diverse group of organisms that obtain food by direct absorption of nutrients. The food is dissolved by enzymes that the fungi excrete, is then absorbed through thin cell walls, and is distributed by simple circulation, or streaming, of the PROTOPLASM, Together with BACTERIA, fungi are the chief saprobes, or saprophytes, that is, organisms responsible for the decay and decomposition of all organic matter. Except for most yeasts, fungi are multicellular. They are found wherever other forms of life exist. Some are parasitic on living matter and cause serious plant and animal diseases (see PARASITE,). The study of fungi is called mycology.

Fungi were at one time classified as a division in the kingdom Plantae. They were thought of as plants that have no stems or leaves and that in the course of becoming food absorbers lost the pigment chlorophyll, which is needed for conducting PHOTOSYNTHESIS,. Today, the organisms that were traditionally called fungi are classified differently. Most are commonly regarded as forming an entirely separate group, a kingdom called Fungi or, sometimes, Eumycota; the rest may then be grouped among the PROTISTA, (see Classification). In terms of evolution, fungi appear to be more closely related to animals than plants; they may have evolved from unpigmented FLAGELLATES, Approximately 100,000 species of fungi are known. The more complex groups are believed to have derived from the primitive types, which have flagellated cells at some stage in their life cycle.

Structure.

Most fungi are composed of delicate protoplasm-containing tubes known as hyphae, which frequently are partitioned by dividing walls called septa. One or two nuclei are found in each hyphal cell, and protoplasm moves through a tiny pore in the center of each septum. In the fungi of the division, or phylum, Zygomycota, however, the hyphae ordinarily do not have septa, and numerous nuclei are scattered throughout the protoplasm. Hyphae grow by elongation at the tips and also by branching. The resulting profusion of hyphae is called the mycelium. Abundant development of mycelium may result in the formation of large fruiting structures, or sporocarps, such as mushrooms and puffballs. Other types of massive hyphal structures enable some fungi to exist under difficult conditions or to spread to suitable nutritional sources. The cordlike strands of mycelium of the honey fungus, Armillaria, enable it to spread from the roots of one tree to another. Some fungi form resistant, more or less spherical masses of mycelium, called sclerotia, which may be smaller than grains of sand or as large as cantaloupes.

Reproduction.

Most fungi reproduce by spores, which are tiny particles of protoplasm enclosed in walls (see SPORE,). The common meadow, or field, mushroom, Agaricus campestris, may form 12 billion or more spores on its fruiting body; the giant puffball, Calvatia gigantea, may produce several trillion. See MUSHROOM,.

Spores are usually formed in one of two ways. In one process the spores form after the union of two or more nuclei within a specialized cell or series of cells. These spores typically germinate into hyphae that have different combinations of the hereditary characteristics of the parent nuclei. The types of spores that are produced in this way—zygospores, ascospores, and basidiospores—are representative of major groups of fungi. Zygospores are formed by conjugation of two similar sex cells. Ascospores are spores (usually eight) that are contained in sacs (asci), and basidiospores (usually four) are contained in clublike structures (basidia).

The other usual method of spore production involves the transformation of hyphae into numerous short segments or into various kinds of more complicated structures. Here, the fusion of two nuclei is not a requirement. The principal reproductive spores formed in this asexual manner include oidia, conidia, and sporangiospores. Sporangiospores are formed inside bladderlike containers called sporangia. Some asexual spores are equipped with a flagellum for locomotion; called zoospores, they are produced in zoosporangia. Most fungi produce spores both sexually and asexually.

Classification.

At the beginning of the 21st century, numerous details of the classification of fungi remained unclear or open to question. However, mycologists commonly distinguish within the kingdom Fungi—the true fungi—several major groups, generally treated as phyla or divisions. One such group is the Chytridiomycota, which produce zoospores and are mostly single-celled organisms. Other groups include the Zygomycota, Ascomycota, and Basidiomycota, which individually produce zygospores, ascospores, and basidiospores. An additional, recently recognized group is the Glomeromycota, which are responsible for a special type of the cooperative association between fungus and plant roots known as mycorrhiza (see SYMBIOSIS); in this type, sometimes referred to as endomycorrhiza, the fungal hyphae actually penetrate the plant roots.

