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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. Fungi other than mushrooms are sometimes collectively called molds, although this term is better restricted to fungi of the sort represented by bread mold.
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. 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. 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.
In 1928 a green mold accidentally grew in a culture dish of Staphylococcus bacteria that the bacteriologist Alexander Fleming was studying in a London hospital. The fungus colony that developed inhibited the growth of the bacteria. Such unavoidable contamination certainly had occurred many times before in laboratories throughout the world, but the people who may have seen such cultures probably regarded them as contaminated plates to be discarded as soon as possible. Fleming, however, carefully recorded his observation and in 1929 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.
In 1951 a strange disease broke out in the small French village of Pont-Saint-Esprit, and several persons died. Doctors were baffled by the mysterious malady until it was recognized as a form of “St.Anthony's fire”—ergotism—that had resulted from eating bread made from contaminated flour. Ergotism was prevalent in northern Europe in the Middle Ages, particularly in regions of high rye-bread consumption; modern grain-cleaning and milling methods have practically eliminated the disease.
The cause of ergotism is ergot—a fungus. More precisely, ergot is a sclerotium (plural sclerotia), a special part of a fungus that develops on grasses and especially on rye. The wind carries the fungal spores to the flowers of the rye, where the spores germinate, infect and destroy the ovaries of the plant, and replace them with masses of microscopic threads cemented together into a hard fungal structure shaped like a rye kernel but considerably larger and darker. This is ergot, and it contains a number of poisonous organic compounds called alkaloids. A mature head of rye may carry several ergots in addition to non infected kernels. When the grain is harvested, much of the ergot falls to the ground, but some remains on the plants and is mixed with the grain. If the ergot is not removed before milling, the ergotized flour would be converted into bread and other food products and consumed; St. Anthony's fire—for which no cure is known—is the result. The ergot that falls to the ground may be the source of more trouble. Cattle put to graze in the rye fields after harvest are likely to consume enough ergot to bring on abortion of fetuses or death. In the spring, when the rye is in bloom, the ergot remaining on the ground produces tiny, black, mushroomlike bodies that expel large numbers of spores to start a new series of infections.
Among the many interesting chemicals in ergot is lysergic acid, the active principle of the psychedelic drug lysergic acid diethylamide ( LSD). Here, then, is a single fungus that can reduce crop yields, cause abortion in cattle, sicken and sometimes kill people, and be used as a source of LSD. On the credit side, ergot provides medical science with drugs useful in inducing labour in pregnant women and in controlling hemorrhage after birth.
The systematic study of fungi began 250 years ago, but humans have been indirectly aware of fungal activity since the first loaf of leavened bread was baked and the first tub of grape must was turned into wine. Yet, even now, few people realize that they are almost constantly either benefited or harmed by these organisms. 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; it is this that makes them so important in the human environment. Together with bacteria, fungi are responsible for the disintegration of organic matter and the release, into the soil or atmosphere, of the carbon, oxygen, nitrogen, and phosphorus that otherwise would be forever locked up in undecomposed organic matter. Fungi are essential to many household and industrial processes, notably the making of bread, wine, beer, and certain cheeses. They are used in the production of a number of organic acids, enzymes (biological catalysts), and vitamins and are the sources of a number of antibiotics besides penicillin. Fungi are also used as food: mushrooms, morels, and truffles are epicurean delicacies.
Studies of fungi have greatly contributed to the accumulation of fundamental knowledge in biology. Current knowledge of biochemistry and cellular metabolism was derived in part from studies of ordinary baker's or brewer's yeast (Saccharomyces cerevisiae ). Some of these pioneering discoveries were made at the end of the 19th century and continued during the first half of the 20th. From 1920 through the 1940s, geneticists and biochemists who studied mutants of the red bread mold, Neurospora , established the one-gene–one-enzyme theory and laid the foundation of modern genetics. These and other fungi continue to be useful for studying cell and molecular biology, genetic engineering, and other basic disciplines of biology.
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