CLASSIFICATION AND NAMING
Every blink of an eye, every beat of a heart, every bloom of a flower, every bit of life in all but a few unusual bacteria is as it is on Planet Earth only because energy from the sun, in the form of photons, has been trapped and converted into chemical bonds. And when those bonds are broken in a truly astounding cascade of reactions, the energy from them is released and used to do the work needed to keep us all alive.
Green plants are crucial players in this scheme, for they are the Earth's primary solar collectors. Within their cells, the energy of light is trapped in the joining of carbon dioxide with water to make more complex compounds such as sugar and starch. And those compounds, together with small amounts of mineral elements from the soil--nitrogen, potassium, phosphorus, iron, calcium, and others--become increasingly more complex, forming amino acids and proteins and enzymes and nucleic acids. They, in turn, direct still more chemical reactions to make wood and flowers and fruits, and leaves and roots and all the other parts that go together to create more plants that are even bigger and better solar collectors.
Moreover, the energy captured by plants is not used only to ensure their own future; the survival of almost every organism that is not a plant also depends on the work that plants do. Many organisms eat plants, thereby "swallowing" the energy and releasing it for their own purposes through digestion and subsequent metabolic processes. Others eat animals that eat plants, and still others eat animals that eat animals that eat plants. No matter how long the chain gets, every link along the way uses some of the sun's energy, originally bound in plant tissue, to do the work needed for its own growth and development. So long as the sun shines and plants keep growing and trapping its energy and feeding the planet's herbivores, life ought to continue ad infinitum.
From the beginning of time, however, there have been several potential problems with this idyllic scenario. First, those great green solar collectors are perhaps a bit too good at what they do. They grow bigger and make more of themselves and feed the world, but as they do so they also trap mineral elements from the soil in such complex chemical webs that these elements are no longer available to aid the growth and development of other plants. Without some means to complete a cycle, the soil could eventually become so depleted that lack of essential elements could bring new plant growth to a screeching halt.
Second, as leaves and flowers and fruit reach the ends of their useful lives, they become nuisances. They're slippery, they smother other plants that are still trying to grow, and they just plain get in the way. Some people would even say they stink. One way or another, spent plant material has to be reduced to a tolerable volume or we would find ourselves, our houses, our roads, and our water courses totally overwhelmed by it.
Third, plant debris is not the only problem. One must also cope with the bodies of all those animals who, through natural processes of their own, have reached the ends of their useful lives. After all, they stink and take up space, too. And, having gotten where they have by eating plants, they have also amassed a significant store of essential elements in their cells.
Obviously, since life has been continuous on Earth for millions of years, a means of making room for future generations and of recycling scarce resources to nourish their growth has been essential.
Enter: The Fungi
I will acknowledge at the outset that there are several different groups of organisms that help to break down complex organic matter into its essential elements. Notable among these are the unicellular--some say primitive--life forms known as bacteria, and the slightly more advanced organisms, though still microscopic in size, known as protozoans. But the fungi are arguably the most important of the lot, and because this book is about fungi they will hold center stage.
These seemingly fragile organisms possess a powerful array of chemicals known as enzymes that ooze out of the fungus body and, like two hands pulling apart Tinkertoys, systematically uncouple some of the bonds that hold atoms of organic molecules together. The reduced chemicals are then absorbed through the walls of the fungus cells, where they are further undone to provide nourishment for the organism's relentless growth.
Cellulose, pectin, and lignin--the stuff plant cells are made of--are particularly vulnerable to attack. But the fungus enzymes also go after flesh and bones, plastic and paint, gas and oil, and many other complex materials. And the best evidence indicates that they have been doing their recycling for a very long time--at least as long as the 400 million years or so that land plants have been around, and probably closer to 900 million years.