Formerly, many fungus species that were thought to reproduce asexually were assigned to a separate phylum of fungi. This group—the so-called Deuteromycota, or Fungi Imperfecti, also known as mitosporic fungi—is no longer generally recognized as a formal classification, but it is still sometimes referred to in discussions of fungi. See Deuteromycetes.

Several groups of organisms once considered fungi or closely related to fungi are now regarded by most authorities as belonging to other classifications because of their lack of certain distinctive fungus characteristics. These groups include such traditional phyla as Actinomycota, Myxomycota, Plasmodiophoromycota, Labyrinthulomycota, Acrasiomycota, and Oomycota. The Actinomycota, with very delicate “hyphae” and reproduction usually by oidia or conidia, are grouped as bacteria. The Myxomycota, or true SLIME MOLDS,, are commonly placed among the animal-like protists. In this group the nutritional phase is an unwalled mass of amoebalike protoplasm, called a plasmodium. The reproductive phase includes swimming cells, called swarm cells, which are propelled by two flagella of unequal length. The Plasmodiophoromycota, also protists, resemble Myxomycota in having swarm cells and a plasmodial stage. The protists Labyrinthulomycota and Acrasiomycota have some slime mold characteristics, but their nutritional stage (the pseudoplasmodium) is different. The Oomycota—which include water molds, white rusts, and downy mildews—are filamentous chromists.

Chytridiomycota.

Chytrids, as members of the Chytridiomycota are commonly called, are primitive fungi. In some ways they seem similar to the Oomycota, and in some old classification systems they were considered protists rather than true fungi. They share, however, a number of characteristic fungus features. See CHYTRIDS,.

Zygomycota.

The zygomycetes are characterized by the formation of sexual, thick-walled zygospores and asexual, nonswimming sporangiospores. Black bread mold (Rhizopus nigricans and R. stolonifer), a well-known representative of this group in the order Mucorales, forms a mass of hyphae on stale bread, fruits, and other foodstuffs. Fungi in the order Entomophthorales are parasitic on flies and other insects; they have single sporangiospores in spore cases, and each spore case develops into a structure that becomes detached and functions as a conidium. The order Zoopagales includes fungi parasitic on other fungi as well as on amoebas, nematodes, and rotifers. All members of the order Dimargaritales are parasites of other fungi. Members of the order Harpellales live in the gut of arthropods.

Ascomycota.

Ascomycetes, also called sac fungi, make up the majority of known fungi. They typically bear a definite number of ascospores inside a bladderlike sac called an ascus. Except for some yeasts and a few other types, ascomycetes have well-developed hyphae, usually with a single nucleus in each hyphal cell. Certain cells become binucleate shortly before formation of the spore sacs. Nuclear union occurs in the young ascus; division usually produces eight daughter nuclei, which become centers of ascospore formation. Some ascomycetes have only one ascospore; others may have up to several hundred.

Ascomycota are divided into three main groups, commonly known as the Pezizomycotina, Saccharomycotina, and Taphrinomycotina. The Pezizomycotina (about the same as the former classification called Euascomycetes), account for the overwhelming majority of Ascomycota. Most of the forms that reproduce sexually have ascomata, or ascocarps (fruiting bodies containing asci). In simple forms, as in the order Eurotiales, asci are scattered throughout the interior of a ball of hyphae called a cleistothecium. Penicillium and Aspergillus are conidial stages of the Eurotiales. The Erysiphales, a group of plant parasites called the powdery mildews, have cleistothecia of specialized form. Some ascomycetes, known usually as Pyrenomycetes, have asci formed inside flask-shaped structures called perithecia. Many perithecia may be borne on a supporting ascocarp. The morels, truffles, and cup fungi are well-known ascocarps, with asci borne at the upper surface. Another Pyrenomycete, the genus Neurospora—familiar as a red bread mold—has been used extensively in the study of heredity.

The Saccharomycotina form ascospores but lack ascomata. In addition to reproducing sexually, they can reproduce asexually by forming protuberances, or buds, which eventually pinch off from the parent cell. This group includes the true yeasts, notably brewer's yeast (Saccharomyces cerevisiae). Another well-known organism belonging to the Saccharomycotina is Candida albicans, the most common cause of the fungal disease candidiasis in humans.

