the fact that the dolipore (inflated) septum has not been found in the rusts or smuts or in any clearly nonbasidiomycetous group poses an interesting question on the origin of the Basidiomycetes. Classification Distinguishing taxonomic features The fungi as a group are distinguished from other organisms by the nature of their somatic (body) and reproductive structures and by the mode of nutrition they employ. Within the division Mycota, the classes are further distinguished by variations of these characteristics, particularly those involving reproductive stages.