For most of recorded human history, the fungi were known largely because of the mushrooms that some produced. These mysterious earthly excrescences, springing up as they did--literally overnight--were viewed by many as the probable work of evil spirits, witches, or the devil, and most assuredly they were able to corrupt the minds and bodies of hapless souls who stooped to pick them. Such attitudes persisted among laypeople well into the nineteenth century, but the scientific community recognized much earlier that the fungi--at least the mushrooms--represented a life form comparable to plants. In fact, the earliest classification schemes devised by biologists in the 1700s listed the fungi on the plant side of a two-kingdom system--plants and animals. Members of the plant kingdom were rooted in place, had rigid cell walls, and could make their own food by way of the poorly understood and not-yet-named process of photosynthesis. Animals, on the other hand, could' move around, had no walls surrounding their cells, and obtained food by eating either plants or one another. The differences between the kingdoms were obvious, and although the fungi were regarded as unusual plants, they were presumed to be plants nonetheless.
When Antony van Leeuwenhoek applied a primitive microscope to botanical study in 1665, and when Robert Hooke and others improved upon the instrument throughout the nineteenth century, biologists discovered a whole new world of tiny, living organisms that shook their previous perceptions of life on Earth. Among other things, microscopic examination of fungi made it quite clear that they were not plants. Not only were they without chlorophyll and unable to make their own food, but the food they did use was first digested outside of the fungus body and then absorbed through the cell walls. And the fungal cells themselves were very simple in structure and function. Each one had a clearly visible central body--a nucleus--and most were tubular in shape, connected end to end, and appeared en masse as circular growths of hairlike material. Roots, stems, leaves, and all of the other tissues that comprise typical plants were nowhere to be seen in the fungi. Thus it was decided that they were unique enough to deserve a kingdom of their own, and in 1784 that concept was proposed to the scientific world. Thus, a third kingdom of living organisms--the Fungi--was born.
The three-kingdom concept for classifying life forms was used by the scientific community for almost two centuries until, in 1969, another system of "natural" classification was introduced. This one was devised by Cornell University ecologist R. H. Whittaker, who proposed an evolutionary scheme involving five kingdoms. The Monera, including bacteria and other single-celled organisms without true nuclei, gave rise to the Protista--simple, often multicelled organisms with nuclei, and occasionally chlorophyll, in their cells. From the Protista arose the Plantae, Animalia, and Fungi. Mycologists blanched a bit as Whittaker moved some of their prized subjects, such as the "fungus" that caused the Irish potato famine, out of the Fungi and into the Protista, but they agreed with his reasoning and were not about to stand in the way of progress. The five-kingdom system seemed to accommodate all of the shortcomings of previous classification efforts, and members of the scientific community heaved a great sigh of relief that finally the issue was settled.
Well ... yes, most members heaved a great sigh of relief, but some remained skeptical, certain that there was still more to learn about the interrelationships of the major groups of organisms. Indeed, with the advent of the new technology that enables scientists to decode the genetic material that makes us all different from one another, still another classification scheme has arisen. This one, originating in the laboratory of Dr. Carl Woese and colleagues at the University of Illinois, suggests that the living world had evolved from the very earliest primordial slime into three domains: the Archaea, or primitive bacteria; the Bacteria; and the Eucarya. Each domain contained any number of kingdoms, each arising independently from other kingdoms in that domain, most likely not in any orderly evolutionary succession. The new concept continues to be debated as I write these words, but the data are sufficiently convincing for most experts to view the arguments favorably. Some find it hard to believe that Homo sapiens may not be a higher form of life than fungi, but that they are instead equal partners on parallel roads. But those of us familiar with the nature of the fungi expected as much all along!
Evolutionary arguments aside, however, today's students of biological classification generally agree that the true fungi are uniquely different from other groups of living organisms in several major ways.
First, the fungus body is usually comprised of cells with nuclei and with walls made of chitin and other polysaccharides but rarely cellulose. These features contrast with the Archaea and Bacteria, which have no clearly defined nucleus; animals, which have no cell walls; and plants, which have walls with cellulose as the major component.
