Introduction
plural fungi any of about 80,000 known species of organisms of the kingdom Fungi, which includes the yeasts, rusts, smuts, mildews, molds, mushrooms, and toadstools. There are also many funguslike organisms, including slime molds and oomycetes, that do not belong to kingdom Fungi but are often called fungi. Many of these funguslike organisms are included in the kingdom Chromista. Fungi are among the most widely distributed organisms on Earth and are of great environmental and medical importance. Many fungi are free-living in soil or water; others form parasitic or symbiotic relationships with plants or animals.
The fungi are eukaryotic organisms; i.e., their cells contain membrane-bound organelles and clearly defined nuclei. Historically, the fungi were included in the plant kingdom; however, because fungi lack chlorophyll and are distinguished by unique structural and physiological features (i.e., components of the cell wall and cell membrane), they have been separated from plants. In addition, the fungi are clearly distinguished from all other living organisms, including animals, by their principal modes of vegetative growth and nutrient intake. Fungi grow from the tips of filaments (hyphae) that make up the bodies of the organisms (mycelia), and they digest organic matter externally before absorbing it into their mycelia.
While mushrooms and toadstools (poisonous mushrooms) are by no means the most numerous or economically significant fungi, they are the most easily recognized fungi. 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, mykēs. Fungi other than mushrooms are sometimes collectively called molds, although this term is better restricted to fungi of the sort represented by bread mold. (For information about slime molds, which exhibit features of both the animal and the fungal worlds, see protist.)
Importance of fungi
Humans have been indirectly aware of fungi since the first loaf of leavened bread was baked and the first tub of grape must was turned into wine. Ancient peoples were familiar with the ravages of fungi in agriculture but attributed these diseases to the wrath of the gods. The Romans designated a particular deity, Robigus, as the god of rust and, in an effort to appease him, organized an annual festival, the Robigalia, in his honour.Fungi are everywhere in very large numbers—in the soil and the air, in lakes, rivers, and seas, on and within plants and animals, in food and clothing, and in the human body. Together with bacteria, fungi are responsible for breaking down organic matter and releasing carbon, oxygen, nitrogen, and phosphorus into the soil and the atmosphere. Fungi are essential to many household and industrial processes, notably the making of bread, wine, beer, and certain cheeses. Fungi are also used as food; for example, some mushrooms, morels, and truffles are epicurean delicacies, and mycoproteins (fungal proteins), derived from the mycelia of certain species of fungi, are used to make foods that are high in protein.Studies of fungi have greatly contributed to the accumulation of fundamental knowledge in biology. For example, studies of ordinary baker's or brewer's yeast (Saccharomyces cerevisiae) led to discoveries of basic cellular biochemistry and metabolism. Some of these pioneering discoveries were made at the end of the 19th century and continued during the first half of the 20th century. From 1920 through the 1940s, geneticists and biochemists who studied mutants of the red bread mold, Neurospora, established the one-gene–one-enzyme theory, thus contributing to the foundation of modern genetics. Fungi continue to be useful for studying cell and molecular biology, genetic engineering, and other basic disciplines of biology.The medical relevance of fungi was discovered in 1928, when Scottish bacteriologist Alexander Fleming noticed the green mold Penicillium notatum growing in a culture dish of Staphylococcus bacteria. Around the spot of mold was a clear ring in which no bacteria grew. Fleming successfully isolated the substance from the mold that inhibited the growth of bacteria. In 1929 he published a scientific report announcing the discovery of penicillin, the first of a series of antibiotics—many of them derived from fungi—that have revolutionized medical practice.Another medically important fungus is Claviceps purpurea, which is commonly called ergot and causes a plant disease of the same name. The disease is characterized by a growth that develops on grasses, especially on rye. Ergot is a source of several chemicals used in drugs that induce labour in pregnant women and that control hemorrhage after birth. Ergot is also the source of lysergic acid, the active principle of the psychedelic drug lysergic acid diethylamide (LSD). Other species of fungi contain chemicals that are extracted and used to produce drugs known as statins, which control cholesterol levels and ward off coronary heart disease. Fungi are also used in the production of a number of organic acids, enzymes, and vitamins.