Notable members of Taphrinomycotina (Archiascomycetes) include the yeasts of the genus Schizosaccharomyces, which divide by fission; Pneumocystis jiroveci (formerly P. carinii), which causes a form of pneumonia; and species of Taphrina responsible for a number of plant diseases, including peach leaf curl (T. deformans). Among Taphrinomycotina only Neolecta has a prominent ascoma.

Basidiomycota.

This phylum comprises numerous and varied types of fungi. Sexual reproduction relies on structures called basidia, located at the tips of the hyphae and usually bearing four basidiospores on stalklike protrusions. Asexual Basidiomycota do not produce basidia. The basidia may be club-shaped, cylindrical, or oval. Three principal classes of Basidiomycota are usually distinguished: the Urediniomycotina, the Ustilaginomycetes, and the Hymenomycetes. The first two groups include some important plant parasites. Rust fungi (Uredinales) make up most of the species in the Urediniomycotina, which also include some yeasts, as well as smutlike and jellylike fungi. True smut fungi belong to the Ustilaginomycetes.

Many rusts, including Puccinia graminis, the black stem rust of wheat and other grains, have a complicated life cycle, requiring growth on two different hosts for production of the various spore forms. In the black stem rust, small, flask-shaped structures, known as the spermagonia, bear numerous tiny, sporelike bodies, called spermatia, on the upper surfaces of barberry leaves. On the lower surfaces develop cup-shaped structures called aecia, from the bases of which arise rows of aeciospores. The aeciospores never reinfect barberry, but attack only grain plants, producing clusters of red, spore-containing pustules called uredia, which give a rustlike appearance to the plant stems and leaves. Later in the season another type of spore, known as the teliospore, or winter spore, which is black and thick walled, is produced on the wheat stem. In the following spring the teliospores develop cylindrical projections, each of which divides into four cells bearing individual basidiospores. Rusts that alternate between two hosts are termed heteroecious; those that have all stages of development confined to one host are known as monecious.

In the smuts the teliospores are known as chlamydospores. These spores may soon reinfect the host plant but usually germinate in the soil the next spring and produce a short filament of approximately four cells, which bear basidiospores called sporidia.

The majority of known Basidiomycota species belong to the Hymenomycetes, nearly all of which are classed as Homobasidiomycetes, which includes most types of mushrooms, including bracket fungi and puffballs. Other groups in the Hymenomycetes—the Tremellomycetidae, the Dacrymycetales, and the Auriculariales—include jelly fungi and a number of yeast-forming species.Homobasidiomycetes fruiting bodies cover a vast range in size, running the gamut from tiny cuplike shapes a few millimeters across to enormous polypore mushrooms, notably Rigidoporus ulmarius, one specimen of which was reported to be up to 170 cm (5.5 ft) long and to weigh 284 kg (625 lb). Some Homobasidiomycetes rank among the largest and oldest single organisms of any type. While the fruiting bodies of the honey mushroom (Armillaria gallica) are not of unusual size, the underlying mycelium network from which they arise may occupy an area as large as 15 ha (37 acres), may weigh as much as 10,000 kg (22,000 lb), and may be as old as 1500 years. Mushroom fruiting bodies occur in many shapes and forms, and certain variations in their morphology underlay the traditional classification scheme for such fungi, which originated in the 19th century but is now obsolete for scientific purposes. For example, the old classification made a distinction between fungi that make their spores internally (Gasteromycetes) and those that produce spores externally (Hymenomycetes—confusingly, the same name as that used in modern schemes for the much larger group comprising not only mushrooms but jelly fungi and certain yeasts).

Glomeromycota.

In some forms of the intimate association between fungi and the roots of higher plants known as mycorrhiza, a portion of the mycelium wraps itself around the tips of roots, forming a velvety white cover. Such fungi typically belong to the Ascomycota or Basidiomycota. In mycorrhizae formed by fungi of the Glomeromycota (formerly regarded as belonging to the Zygomycota), the hyphae instead penetrate into the root cortex. Such mycorrhizae are said to be arbuscular. Like most Zygomycota, but unlike the Ascomycota and Basidiomycota, hyphae in the Glomeromycota lack septae. The spores of Glomeromycota are relatively large, containing hundreds or thousands of nuclei, and appear to be formed asexually, either singly, in clusters, or in distinct fruiting bodies.

Fossil Record.