Second, fungi are heterotrophic. This means that they cannot make their own food as plants can, through photosynthesis. They obtain much of their nourishment by "digesting" the complex molecules in plants or plant products or in organisms that eat plants. The majority of that digestion takes place outside of the fungus body.
Third, the fungus body is structurally simple; there is no division of cells into various organs such as roots, stems, and leaves, or tissues such as xylem, phloem, and epidermis. As you will see later, fungi become more complex when they reproduce, but even the most elaborate are still much simpler than most members of the plant or animal kingdom.
Fourth, fungi reproduce by way of microscopic, seedlike propagules called spores. Each spore has the capacity to germinate and develop into a new colony of fungus cells. There are many different types of spores, but for purposes of this discussion, we'll consider just a few. One of these is known as a conidium (pl. conidia)--a spore that is little more than a fragment of the parent fungus. Many different species of fungi produce conidia; for each species they are unique in size, shape, color, and/ or generative cells. Some fungi even produce more than one kind of conidium. In all cases, these spores are produced without any cross-fertilization, and, except for the occasional mutation or other aberration in cell division, each is genetically identical to the parent.
Other spores worthy of mention here are ascospores and basidiospores. Each of these spore types is produced only after two compatible nuclei from the same species merge as one and then separate during the process of meiosis. In some fungi, the two nuclei may originate within one fungus body, and a process of self-fertilization leads to ascospore or basidiospore formation. Opportunities for evolution of such fungi are largely limited to genetic changes triggered by spontaneous mutations within the fungus culture or gene exchange during simple cell division, and both are relatively uncommon.
In other fungi, the nuclei must come from two separate but compatible mating types of the same species. For these, the capacity to adapt to environmental changes is far greater because they enjoy the advantage of outcrossing with relatives that have evolved in the face of various selection pressures. For plant pathogenic fungi, for instance, the pressures working against them might be toxic fungicides or resistant plant hosts. If those fungi are obligated to mate with other members of the same species, they seem to produce offspring with tolerance to fungicides or to disease-resistant hosts much more quickly. They are better pathogens, if you will--an issue to be revisited later.
Spores can be single- or multicelled, and they vary greatly in shape and wall ornamentation from one species to another (fig. 1.1). The only consistent feature of virtually all spores is that they are small. They rarely exceed a length of 100 micrometers in any one dimension, and most are less than 20 micrometers. (One micrometer equals one one-millionth of a meter; 100 micrometers equal about one-tenth of the thickness of a dime.) Some spores are borne singly on specialized generative cells, and the whole reproductive apparatus is visible only with a microscope. In other cases, however, spores are produced by the millions in "fruit-bodies" such as mushrooms and puffballs. Other members of the kingdom fall between these two extremes with a vast array of sizes, shapes, and colors.
Students of the fungi are interested in the structure of reproductive bodies and how spores are produced because these features form the basis for the classification and naming of fungi. For this convention, we are indebted to the Swedish botanist Carolus Linnaeus. Linnaeus's efforts, beginning with his Species Plantarum in 1753, brought order to a chaotic and frenzied era when describing and naming life forms were a biologist's main tasks. By focusing his attention on physical features of reproductive bodies such as flowers and mushrooms, Linneaus developed a system for naming plants--including, at the time, fungi--that was not only more logical but much simpler than preceding taxonomic schemes. For instance, before Linneaus, the common buttercup was technically known as Ranunculus tripartitis foliis peltatis quinquangularibus multipartitis laciniis linearibus caule multifloro. He shortened it to Ranunculus acris.
After Linneaus, two pioneers in the field of mycology, Elias Fries and Christian Hendrick Persoon, published the first extensive works on fungal taxonomy, which still stand as the foundations for all that has followed. Fries, a Swede, was most interested in fleshy fungi--especially mushrooms--and lichens. He became famous not only in his home country but well beyond its borders for his many publications that named and described new species. Perhaps his greatest contribution was the three-volume Systema Mycologicum, which still stands as a valuable, authoritative resource on shelves of botanical libraries throughout the world. Thousands of species described by Fries still have the name he gave them over 170 years ago. To colleagues in Sweden and elsewhere he was--and still should be--known as "the grand old man of mycology."