Form and function of fungi
Size range
The mushrooms, because of their size, are easily seen in fields and forests and consequently were the only fungi known before the invention of the microscope in the 17th century. The microscope made it possible to recognize and identify the great variety of fungal species living on dead or live organic matter. The part of a fungus that is generally visible is the fruiting body, or sporophore. Sporophores vary greatly in size, shape, colour, and longevity. Some are microscopic and completely invisible to the unaided eye; others are no larger than a pin head; still others are gigantic structures. Among the largest sporophores are those of mushrooms, bracket fungi, and puffballs. Some mushrooms reach a diameter of 20 to 25 cm (8 to 10 inches) and a height of 25 to 30 cm (10 to 12 inches). Bracket fungi can reach 40 cm (16 inches) or more in diameter, and puffballs often exceed that size. The largest puffballs on record measured 150 cm (5 feet) in diameter. The number of spores within such giants reaches several trillion.
Distribution and abundance
Fungi are either terrestrial or aquatic, the latter living in freshwater or marine environments. Freshwater species are usually found in clean, cool water because they do not tolerate high degrees of salinity. However, some species are found in slightly brackish water, and a few thrive in highly polluted streams. Soil that is rich in organic matter furnishes an ideal habitat for a large number of species; only a small number of species are found in drier areas or in habitats with little or no organic matter. Fungi are found in all temperate and tropical regions of the world where there is sufficient moisture to enable them to grow. A few species of fungi live in the Arctic and Antarctic regions, although they are rare and are more often found living in symbiosis with algae in the form of lichens (see below Lichens). About 80,000 species of fungi have been identified and described, but mycologists estimate that there may be as many as 1.5 million total species.
Basic morphology
A typical fungus consists of a mass of branched, tubular filaments enclosed by a rigid cell wall. The filaments, called hyphae (singular hypha), branch repeatedly into a complicated, radially expanding network called the mycelium, which makes up the thallus, or undifferentiated body, of the typical fungus. The mycelium grows by utilizing nutrients from the environment and, upon reaching a certain stage of maturity, forms—either directly or in special fruiting bodies—reproductive cells called spores. The spores are released and dispersed by a wide variety of passive or active mechanisms; upon reaching a suitable substrate, the spores germinate and develop hyphae that grow, branch repeatedly, and become the mycelium of the new individual. Fungal growth is mainly confined to the tips of the hyphae, and all fungal structures are therefore made up of hyphae or portions of hyphae.Some fungi, notably the yeasts, do not form a mycelium but grow as individual cells that multiply by budding or, in certain species, by fission.
Structure of the thallus
In almost all fungi the hyphae that make up the thallus have cell walls. (The thalli of the true slime molds lack cell walls and, for this and other reasons, are classified as protists rather than fungi.) A hypha is a multibranched tubular cell filled with cytoplasm. The tube itself may be either continuous throughout or divided into compartments, or cells, by cross walls called septa (singular septum). In nonseptate (i.e., coenocytic) hyphae the nuclei are scattered throughout the cytoplasm. In septate hyphae each cell may contain one to many nuclei, depending on the type of fungus or the stage of hyphal development. The cells of fungi are similar in structure to those of many other organisms. The minute nucleus, readily seen only in young portions of the hypha, is surrounded by a double membrane and typically contains one nucleolus. In addition to the nucleus, various organelles—such as the endoplasmic reticulum, Golgi apparatus, ribosomes, and liposomes—are scattered throughout the cytoplasm.
Hyphae usually are either nonseptate (generally in the more primitive fungi) or incompletely septate (meaning that the septa are perforated). This permits the movement of cytoplasm (cytoplasmic streaming) from one cell to the next. In fungi with perforated septa, various molecules are able to move rapidly between hyphal cells, but the movement of larger organelles, such as mitochondria and nuclei, is prevented. In the absence of septa, both mitochondria and nuclei can be readily translocated along hyphae. In mating interactions between filamentous Basidiomycota, the nuclei of one parent often invade the hyphae of the other parent, because the septa are degraded ahead of the incoming nuclei to allow their passage through the existing hyphae. Once the incoming nuclei are established, septa are re-formed.
Variations in the structure of septa are numerous in the fungi. Some fungi have sievelike septa called pseudosepta, whereas fungi in other groups have septa with one to few pores that are small enough in size to prevent the movement of nuclei to adjacent cells. Basidiomycota have a septal structure called a dolipore septum that is composed of a pore cap surrounding a septal swelling and septal pore. This organization permits cytoplasm and small organelles to pass through but restricts the movement of nuclei to varying degrees.
