Encyclopedia Britannica definitions related to fungi

plural FUNGI, any of about 50,000 species of organisms of the kingdom Fungi, or Mycota--including 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 mycology--a 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

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.

Annotated classification

The following classification is adapted from G.W. Martin in Ainsworth's Dictionary of the Fungi, 5th ed. (1961).

Kingdom Mycota (fungi)

Eukaryotic (with true nuclei), achlorophyllous (without chlorophyll), acellular, unicellular, or multicellular organisms; microscopic or macroscopic in size; usually with cell walls and filaments; typically reproducing by spores produced asexually or sexually; walls containing chitin, cellulose, or both, among other substances; about 50,000 living species; fewer than 500 fossil species known.

Division Eumycota (true fungi)

Assimilative stage walled, typically filamentous (a mycelium), sometimes unicellular, usually eucarpic (having only part of the thallus forming a fruiting structure); asexual reproduction by fission, budding, fragmentation, or, more typically, by spores; sexual reproduction by various means, usually resulting in the formation of resting structures or meiospores.

Class Chytridiomycetes

Unicellular or filamentous, holocarpic (having all of the thallus involved in the formation of the fruiting body) or eucarpic; motile cells (zoospores or planogametes) characterized by a single, posterior, whiplash flagellum; mostly aquatic fungi saprobic or parasitic on algae, fungi, or, less often, on flowering plants.

Order Chytridiales

Mycelium lacking but rhizoids (short absorbing filaments) or rhizomycelium often present; chiefly freshwater saprobes or parasites of algae and fungi; some terrestrial species, such as Olpidium brassicae and Synchytrium endobioticum, cause plant disease; about 550 species.

Order Blastocladiales

Water molds with a restricted thallus, characterized by the production of thick-walled, pitted, resistant sporangia; sexual reproduction by isogamous (equal in size and alike in form) or anisogamous (unequal in size but still similar in form) planogametes; Allomyces exhibits an alternation of 2 equal generations; most are saprobes, but various species of Coelomomyces are parasitic in mosquito larvae; uniquely, their hyphae are devoid of cell walls; more than 50 species.

Order Monoblepharidales

Water molds with an extensive, foamy mycelium; sexual reproduction by a motile male gamete (antherozoid) fertilizing a nonmotile differentiated egg, resulting in a thick-walled oospore; about 20 species.

Class Hyphochytridiomycetes

A small group of mostly marine fungi very similar to the order Chytridiales but with motile cells bearing a single tinsel flagellum (i.e., a flagellum with short side branches along the central axis, comblike).

Order Hyphochytriales

Characters of the class; about 15 species.

Class Plasmodiophoromycetes

Endoparasites (internal parasites) of fungi or plants often causing hypertrophy (excessive abnormal growth); assimilative stage an endophytic (living within plant tissue) plasmodium that becomes converted into a group of zoosporangia (structures producing motile asexual spores) or a large number of small, walled spores; motile cells with 2 unequal, anterior, whiplash flagella.

Order Plasmodiophorales

Characters of the class; Plasmodiophora and Spongospora cause serious plant diseases; about 35 species.

Class Oomycetes

Aquatic, amphibious, or terrestrial fungi; saprobic, facultatively (occasionally) or obligately (invariably) parasitic on plants, a few on fish; asexual reproduction typically by zoospores with 2 anterior or lateral flagella, 1 whiplash, 1 tinsel; sexual reproduction usually by contact of differentiated gametangia (gamete- or sex-cell-producing structures) with nuclei from the male fertilizing differentiated eggs and resulting in thick-walled oospores; thallus probably diploid with meiosis occurring in the gametangia.

Order Lagenidiales

Holocarpic, unicellular or filamentous water molds, parasitic on algae and fungi or saprobic; oogonium (egg-producing structure) typically containing a single egg; about 85 species.

Order Saprolegniales

Mostly eucarpic, filamentous water molds or soil fungi; saprobic or parasitic; hyphae without constrictions or cellulin plugs; oogonia containing 1 to many free eggs; some species are diplanetic, i.e., they produce 2 types of zoospores, primary (pear-shaped with anterior flagella) and secondary (kidney-shaped with lateral flagella); some (Aphanomyces) cause root rots; others (Saprolegnia) infect fish and fish eggs; about 200 species.