Persoon, on the other hand, was mired in obscurity and financial hardship throughout his life. Born in South Africa and raised as an orphan in the care of the Dutch government, he somehow became interested in--and then obsessed with--the study of fungi. Plant pathogens known as rusts and smuts were his forte, and at occasional times of good fortune he traveled to the far reaches of the world collecting and identifying various species. Unfortunately, Persoon's life was also fraught with times of illness and bad fortune, and it was indeed remarkable that he was able to contribute as much as he did to the scientific world. A friend visiting him near the time of his death noted that he "was startled to see a shrunken man with gray hair and a tangled beard. His eyes were watery, inflamed, and blinking." Shortly thereafter, he sold his herbarium of 14,000 specimens to the Dutch government. In tribute to Persoon's life, one admirer penned these words: "Thus lived and died perhaps the greatest genius Mycology has ever known, for Persoon was a builder. He began the work with practically nothing and left a system of which others have availed themselves with much too little acknowledgement."
Many others followed in the footsteps of Fries and Persoon, and students of the fungi quickly learn the names of the major contributors. Early experts on North American fungi were L. D. von Schweinitz (1780-1834), J. B. Ellis (1829-1905), C. H. Peck (1833-1917), J. C. Arthur (1850-1942), J. N. Dearness (1852-1954), W. A. Murrill (1869-1957), F. J. Seaver (1877-1970), E. S. Luttrell (1916-1988), and R. Singer (1906-1994).
Of the American mycologists, perhaps one of the most colorful was Curtis Gates Lloyd (1859-1926). Lloyd was educated to be a businessman and, together with his two older brothers, made a tidy fortune in pharmaceuticals. Unfortunately, his aptitude for business didn't match that of his brothers, and when he was thirty-six, they convinced him to retire. This he did with pleasure, because he knew he would then have time to pursue his lifelong hobby of collecting and identifying fungi. Lloyd favored the larger fungi and in a relatively short time he became one of the leading experts of his day in the identification of puffballs. He had the resources to support occasional expeditions to add new specimens to his large personal herbarium and to publish his own private notes. At one time he even purchased the inventory of a failing shoe store just to get some boxes for storing his burgeoning herbarium.
In addition to the contributions he made to taxonomy, Lloyd was well known for objecting to the practice of adding the name of the discoverer of a fungus after its Latin name. To Lloyd, this was "advertising," and he contended that such labels were the chief causes of careless work, meaningless splitting, and what he called "name-juggling." As part of his satirical attack on the publication of authors' names, Lloyd created Professor N. J. McGinty, through whom he named several fungi, including the fictitious Lycoperdon anthropomorphus.
In what was perhaps his final act of distinction, Lloyd prepared his own tombstone with this epitaph:
Curtis Gates Lloyd
Monument erected in 1922 by
himself, for himself during his
life to gratify his own vanity.
What fools these mortals be.
* Classification of Fungi
Historically, fungi within the kingdom have been further separated into lesser groups with similar characters according to the following hierarchy:
Plant pathologists interested in fungi as pathogens of plants also use classifications such as "forma specialis" and "race" to describe fungi that are morphologically similar but have different hosts or different reactions on the same hosts, respectively.
Most recently, the kingdom Fungi has been considered to be divided into four phyla based on differences in the morphology of reproductive structures, as follows.
Chytridiomycota. With a single class, Chytridiomycetes. There are about 100 genera and 1,000 species in the phylum. They are the only true fungi with swimming spores, or zoospores. Each vegetative colony is a single cell with many nuclei but with no crosswalls to separate the colony into individual cells except when spores are produced. For some species in the group, sexual reproduction has been described. In these species, fusion of compatible nuclei results in a zygote in which meiosis occurs and swimming spores are eventually produced. Mostly, chytrids are decayers of aquatic vegetation, but some parasitize plants, other fungi, and animals.
