A Natural History of the Central Appalachians

Chapter 08: Mushrooms and Other Fungi

CHAPTER 08 MUSHROOMS AND OTHER FUNGI WHEN ONE THINKS OF A FOREST, the first things that come to mind are the trees and other plants that make it up. We recognize different types of forests (e.g., oak forests or pine forests) on the basis of the dominant trees present. After trees, what one would ordinarily associate with a forest is the animal life, particularly some of the more conspicuous birds and mammals. But forests as we know them really could not exist without the contributions of a large and exceedingly diverse group of organisms—the fungi. The fungi (singular: fungus) are found everywhere on earth and constitute a separate kingdom, distinct from both plants and animals, from which they appear to have diverged more than one billion years ago. With a few exceptions, the vegetative body of a fungus is made up of microscopic threadlike filaments called hyphae (singular: hypha). They are usually extensively branched, have a cell wall consisting largely of a substance known as chitin, and are either septate (having crosswalls that delimit separate cell-like compartments in a given hypha) or aseptate (lacking crosswalls), depending upon the group of fungi. Collectively the system of hyphae making up a single fungus is referred to as a mycelium (plural: mycelia). Because fungi lack the photosynthetic pigments found in green plants, their mode of nutrition is heterotrophic (literally “feeding upon others”). In contrast to animals, which feed by ingesting organic matter, fungi obtain their nutrition by extracellular digestion accomplished through the activity of enzymes that they secrete into their surrounding environment. These enzymes break down the organic matter into molecules that can be absorbed by the fungus. The organic matter includes dead wood, litter, and other types of plant debris. In most traditional taxonomic treatments, five major groups (or phyla) have been accepted as being “true” fungi, there are the Chytridiomycota (informally known as chytrids), Zygomycota (zygomycetes), Glomeromycota (glomeromycetes), Ascomycota (ascomycetes), and Basidiomycota (basidiomycetes). In addition to these “true” fungi are three other groups of organisms—the water molds, slime molds, and lichens—that have long been studied almost exclusively by mycologists, since they are “fungus-like” in some respects. Reproduction in fungi generally involves the generation of microscopic spores on specialized hyphae on or within some type of fruiting structure. If the fruiting structure is of sufficient size to be easily noticed in nature, it is usually referred to as a fruiting body. The vast majority of fungi are microscopic and thus unlikely to be observed directly in nature except under special circumstances. This is true of chytrids, zygomycetes, glomeromycetes, and most ascomycetes, which are often referred to as microfungi because of their small size. Yet other ascomycetes, along with many basidiomycetes, do produce fruiting bodies that are often readily apparent in nature. Such fungi are known as macrofungi because of the large size of their fruiting bodies. Some slime molds and most lichens also can be observed directly in the field, but members of the third group of fungus-like organisms— the water molds—are microscopic. ROLES OF FUNGI IN NATURE Fungi play important but often underappreciated roles in nature. Fungi called saprobes are the organisms responsible for breaking down dead organic matter, particularly dead plant materials. Dead organic matter derived from animals, by contrast, is for the most part broken down by bacteria. The degradation of dead plant material by fungi is essential for the continuation of life on the earth. Otherwise the nutrients contained in the dead plant material could not be recycled for use by living organisms. The photosynthesis that takes place in living green plants involves the fixation of atmospheric carbon dioxide into organic molecules. The plants use some of these organic molecules to meet their own energy needs, but other organic molecules go into storage (as starch) or are incorporated into new cells or tissue (as a result of growth or reproduction). Animals meet their energy needs by obtaining some of these organic molecules, usually by directly or indirectly consuming plants or plant parts. As a rule, perhaps 10 percent of the biologically fixed atmospheric carbon is passed along to animals, which release it back into the atmosphere through respiration or ultimately by dying. Something must happen for the other 90 percent of the biologically fixed atmospheric carbon contained in plants to be returned to the atmosphere, and this is where fungi become involved. As a result of their activities...