Order Leptomitales

Aquatic saprobes found often in polluted waters; eucarpic; hyphae constricted, with cellulin plugs, arising from a well-defined basal cell; oogonium typically containing a single egg, which may be free or embedded in periplasm (a peripheral layer of protoplasm); 20 species.

Order Peronosporales

Aquatic or terrestrial; parasitic on algae or vascular plants, the latter mostly obligate parasites causing downy mildews; zoosporangia, in advanced species, borne on well-differentiated sporangiophores, deciduous and behaving as conidia (asexually produced spores); about 250 species.

Class Zygomycetes

Terrestrial saprobes or parasites of plants, animals, or humans; asexual reproduction by aplanospores (nonmotile spores) in sporangia or by conidia; sexual reproduction by fusion of morphologically similar gametangia, sometimes differing in size, resulting in thick-walled zygospores.

Order Mucorales

Often called the bread molds; saprobic, weakly parasitic on plants, or parasitic on humans and then causing mucormycosis (a pulmonary infection); asexual reproduction by sporangiospores, 1-spored sporangiola (a small deciduous sporangium), or conidia; in the genus Pilobolus the heavily cutinized sporangium is forcibly discharged; about 360 species.

Order Entomophthorales

Insect parasites or saprobes, some implicated in animal or human diseases; asexual reproduction by modified sporangia functioning as conidia, forcibly discharged; about 150 species.

Order Zoopagales

Parasitic on amoebas, rotifers, nematodes, or other small animals, which they trap by various specialized mechanisms; asexual reproduction by conidia borne singly or in chains, not forcibly discharged; about 60 species.

Class Trichomycetes

Commensals (organisms living parasitically on another organism but conferring some benefit in return, or at least not harming the host) with a filamentous thallus attached by a holdfast or basal cell to the digestive tract or external cuticle of living arthropods; asexual reproduction by sporangiospores (a spore borne within a sporangium), trichospores (zoospores or ciliated spores), arthrospores (a spore resulting from fragmentation of a hypha), or amoeboid cells; sexual reproduction, where known, zygomycetous.

Order Amoebidiales

Thallus coenocytic (without cross walls, with numerous freely distributed nuclei) arising from a holdfast; amoeboid cells formed; about 12 species.

Order Eccrinales

Thallus coenocytic, attached by a holdfast to the digestive tract of arthropods; aplanosporangia produced in succession; more than 50 species.

Order Asellariales

Thallus branched, septate, attached by a basal coenocytic cell; asexual reproduction by arthrospores; 6 species.

Order Harpellales

Thallus simple or branched, septate; asexual reproduction by trichospores; sexual reproduction zygomycetous; about 35 species.

Class Ascomycetes

Saprobic or parasitic on plants, animals, or humans; some are unicellular but most are filamentous, the hyphae septate with 1, rarely more, perforations in the septa; cells uninucleate or multinucleate; asexual reproduction by fission, budding, fragmentation, or, more typically, by conidia usually produced on special sporiferous (spore-producing) hyphae, the conidiophores, which are borne loosely on somatic (main-body) hyphae or variously assembled in asexual fruiting bodies; sexual reproduction by various means resulting in the production of meiosphores (ascospores) formed by free-cell formation in saclike structures (asci), which are produced naked or, more typically, are assembled in characteristic open or closed fruiting bodies (ascocarps); among the largest and most commonly known ascomycetes are the morels, cup fungi, saddle fungi, and truffles.

Subclass Hemiascomycetidae

Asci naked, formed from single cells or on hyphae; no ascocarps or ascogenous hyphae produced; saprobic or parasitic.

Order Protomycetales

Spore sac compound (a synascus); a poorly known small group of plant-parasitic ascomycetes; 20 or more species.

Order Endomycetales

Mostly saprobic, a few parasitic; zygote or single cell transformed directly into the ascus; mycelium sometimes lacking; this group includes the yeasts and their relatives.

Order Taphrinales

Parasites on vascular plants; asci produced from binucleate ascogenous (ascus-producing) cells formed from the hyphae in the manner of chlamydospores (thick-walled spores); 90 or more species.

Subclass Euascomycetidae

Asci unitunicate (ascus wall single-layered), borne in various types of ascocarps; saprobic or parasitic on plants, animals, or humans.