Zygomycota. With two classes, Zygomycetes and Trichomycetes. This phylum has about 175 genera and 1,050 species. Asexual reproduction is by sporangiospores, at first contained within a membrane at the tip of a generative cell. When the membrane ruptures, the spores are blown away. Sexual reproduction, if it does occur, is typically by fusion of nuclei from compatible colonies followed by formation of a zygote--a zygospore--then by meiosis and the production of uninucleate spores. Many members of the Zygomycota are decayers of dead organic matter, but some are pathogens of arthropods--with promise as biopesticides--and others form symbiotic relationships with roots of higher plants.
Ascomycota. With at least 3,200 genera and about 32,000 species. Fungi in this phylum are usually referred to as ascomycetes. The approximate number of classes within the phylum is uncertain, and some authors are reluctant to delineate classes at all. Vegetative cells of ascomycetes do have crosswalls that divide the hyphae into many cells, each cell containing one or more nuclei. Asexual reproduction is by production of conidia on generative hyphae that may or may not be contained in or on a fruit-body.
Sexual reproduction, which occurs in most members of the phylum, is by fusion of compatible nuclei to yield a zygote that undergoes meiosis, often followed by a mitotic division, to yield eight spores, ascospores, contained in a saclike cell, an ascus (pl. asci). There are several variations on this general theme. One has compatible nuclei arising from the same colony in what is called homothallic sexual reproduction, while another has compatible nuclei obligately originating in different mycelia, called heterothallic sexual reproduction. Compatible nuclei may also coexist in the same mycelium without fusing for many hours, days, or weeks until environmental conditions suitable for sexual reproduction occur. Asci may be produced directly on the substrate on which the fungus is feeding, but more often they are contained in a cup- or flask-shaped fruit-body. There are numerous variations in shapes, sizes, and colors, each unique to a particular species. Many ascomycetes decay organic matter, but others are important pathogens of plants and animals. In addition, most lichens are associations of ascomycetous fungi with algae or blue-green algae. Some yeasts are ascomycetes, uniquely different from others in the phylum in both physical and physiological properties.
Basidiomycota. With three classes--Basidiomycetes, Teliomycetes, and Ustomycetes--and approximately 22,300 species. As with the Ascomycota, the delineation of higher levels of classification, above orders, is currently being reviewed and debated. Vegetative cells in the Basidiomycota have septa and the formation and structure of those septa is diagnostic for some taxa. Sexual reproduction is by fusion of compatible nuclei followed by meiosis and production of (usually) four basidiospores on the outside of a generative cell, the basidium, which may or may not have crosswalls of its own. Compatible nuclei may coexist in the same cell or spore for many weeks or months before fusing to allow continued progression of sexual reproduction. Asexual reproduction is highly variable within this phylum. Some species produce conidia like those of the Ascomycota, but many others are not known to produce them at all. Members of the Basidiomycota occupy a broad range of ecological niches. They decay organic matter, cause diseases in plants and animals, and form symbiotic relationships with higher plants. Some, particularly those known as "rusts," are obligate parasites and must have a living plant host if they are to complete their life cycles.
In addition to those organisms in the kingdom Fungi, there is one other phylum, the Oomycota, in the relatively newly proposed kingdom Stramenopila, that deserves mention here. The Oomycota contains organisms that look very much like fungi on both macroscopic and microscopic levels but have some fundamental structural and biochemical differences from them. They were long thought to be true fungi, and are still considered to be legitimate subjects for teaching and research by mycologists. In addition, some members of the Oomycota are serious plant pathogens. Most Oomycota have hyphae without crosswalls, they reproduce asexually by way of zoospores and they reproduce sexually by fusion of compatible nuclei, meiosis, and production of oospores. Oomycota are largely soil- or water-borne fungi, but one, Phytophthora infestans, played an important role in precipitating the legendary Irish potato famine.
Return to Book Description
File created: 8/7/2007
Questions and comments to: email@example.com
Princeton University Press