The wall of the hypha is complex in both composition and structure. Its exact chemical composition varies in different fungal groups. In some funguslike organisms the wall contains considerable quantities of cellulose, a complex carbohydrate that is the chief constituent of the cell walls of plants. In most fungi, however, two other polymers—chitin and glucan (a polymer of glucose linked at the third carbon and branched at the sixth), which forms an α-glucan layer and a special β-1,3-1,6-glucan layer—form the main structural components of the wall. Among the many other chemical substances in the walls of fungi are some that may thicken or toughen the wall of tissues, thus imparting rigidity and strength. The chemical composition of the wall of a particular fungus may vary at different stages of the organism's growth—a possible indication that the wall plays some part in determining the form of the fungus. In some fungi, carbohydrates are stored in the wall at one stage of development and are removed and utilized at a later stage. In some yeasts, fusion of sexually functioning cells is brought about by the interaction of specific chemical substances on the walls of two compatible mating types.When the mycelium grows in or on a surface, such as in the soil, on a log, or in culture medium, it appears as a mass of loose, cottony threads. The richer the composition of the growth medium, the more profuse the threads and the more feltlike the mass. On the sugar-rich growth substances used in laboratories, the assimilative (somatic) hyphae are so interwoven as to form a thick, almost leathery colony. On the soil, inside a leaf, in the skin of animals, or in other parasitized plant or animal tissues, the hyphae are usually spread in a loose network. The mycelia of the so-called higher fungi does, however, become organized at times into compact masses of different sizes that serve various functions. Some of these masses, called sclerotia, become extremely hard and serve to carry the fungus over periods of adverse conditions of temperature and moisture. One example of a fungus that forms sclerotia is Claviceps purpurea, which causes ergot, a disease of cereal grasses such as rye. The underground sclerotia of Poria cocos, an edible pore fungus also known as tuckahoe, may reach a diameter of 20 to 25 cm.Various other tissues are also produced by the interweaving of the assimilative hyphae of some fungi. Stromata (singular stroma) are cushionlike tissues that bear spores in various ways. Rhizomorphs are long strands of parallel hyphae cemented together. Those of the honey mushroom (Armillaria mellea), which are black and resemble shoestrings, are intricately constructed and are differentiated to conduct water and food materials from one part of the thallus to another.
Sporophores and spores
When the mycelium of a fungus reaches a certain stage of growth, it begins to produce spores either directly on the somatic hyphae or, more often, on special
sporiferous (spore-producing) hyphae, which may be loosely arranged or grouped into intricate structures called fruiting bodies, or sporophores.
Photograph:Fungi can produce spores on somatic hyphae or on special spore-producing hyphae, which are arranged …
* Fungi can produce spores on somatic hyphae or on special spore-producing hyphae, which are arranged …
The more primitive fungi produce spores in sporangia, which are saclike sporophores whose entire cytoplasmic contents cleave into spores, called sporangiospores.
Thus, they differ from more advanced fungi in that their asexual spores are endogenous. Sporangiospores are either naked and flagellated (zoospores) or walled and nonmotile (aplanospores). The more primitive aquatic and terrestrial fungi tend to produce zoospores. The zoospores of aquatic fungi and funguslike organisms swim in the surrounding water by means of one or two variously located flagella (whiplike organs of locomotion). Zoospores produced by terrestrial fungi are released after a rain from the sporangia in which they are borne and swim for a time in the rainwater between soil particles or on the wet surfaces of plants, where the sporangia are formed by parasitic fungi. After some time, the zoospores lose their flagella, surround themselves with walls, and encyst. Each cyst germinates by producing a germ tube. The germ tube may develop a mycelium or a reproductive structure, depending on the species and on the environmental conditions. The bread molds, which are the most advanced of the primitive fungi, produce only aplanospores (nonmotile spores) in their sporangia.
The more advanced fungi do not produce motile spores of any kind, even though some of them are aquatic in fresh or marine waters. In these fungi, asexually produced
spores (usually called conidia) are produced exogenously and are typically formed terminally or laterally on special spore-producing hyphae called conidiophores. Conidiophores may be arranged singly on the hyphae or may be grouped in special asexual fruiting bodies, such as flask-shaped pycnidia, mattresslike acervuli, cushion-shaped sporodochia, or sheaflike synnemata.
Sexually produced spores of the higher fungi result from meiosis and are formed either in saclike structures (asci) typical of the Ascomycota or on the surface of club-shaped structures (basidia) typical of the Basidiomycota. Asci and basidia may be borne naked, directly on the hyphae, or in various types of sporophores, called ascocarps (also known as ascomata) or basidiocarps (also known as basidiomata), depending on whether they bear asci or basidia, respectively. Well-known examples of ascocarps are the morels, the cup fungi, and the truffles. Commonly encountered basidiocarps are mushrooms, brackets, puffballs, stinkhorns, and bird's-nest fungi.