The identification of fossils of fungi is often a daunting task. The remains or traces of fungi are typically very small or microscopic in size and in some cases can be readily confused with fossils from other small organisms, such as algae, protozoa, or bacteria. Be that as it may, scientists report that all the principal modern classes of fungi were represented by the end of the Carboniferous period, roughly 300 million years ago. Furthermore, available fossil evidence, coupled with molecular studies of genetic material, indicates that some types of fungi existed as early as the late Proterozoic eon, which ended about 542 million years ago. Fossil finds dating from more than 450 million years ago suggest that fungi made the transition from water to land at about the same time as plants did. Some evidence raises the possibility that the move to land was made by fungi and plants allied in a mycorrhizal relationship, which may have facilitated, it has been suggested, plants' adaptation to a terrestrial existence.

Fungus Physiology.

Most fungi have hyphal walls consisting primarily of a white, horny substance known as chitin and also containing some hemicelluloses. Cellulose is found in only a few species, belonging to the Ascomycota, and some chytrids lack walls. The water content of jelly fungi often is more than 90 percent. Spores may have less than 50 percent water content, and dormant structures such as sclerotia contain even less. Fungi require free oxygen and large amounts of water and of carbohydrates or other carbon sources for growth. Sugars such as glucose and levulose are usable by most fungi, but the use of other carbon sources depends on the ability of the fungus to produce suitable enzymes. Some of the mycorrhizal fungi may use nitrogen from the atmosphere, but all of the others depend on nitrates, ammonium salts, or other inorganic or organic nitrogen compounds. Other elements necessary for fungus growth include potassium, phosphorus, magnesium, and sulfur. Traces of iron, manganese, copper, molybdenum, zinc, and gallium and small amounts of growth substances also are necessary. Some fungi are at least partially deficient in one or more of these growth substances.

The enzymes of fungi enable them to act upon a variety of substances. A group of enzymes, called the zymase complex, permits yeasts to carry on alcoholic FERMENTATION,. Other enzymes, including protopectinase, pectase, and pectinase, which are elaborated by such fungi as Botrytis cinerea and Aspergillus oryzae, hydrolyze pectic substances contained in the middle layers of plant-cell walls. Amylase, cellobiase, cytase, dextrinase, invertase, lactase, maltase, protease, and tannase are among the other enzymes produced by fungi.

Glycogen, a substance related to starch and dextrin, is the most common reserve carbohydrate of fungi. In addition, some varieties of fungi form polysaccharides and polyhydroxy alcohol such as mannitol and GLYCEROL. Proteins and fats are produced in abundance by some fungi. Oxalic acid and other organic acids such as citric, formic, pyruvic, succinic, malic, and acetic acids are formed by many fungi, but lactic acid production is largely confined to the Mucoraceae. Other fungus products include complex sulfur compounds, chlorine-containing substances, and numerous pigments. A few fungi have the ability to form volatile arsenic compounds when they are growing on arsenic-containing substrates.

Fungus Ecology.

Spores and hyphal fragments of fungi are carried for long distances in the atmosphere. Spores of Cladosporium are especially abundant in the air.

Water habitats often abound with chytrids and water molds. A number of ascomycetes and deuteromycetes also frequent either fresh or salt water. In addition, fungi have been discovered in polluted rivers and streams. These fungi participate in the natural decomposition of sewage. Of special interest, because they cause disease in humans, are the species Aspergillus fumigatus, which can produce infection in a variety of organs, especially in immunocompromised persons, and Geotrichum candidum, which gives rise to geotrichosis, an infection that can resemble candidiasis and may affect the mouth, intestinal tract, bronchi, or lungs.

Soil is a natural habitat for saprophytic fungi, which live on organic remains, as well as a reservoir for parasitic fungi, which infect living plants and animals. The Mucorales are common soil inhabitants, as are various Eurotiales, other ascomycetes, and many deuteromycetes. Many such fungi decompose cellulose and proteins and thus are active in the formation of HUMUS,.

Most plants exist in a mycorrhizal relationship with fungi. Some species seem to be dependent on this association for satisfactory development. The plant receives water and minerals from the fungus, whose hyphae have good absorption capabilities. Because it performs photosynthesis, the plant in its turn is able to supply the fungus with carbohydrate food. Mycorrhizae are not the only major form of symbiosis involving fungi. Certain fungi live in a symbiotic association with algae, forming characteristic structures known as lichens (see LICHEN,). Most lichen fungi are ascomycetes, but the fungus components of Cora pavonia and a few other species are basidiomycetes.