Order Eurotiales

Asci globose to broadly oval, typically borne at different levels in cleistothecia (completely closed ascocarp or fruiting structure); most of the human and animal dermatophytes belong here, also many saprobic soil or coprophilous fungi; possibly up to 150 species.

Order Microascales

Asci evanescent (quickly deteriorating), borne at different levels in perithecia (closed ascocarps with a pore in the top) with ostioles (the opening of the perithecium), or sometimes a long necklike structure terminating in a pore; some serious plant parasites such as Ceratocystis ulmi (Dutch elm disease) and C. fagacearum (oak wilt) belong here; about 100 species.

Order Onygenales

Asci formed in a mazaedium (a fruiting body consisting of a powdery mass of free spores interspersed with sterile threads, enclosed in a peridium or wall structure), evanescent, and liberating the ascospores as a powdery mass among sterile threads; about 25 species.

Order Erysiphales

Obligate parasites on flowering plants causing powdery mildews; mycelium white, superficial in most, feeding by means of haustoria sunken into the epidermal cells of the host; 1 to several asci in a cleistothecium, if more than 1, in a basal layer at maturity; asci globose to broadly oval; cleistothecia with appendages; about 150 species.

Order Meliolales

Mycelium dark, superficial on leaves and stems of vascular plants, typically bearing appendages (termed hyphopodia or setae); asci in basal layers in ostiolate perithecia without appendages; mostly tropical fungi; more than 1,000 species.

Order Chaetomiales

Asci in basal layers in superficial perithecia that bear conspicuous, straight or curly, simple or branched hairs on the surface; asci evanescent; about 110 species.

Order Xylariales

Perithecia with dark, membranous or carbonous (appearing as black burned wood) walls, with or without a stroma (a compact structure on or in which fructifications are formed); asci persistent, borne in a basal layer among paraphyses (elongate structures resembling asci but sterile), which may ultimately gelatinize and disappear; a rather large group of fungi one of which, Neurospora crassa, has been used extensively in genetic and biochemical studies; approximately 4,500 species.

Order Diaporthales

Perithecia immersed in plant tissue or in a stroma with their long ostioles protruding; ascal stalks gelatinizing, freeing the asci from their basal attachment; paraphyses lacking; the chestnut blight fungus (Endothia parasitica) belongs here; close to 500 species.

Order Hypocreales

Perithecia and stromata when present, brightly coloured, soft, fleshy, or waxy, when fresh; asci borne in a basal layer among apical paraphyses; about 800 species.

Order Clavicipitales

Perithecia immersed in a stroma that issues from a sclerotium (a hard-resting body resistant to unfavourable environmental conditions); asci with a thick apex penetrated by a central canal through which the septate, threadlike ascospores are ejected; the ergot fungus (Claviceps purpurea), cause of ergotism in plants, animals, and humans, and the original source of LSD, belongs to this order; some other Clavicipitales parasitize insect larvae; about 170 species.

Order Coryneliales

Asci in ascostromata with funnel-shaped ostioles at maturity; about 20 species.

Order Coronophorales

Asci in ascostromata with irregular or round, never funnel-shaped, openings; about 30 species.

Order Laboulbeniales

Ascomycetes of uncertain affinity; minute parasites of insects and arachnids with mycelium represented only by haustoria and stalks; about 1,635 species.

Order Ostropales

Ascocarp a loculelike apothecium (an open, often cuplike ascocarp); asci inoperculate (without a terminal pore) constructed as in the Clavicipitales; ascospores septate, threadlike; about 80 species.

Order Phacidiales

Ascocarp an apothecium immersed in a black stroma, the upper covering of which splits in stellate (star-shaped) or irregular fashion when ascospores mature; about 150 species.

Order Helotiales

Ascocarp an apothecium bearing inoperculate asci exposed from an early stage; some important plant diseases are caused by members of this group (for example, Monilinia fructicola causes brown rot of stone fruits), the earth tongues (Geoglossaceae) also belong here; more than 1,500 species.

Order Pezizales

Ascocarp an apothecium bearing operculate (with a hinged cap) asci above the ground; apothecia often large, cup- or saucer-shaped, spongy, brainlike, saddle-shaped, etc.; this group includes the morels, the false morels, and the saddle fungi among others; about 700 species.

Order Tuberales (truffles)

The ascocarps, mostly closed and borne below the ground, are considered to be modified apothecia; the asci are globose, broadly oval, or club-shaped; about 230 species.