Growth
Under favourable environmental conditions, fungal spores germinate and form hyphae. During this process, the spore absorbs water through its wall, the cytoplasm becomes activated, nuclear division takes place, and more cytoplasm is synthesized. The wall initially grows as a spherical structure. Once polarity is established, a hyphal apex forms, and from the wall of the spore a germ tube bulges out, enveloped by a wall of its own that is formed as the germ tube grows.The hypha may be roughly divided into three regions: (1) the apical zone about 5–10 micrometres (0.0002–0.0004 inch) in length, (2) the subapical region, extending about 40 micrometres back of the apical zone, which is rich in cytoplasmic components, such as nuclei, Golgi apparatus, ribosomes, mitochondria, the endoplasmic reticulum, and vesicles, but is devoid of vacuoles, and (3) the zone of vacuolation, which is characterized by the presence of many vacuoles and the accumulation of lipids.
Growth of hyphae in most fungi takes place almost exclusively in the apical zone (i.e., at the very tip). This is the region where the cell wall extends continuously to produce a long hyphal tube. The cytoplasm within the apical zone is filled with numerous vesicles. These bubblelike structures are usually too small to be seen with an ordinary microscope but are clearly evident under the electron microscope. In higher fungi the apical vesicles can be detected with an ordinary microscope equipped with phase-contrast optics as a round spot with a somewhat diffuse boundary. This body is universally known by its German name, the Spitzenkörper, and its position determines the direction of growth of a hypha.
The growing tip eventually gives rise to a branch. This is the beginning of the branched mycelium. Growing tips that come in contact with neighbouring hyphae often fuse with them to form a hyphal net. In such a vigorously growing system, the cytoplasm is in constant motion, streaming toward the growing tips. Eventually, the older hyphae become highly vacuolated and may be stripped of most of their cytoplasm. All living portions of a thallus are potentially capable of growth. If a small piece of mycelium is placed under conditions favourable for growth, it develops into a new thallus, even if no growing tips are included in the severed portion.
Growth of a septate mycelium (i.e., with cross walls between adjacent cells) entails the formation of new septa in the young hyphae. Septa are formed by ringlike growth from the wall of the hypha toward the centre until the septa are complete. In the higher fungi the septum stops growing before it is complete; the result is a central pore through which the cytoplasm flows, thus establishing organic connection throughout the thallus. In contrast to plants, in which the position of the septum separating two daughter cells determines the formation of tissues, the fungal septum is always formed at right angles to the axis of growth. As a result, in fungal tissue formation, the creation of parallel hyphae cannot result from longitudinal septum formation but only from outgrowth of a new branch. In fungi, therefore, the mechanism that determines the point of origin and subsequent direction of growth of hyphal branches is the determining factor in developmental morphogenesis.
The individual fungus is potentially immortal, because it continues to grow at the hyphal tips as long as conditions remain favourable. It is possible that, in undisturbed places, mycelia exist that have grown continuously for many thousands of years. The older parts of the hyphae die and decompose, releasing nitrogen and other nutrients into the soil.
Some species of endophytic fungi, such as Neotyphodium and Epichloë, which invade the seeds of grasses (e.g., ryegrass and fescue) and grow within the plant, grow
not through extension of the hyphal tips but by intercalary growth, in which the hyphae attach to the growing cells of the plant. This type of growth enables the hyphae
of the fungus to grow at the same rate that the plant grows. Intercalary growth of endophytic fungi was discovered in 2007, although for many years scientists
suspected that these fungi possessed unique adaptations that allow them to grow as if they were natural parts of their hosts.
The underground network of hyphae of a mushroom can grow and spread over a very large area, often several metres (yards) in diameter. The underground hyphae
obtain food from organic matter in the substratum and grow outward. The hyphal branches at the edge of the mycelium become organized at intervals into elaborate
tissues that develop aboveground into mushrooms. Such a circle of mushrooms is known as a fairy ring, because in the Middle Ages it was believed to represent the
path of dancing fairies. The ring marks the periphery of an enormous fungus colony, which, if undisturbed, continues to produce ever wider fairy rings year after year.
Fungi can grow into enormous colonies. Some thalli of Armillaria species, which are pathogens of forest trees, are among the largest and oldest organisms on Earth.
Nutrition