Some fungi, which ordinarily grow on dead organic matter, are capable of infecting live plants when given the opportunity. Others cannot exist except as parasites of living plants. Notable plant disorders produced by infection with fungi include potato wart, caused by the chytrid Synchytrium endobioticum, along with several diseases caused by ascomycetes, among them the spot anthracnoses, chestnut blight, Dutch elm disease, oak wilt, ergot, and brown rot of stone fruits. The rusts and smuts are basidiomycetes. See DISEASES OF PLANTS,; articles on some individual diseases.

Some soil-inhabiting fungi trap microscopic organisms such as amoebas and nematodes. Nematodes are trapped by networks of hyphae covered by an adhesive substance, by knoblike outgrowths that come into contact with the prey, or by hyphal rings that in some instances swell shut abruptly after the nematodes have entered. After an amoeba or nematode is trapped, special hyphae grow into its body and deplete it of protoplasm.

Many small animals, insects, and millipedes eat fungi and thus are instrumental in spore distribution. Some groups of insects cultivate fungi as food. Notable among these are the ambrosia beetles, tropical leaf-cutting ants, and certain groups of termites. Numerous fungi, including Entomophthora muscae and other members of the order Entomophthorales of the Zygomycota, are parasites of insects. Septobasidium, in the Urediniomycetes of the Basidiomycota, has a relationship with scale insects that is partly parasitic, partly symbiotic. Diseases of animals due to fungi include the sometimes deadly infection of amphibians known as chytridiomycosis, caused by the chytrid Batrachochytrium dendrobatidis.

Uses of Fungi.

The hydrolytic enzymes of fungi are useful for a number of industrial processes. When grown on steamed wheat bran or rice bran, Aspergillus oryzae produces an amylase product useful in alcoholic fermentation. Proteases obtained from A. flavus are used in the manufacture of liquid glue. Commercial production of industrial ethyl alcohol is accomplished by fermentation of sugarcane molasses or hydrolyzed starch by means of enzymes formed by Saccharomyces cerevisiae. In the process of making bread, yeast is added to dough to produce carbon dioxide.

A. niger is used for the commercial production of CITRIC ACID, and in the production of gluconic acid and of gallic acid, which is used in the manufacture of inks and dyes. Synthetic resins are manufactured from itaconic acid produced by A. terreus and from fumaric acid formed by the black bread mold Rhizopus stolonifer. Gibberellic acid, which promotes increased growth of plant cells, is formed by Gibberella fujikuroi, an ascomycetous fungus causing disease in rice plants. Also among the ascomycetes, commercially usable oils have been obtained from species of Fusarium, Endomyces, and other genera, and the species Torulopsis utilis is a practical source of edible proteins. Vitamin D is prepared by irradiation of ergosterol, a substance which may be obtained from the waste brewer's yeast. A yeastlike ascomycetous fungus called Eremothecium ashbyii is a source of riboflavin, and biotin accumulates during production of fumaric acid by Rhizopus nigricans. Penicillium roqueforti is used to produce Roquefort and other blue cheeses and P. camemberti to ripen Camembert and Brie cheeses.

Fungi have been used medicinally since ancient times. The basidiomycetous fungus Polyporus officinalis was once commonly used as a purgative (larch, or white, agaric), and the alkaloid in the sclerotium of ergot, Claviceps purpurea, was frequently used to produce uterine contractions in childbirth. Ergot alkaloids are also a source of LYSERGIC ACID DIETHYLAMIDE, commonly known as LSD, which produces hallucinogenic effects, often of a severe nature. The use of antibiotics (see ANTIBIOTIC,) in medical practice dates from recognition of the antibiotic properties of PENICILLIN,. Many antibiotics today are produced by nonfungal microorganisms or are made synthetically. Griseofulvin is an example of an antifungal antibiotic formed by a fungus (Penicillium).         R.Ha., RICHARD HANTULA, B.A., A.M., Ph.D.

See also FUNGICIDES,; FUNGUS INFECTIONS,

For further information on this topic, see the Bibliography, section 457. Ferns, algae, fungi.