Fungi

The fungi, which make up the kingdom Mycophyta, embrace a wide variety of organisms. Nevertheless, they have several characteristics in common: they have no chlorophyll and, therefore, cannot photosynthesize, but obtain food by absorbing soluble materials as saprophytes (feeding on decaying matter) or parasites (feeding on living organisms); they reproduce by spores sexually or asexually; and they usually have cell walls that contain chitin or cellulose. Scientists now believe there are more than 100,000 species of fungi.

This broad group is divided into the division Myxomycota, which contains the slime molds (related to protozoa), and Eumycota, the “true fungi.” Unlike slime molds, eumycotes are typically nonmobile and composed of branching filaments called hyphae. They include the aquatic molds and downy mildews (Phycomycota, regarded by many botanists as colorless algae) and three other groups of fungi: the so-called bread molds (Zygomycota); the powdery mildews, flask fungi, cup fungi, yeasts, and truffles (Ascomycetes); and the rusts, smuts, mushrooms, and toadstools (Basidiomycetes). In addition, there is a group of fungi—the “fungi imperfecti”—that have never been observed to have a sexual stage in their life cycle, so are difficult to classify precisely, but are usually classed as Deuteromycetes.

Fungi, classified botanically as the kingdom Mycophyta, include a range of outwardly dissimilar organisms, from yeasts and molds to mushrooms. The major classes are shown in the diagram. (Remember that this system of classification may differ somewhat from systems used elsewhere.)

Myxomycota—the slime molds

These protozoalike organisms live on land in moist environments, growing on damp soil, rotting logs, and leaf mold.

The true slime molds consist of a white, yellow, or red ameboid slime called a plasmo-dium. An inch or so across, it moves imperceptibly, feeding on bacteria and particles of organic matter. The molds feed by engulfing their food (phagocytosis), a method which is atypical of fungi. They also differ from most other fungi by lacking a hyphal structure at any stage of their life cycle. Eventually movement ceases and the organism develops fruiting bodies known as sporangia, which produce spores. When the spores germinate, they release naked flagellated sex cells (gametes), which fuse in pairs to produce ameboid zygotes. The ameba grows into a plasmodium by feeding and nuclear division.

Resembling the antlers of a deer, a club fungus (Xy-laria hypoxy Ion) grows from a rotting log in the leaf litter of a woodland floor. Mosses also take advantage of the nutrients readily available in this damp environment.

In cellular slime molds, the spores give rise to free-living soil amebas. When there is a large population of amebas they aggregate (but do not fuse) to form a “slug,” which finally produces a sporangium. The cellular slime mold Dictyostelium is a common laboratory organism and has been used extensively in research on the physiology of ameboid movement.

Oomycota—the water molds

This group includes aquatic single-celled and filamentous forms and the downy mildews which attack some land plants. The spores or gametes are the only motile stage in the life cycles of these plants. Like the rest of the eu-mycotes (but unlike slime molds) they have cell walls at every phase.

Many species of those fungi known as chytrids are parasitic on algae, other fungi, aquatic angiosperms, and even fish. Among this group asexual reproduction is by uniflagellate swimming cells (zoospores), whereas sexual reproduction is by the fusion of two flagellated gametes to form a thick-walled survival spore.

Unlike most other aquatic fungi the water molds (class Oomycetes) develop an extensive branching system of hyphae, known as a mycelium. The hyphae usually contain several nuclei not separated by cell walls and are termed coenocytic. The water molds grow in water or damp soil, on seeds, dead insects, frogs, fish, and fish eggs. The downy mildews (Perono-sporales) are devastating plant pathogens; Phy-tophthora infestans, for example, causes potato blight and Pythium attacks plant seedlings.

The oomycetes are also characterized by having cellulose only in their cell walls, asexual reproduction by biflagellated zoospores, and sexual reproduction by oogamy. This process involves the fertilization of eggs in the female sexual organs (oogonia) by male nuclei from antheridia. These and other features— typical of some algae—suggest that the oomycetes derive from algae that have lost their chlorophyll.

Despite its attractive appearance the fly agaric (Amanita muscaria, right) contains a toxin that is deadly poisonous to human beings. Like the club fungus illustrated on the opposite page, a yellow slime fungus (left) has colonized a fallen branch as its food supply.

Zygomycota—the bread molds

These fungi are composed of the classes Zygomycetes and Trichomycetes, both of which reproduce sexually by conjugation of gametes attached to a mycelium. They form extensive molds in terrestrial habitats, occurring as bread molds, plant parasites, fungal parasites, insect parasites (the trichomycetes are parasites in the gut of arthropods only), and dung fungi; those in the order Zoopagales prey on amebae and nematodes.

The reproductive processes of the common bread molds Rhizopus and Mucor exemplify the group. In asexual reproduction, the hyphae, which grow on damp bread, produce globular sporangia that release thousands of nonmotile spores into the air. The spores land and germinate on a suitable surface to produce new, haploid mycelia. In sexual reproduction, short branches from two hyphae touch at the tips and each separates off a haploid gamete by a cross wall. The gametes fuse (conjugate) to form a diploid zygote that develops a thick protective coat, and is then known as a zygospore. After dormancy for several months the zygospore undergoes meiosis (reduction division) and germinates to produce a haploid sporangium. The spores from this structure grow into new mycelia, and thus complete the sexual cycle. The many different species of these molds are classified by the shapes and patterns of their sporangia.

Ascomycetes—cup and flask fungi

The class Ascomycetes is the largest group of fungi, with more than 30,000 named species.

They are mainly terrestrial and are saprophytic or parasitic. They include edible fungi, such as morels and truffles; some of the yeasts; plant parasites, such as those that cause Dutch elm disease; ergot; powdery mildews; animal para- s sites such as ringworm; most of the fungal components of lichens; and most marine fungi.