Subclass Loculoascomycetidae

Asci bitunicate, borne in ascostromata; saprobic or parasitic on plants.

Order Myriangiales

Asci borne singly in locules arranged at various levels in a more or less globose stroma; about 100 species.

Order Dothideales

Asci borne in fascicles (clusters) in a locule devoid of sterile elements; about 600 species.

Order Pleosporales

Asci borne in a basal layer among pseudoparaphyses; more than 2,000 species.

Order Microthyriales

Stroma flattened, hemispherical, opening by a pore or tear; base usually lacking; asci borne among pseudoparaphyses; mostly tropical fungi; about 1,200 species.

Order Hysteriales

Stroma boat-shaped, opening by a longitudinal slit, which renders it apothecium-like; asci borne among pseudoparaphyses; about 110 species.

Class Basidiomycetes

Saprobic or parasitic on plants or insects; filamentous; the hyphae septate, the septa typically inflated (dolipore) and centrally perforated; mycelium of 2 types, primary of uninucleate cells, succeeded by secondary, consisting of dikaryotic cells, this often bearing bridgelike clamp connections over the septa; asexual reproduction by fragmentation, oidia (thin-walled, free, hyphal cells behaving as spores), or conidia; sexual reproduction by fusion of hyphae (somatogamy), fusion of an oidium with a hypha (oidization), or fusion of a spermatium (a nonmotile male structure that empties its contents into a receptive female structure during plasmogamy--a kind of gamete) with a specialized receptive hypha (spermatization), resulting in dikaryotic hyphae that eventually give rise to basidia, either singly on the hyphae or in variously shaped basidiocarps; meiospores (basidiospores) borne on basidia; in the rusts (Uredinales) and smuts (Ustilaginales), the dikaryotic hyphae produce teleutospores (thick-walled resting spores), which are a part of the basidial apparatus; this is a large class of fungi containing the rusts, smuts, jelly fungi, club fungi, coral and shelf fungi, mushrooms, puffballs, stinkhorns, and bird's-nest fungi.

Subclass Heterobasidiomycetidae

Basidia septate or deeply divided or arising from a teleutospore or cyst; basidiospores often germinating by repetition, budding, or production of conidia; includes the jelly fungi, the rusts, and the smuts.

Order Tremellales (jelly fungi)

Fruiting bodies (basidiocarps) well-formed, appearing as inconspicuous horny crusts when dry but usually bright-coloured to black gelatinous masses after a rain; a few are parasitic on mosses, vascular plants, or insects; most are saprobes; about 500 species.

Order Uredinales (rusts)

Parasitic on vascular plants; basidial apparatus consists of a thick-walled teleutospore (probasidium), which either gives rise to a 4-celled tube (metabasidium) on which basidiospores are borne or which itself becomes 4-celled and produces basidiospores directly; basidiospores forcibly discharged; many rusts are heteroecious, i.e., they require 2 species of host to complete their life cycle; rusts are among the fungi most destructive to agriculture; about 4,600 species.

Order Ustilaginales (smuts)

Called smuts because the masses of spores (sori) are usually black and dusty; basidial apparatus consisting of a thick-walled teleutospore (probasidium), which, upon germination, gives rise to a septate or nonseptate tube (metabasidium), which bears the basidiospores; basidiospores not forcibly discharged, germinating usually by budding or by fusing and then producing a mycelial germ tube; various cereal smuts are of great economic importance; about 700 species.

Subclass Homobasidiomycetidae

Includes the great majority of the Basidiomycetes; most produce conspicuous, large-fruiting bodies, which bear the spores on basidia; basidia are simple, cylindrical, or club-shaped; basidiospores, which may or may not be forcibly discharged, germinate directly into a mycelium.

Order Exobasidiales

Basidiocarps lacking; basidia produced in a layer on the surface of parasitized vascular plants; 15 species.

Order Polyporales

Basidiocarps present; a large and probably heterogeneous order of fungi in which the basidia are borne in various ways but rarely on gills; includes the coral fungi, the club fungi, the chanterelles, and the pore (shelf or bracket) fungi among others; common genera include Stereum, Clavaria, Hydnum, Cantharellus, Polyporus, Fomes; Schizophyllum has been used extensively for genetic research; up to 2,500 species.

