Forms of Fungi

Mushrooms

Mushroom is a family of fungi with gills, but in popular usage any of the larger fleshy or woody fungi. The application of the term mushroom is  to edible species only and the term toadstool to those considered poisonous or otherwise objectionable has no scientific basis. For example, two poisonous fungi may be less closely related than are a poisonous species and an edible one.

Fly Agaric Mushroom. The brightly colored fly agaric mushroom is a deadly poisonous mushroom. Containing ibotenic acid and a number of other organic poisons, the fly agaric can cause severe damage to the central nervous system, blood vessels, kidneys, liver, and muscles. Symptoms, which may not become apparent for 8 to 12 hours or longer, include nausea, vomiting, and severe diarrhea and can lead to coma and death.
A systemic mycosis, which is an infection of the entire body, is typically more serious and can be fatal for individuals whose immune system has been weakened by diseases such as acquired immunodeficiency syndrome AIDS or cancer. Fungal infections are typically spread by spores that enter the body through inhalation or through an opening in the skin. Some infections are passed from animals to humans or between humans.

Life Cycle of a Mushroom. Most mushrooms undergo a complex reproductive cycle in which they produce reproductive spores that undergo sexual reproduction under proper environmental conditions. Mushrooms belong to the phylum Basidiomycetes, named for the tiny, club-shaped structures called basidia that these fungi use to produce spores. The basidia line the undersides of a mushroom cap on thin gills that radiate from the mushroom’s center.

In the reproductive cycle of mushrooms, the mycelium contain hyphae of two mating types, sometimes called plus and minus strains, with no obvious anatomical differences distinguishing them. If plus and minus strains of hyphae fuse, sexual reproduction begins. Initially the nuclei of the two hyphae remain separate, producing an intermediate stage called the dikaryon, meaning “two nuclei.” The dikaryon stage can last from weeks to years, depending upon the species. The two nuclei in the dikaryon eventually fuse to produce a diploid cell—that is, a cell that contains one nucleus with two sets of chromosomes. This cell immediately undergoes meiosis, a type of nuclear cell division that produces offspring with half the genetic material as the parents. Meiosis usually produces four genetically unique haploid spores and the reproductive cycle begins again. This population of genetically different spores has a better chance of surviving environmental changes, such as disease or temperature changes, that may wipe out an entire population of genetically identical spores.

Of the thousands of species of mushrooms known throughout the world, the great majority are tough, woody, bitter, tasteless, or of such rare occurrence that they are of no interest as food. A few species produce death or serious illness when eaten. No simple rule exists for distinguishing edible and poisonous mushrooms, but the characteristics of the more common edible species can be readily learned, and collecting activities should be confined to such species. Morels, puffballs, and other species described below are not ordinarily confused with dangerous types; whenever doubt arises, the only safe procedure is to discard all suspicious mushrooms. Fresh commercially grown mushrooms can always be eaten with safety.

Yeasts

Yeast is any of a number of microscopic, one-celled fungi important for their ability to ferment carbohydrates in various substances. Yeasts in general are widespread in nature, occurring in the soil and on plants. Most cultivated yeasts belong to the genus Saccharomyces, those known as brewer’s yeasts are strains of S. cerevisiae.

Yeasts have been used since prehistoric times in the making of breads and wines, but their cultivation and use in large quantities was put on a scientific basis by the work of the French microbiologist Louis Pasteur in the 19th century. Today they are used industrially in a wide range of fermentation processes; medicinally, as a source of B-complex vitamins and thiamine and as a stage in the production of various antibiotics and steroid hormones; and as feed and foodstuffs. Pure yeast cultures are grown in a medium of sugars, nitrogen sources, minerals, and water. The final product may take the form of dried yeast cells, or the yeast may be pressed into cakes with some starchy material.

Molds

Mold is the fuzzy, cobweblike growth produced on organic matter by several types of fungi. Mold and mildew are commonly used interchangeably, although mold is often applied to black, blue, green, and red fungal growths, and mildew to whitish growths.

Black bread mold, Aspergillus niger, one of the most familiar molds, begins as a microscopic, airborne spore that germinates on contact with the moist surface of nonliving organic matter. It spreads rapidly, forming the mycelium (fungal body), which is made up of a fine network of filaments. The mycelium produces other clusters of rootlike hyphae, called rhizoids, which penetrate the organic material, secreting enzymes and absorbing water and the digested sugars and starches. Other clusters of hyphae called sporangiophores then reach upward, forming sporangia, which bear the particular color of the mold species. Upon ripening, the sporangia break open and the windborne spores land elsewhere to reproduce asexually. Some molds also reproduce sexually through conjugation of gamete cells by the joining of two specialized hyphae. The resulting zygote matures into a zygospore that germinates after a dormant period.

Lichens

Lichen is a living partnership of a fungus and an alga. The fungus component is called the mycobiont and is composed of intertwined, threadlike fibers called hyphae that are tightly packed into a tissuelike sheet. The fungus uses these hyphae to absorb food from its surroundings. The algal component, called the photobiont, makes its own food through photosynthesis and grows as a mass of green cells dispersed among the fungal hyphae. Lichens survive in a wide variety of environments, either forming small, circular crusts or leaflike structures attached to bark, rocks, or soil, or as hairlike structures hanging from tree branches.