The unique characteristic of the Ascomycetes is the ascus—a saclike cell. This structure produces eight haploid spores (ascospores) internally by “free cell formation,” which involves the spores being cut out from the substance of the cell. The ascus is formed during sexual reproduction after fusion of male and female gametes. The asci may be surrounded by a large mycelium of tightly packed hyphae that are able to form macroscopic fruiting bodies of various forms. Typical fruiting ascomycetes are the morels (Morchella sp.), truffles (Tuberales), and such common woodland forms as the orange-peel fungus (Aleuria au-rantia). In these forms the asci are produced by disk-, cup-, or flask-shaped ascocarps.

Aspergillus is an important Ascomycete, which has a moldlike form and reproduces asexually by single-spored sporangia (conidia). It causes considerable damage to stored grain, cloth, and other goods. It also produces afla-toxin, one of the most potent of the poisons and carcinogens.

Most yeasts are specialized Ascomycetes that grow as single oval cells. Some multiply by fission, but most undergo budding, as do baker’s and brewer’s yeast (Saccharomyces cereviseae). In the sexual phase, two haploid yeast cells fuse to form a diploid colony in which individual cells act as asci and cleave out four or eight haploid ascospores. Yeasts can ferment sugars to produce alcohol and carbon dioxide, a property that is exploited in brewing beer, making wine, and raising bread.

The edible field mushroom (Agaricus campestris) is a typical member of the class Basidiomycetes. The fruiting body—the part we eat—consists of a cap lined underneath with gills, in which the spores are formed. The development of the fruiting body, from the underground “button” stages, is illustrated above.

Imperfect fungi

The 25,000 fungi in the class Deuteromycetes are known as imperfect fungi because they only reproduce asexually. Reproduction usually involves the production of conidia (on co-nidiophores) or by budding. Many of these fungi probably represent the asexual stages of ascomycetes and basidiomycetes.

The Deuteromycetes include yeastlike and mycelial forms, many of which cause diseases. A typical deuteromycete genus is Penicillium, which grows as a gray-green mold on rotting fruit. It reproduces asexually by the erect co-nidiophores producing chains of conidia. Species of this genus are well known for their commercial use in the production of the antibiotic penicillin.

Yeasts, in the class Asco-mycetes, consist of microscopic single-celled organisms. They reproduce asexually by budding, as explained in the diagram below. Yeast was probably the first plant to be domesticated, and it is still used in making bread, beer, and wine, as well as for other fermentation processes.

Basidiomycetes

Most of the large conspicuous fungi—mushrooms, toadstools, puffballs, and brackets—are grouped in the class Basidiomycetes, of which there are more than 38,000 species. The vegetative mycelium of these fungi is an extensive, usually underground, system of septate (rather than coenocytic) hyphae. The aerial structures, such as those of toadstools, are composed of interwoven hyphae and are the spore-bearing portions of the fungus. These fruiting bodies are often brightly colored due to pigments in their cell walls.

The basidium, a club-shaped hypha specialized for reproduction, is the common characteristic of the Basidiomycetes. Single basidio-spores form in four small protrusions on each basidium. Formation of the basidiospores is the end result of a type of sexual reproduction that begins when the mycelia of two different basidiomycetes merge. In each cell of this merged mycelium, called a heterokaryon, the haploid nuclei of each parent persist, sometimes for many years. Biologists think that the presence of two genetically distinct nuclei may give the heterokaryon greater ability to grow or use nutrients than either parent fungus has alone.

Antibiotics such as penicillin are produced naturally by molds, although many can now also be synthesized in the chemical laboratory. The photograph shows three colonies of Penicillium chryso-genum cultured on a nutrient jelly.

In the basidium, the two nuclei fuse and the resulting zygote undergoes meiosis, forming four haploid spores. The basidia may be borne on gills, in tubes, or in fleshy masses inside the fruiting bodies as in puffballs (Lycoper-dales) or outside as in stinkhorns (Phallales). A typical mushroom discharges 10 million spores per hour for several days. These are usually dispersed by wind or insects. The basidia discharge their spores by means of a little explosion that carries them a distance of 1 millimeter or so. They then drop down the space between the gills or the inside of the tubes until they reach the open air and are wafted away.

When a single raindrop falls on top of an earth star fungus (Geastrum triplex) it explosively ejects a cloud containing many thousands of spores that are dispersed by the wind, before germinating to produce new plants.

Mushrooms are often found in “fairy rings.” As the underground mycelium uses the organic matter in the soil, it grows outward to find fresh food. This results in a ring of hy-phae, enlarging all the time, producing fruiting bodies as it goes.

Rust (Uredinales) and smut fungi (Ustilagi-nales) are characterized by a variety of spore types and complex life cycles involving parasitism of several host plants. There are about 6,000 species of rusts and 1,000 species of smuts. Many live as parasites on cereals, vegetables, and flowers.

Basidiomycetes are of immense ecological and industrial importance in their mycorrhizal associations with forest trees and many other plants. Mycorrhiza is a symbiotic association between fungi and plant roots, from which both benefit. The advantages for the fungi are that the plants with which they are associated pass on photosynthetic products to them, and in turn the fungi provide the plants with mineral nutrients, which they can extract more efficiently from the soil. Many pioneer plants of poor soils are extremely dependent on these mycorrhizal fungi. Plants may form mycorrhizal associations with many fungal species; birch, for example, has a large number of basidiomycetes with which it will grow.

A bracket fungus is named after its horizontal fruiting bodies, which stick out from the bark of a tree. The yellow, or sulfur, bracket fungus Laetipurus sulphurus forms a cascade of brackets that grow up to 1 foot (30 centimeters) across.