Order Agaricales (mushrooms and boletes)

Basidia produced in layers (hymenia) on the underside of fleshy fruiting bodies (basidiocarps), in tubes (boletes) or on gills (mushrooms); some of these fungi form mycorrhizae, some are parasitic and cause root rots; most are saprobic; 4,000 to 5,000 species.

Order Hymenogastrales

Basidiocarps underground or on the surface but usually buried in humus, remaining closed, the interior (gleba) disintegrating into a slimy mass containing the spores; about 225 species.

Order Lycoperdales (puffballs)

Gleba dry and powdery at maturity; consisting of small, pale spores and well-developed capillitium; about 160 species.

Order Sclerodermatales

These are puffballs with a hard peridium enclosing a dry, powdery gleba consisting of large, dark spores and some capillitium; about 120 species.

Order Phallales (stinkhorns)

Gleba slimy and fetid at maturity; exposed on an elongated or net-shaped receptacle; Phallus, Mutinus, Dictyophora, Simblum, Clathrus are temperate-zone genera; about 70 species.

Order Nidulariales (bird's-nest fungi)

The gleba separates into chambers, which become thick-walled, waxy, and hard--these are the peridioles ("eggs"), which are evident within a cuplike or gobletlike basidiocarp, the whole resembling a bird's nest at maturity--Cyathus and Crucibulum are the 2 most widely distributed genera; about 60 species.

Form-class Deuteromycetes

Fungi with septate mycelium reproducing only asexually and resembling asexual stages of Ascomycetes and Basidiomycetes.

Form-order Sphaeropsidales

Conidia borne in pycnidia; about 5,500 species.

Form-order Melanconiales

Conidia borne in acervuli; about 1,000 species.

Form-order Moniliales

Conidia borne on variously assembled conidiophores but never in pycnidia or acervuli; 10,000 or more species.

Form-order Mycelia Sterilia

No conidia produced; probably mycelial stages of Basidiomycetes; about 200 species.

Critical appraisal

Although the fungi were traditionally classified in the plant kingdom, some of their characteristics argue strongly against such affinities. Chief among them are their heterotrophy and the almost universal presence of chitin, which is also found in the skin of many invertebrate animals such as insects, in their walls. The classification followed in this article and in other articles removes the fungi from the plant kingdom and includes them instead in a separate kingdom--Fungi. The slime molds (Myxomycetes, or Mycetozoa) are placed in the kingdom Protista. Older classification systems applied the name Phycomycetes (algal fungi) to the six classes of "lower fungi" (Chytridiomycetes, Hyphochytridiomycetes, Plasmodiophoromycetes, Oomycetes, Zygomycetes, and Trichomycetes). Although no longer recognized as a formal taxonomic category, the name Phycomycetes is still a useful term for designating those fungi that produce their spores in sporangia and have, in most cases, coenocytic hyphae. These "lower fungi" have been included by some protistologists in the kingdom Protista (see protist). A definitive classification of the "lower fungi" and protists remains to be agreed upon, however, and the classifications presented above and in the article protist reflect differences among scientists concerning taxonomy.

The groups that are designated as the form-class Deuteromycetes (also sometimes called "Fungi Imperfecti") consist of organisms whose sexual stages either have not been found or do not exist, together with the asexual stages of some sexually reproducing species. It has happened that sexual stages have been found for some species in this group, and, whenever this occurs, the species is immediately given a name appropriate to its proper taxonomic group. Sometimes, however, it has been found advisable to maintain the form-category name even though sexual stages are known, and this has led to the unusual condition of one species having been assigned two scientific names, only one of which, of course, is valid--the one applying to the sexual stage. The practice is justified mainly for the convenience of having a classification system, artificial though it is, into which conidial (asexual) forms may be grouped and studied. It has even been found useful to extend the concept of form categories to include the conidial stages of known sexually reproducing fungi, mainly Ascomycetes. Since many of these fungi, particularly the parasitic ones, are usually encountered only in the conidial stage, they can be easily identified by making use of the form-category classification. The convenience is great and the practice so widespread that in 1950 the International Botanical Congress legalized the use of form-names for conidial stages, recognizing, of course, the name of the perfect (sexual) stage as the official name of the whole organism.

(C.J.Al.) (Ed.) ? 1999-2000 Britannica.com Inc.

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