The first thing to learn about fungi is that mushrooms, although commonly conflated with fungi, are actually only the fruit body of a fungus. The rest of the fungus is a network of filaments one-cell thick (between 2 to 10 µm in diameter!). These filaments are called hyphae or hyphal threads (singular: hypha) and the network is called mycelium (plural: mycelia). Hyphae grow radially in all directions in branching formations. The dense network of mycelia that grows in soil is called a mycelial mat.
The mushroom is the means by which some fungi disperse their spores, much as a fruit is a means by which a plant disperses its seeds. Of course, one crucial difference between the spores of a fungus and the seeds of a plant is that spores are microscopic. Different kinds of mushrooms differ in the way they disperse their spores. The classic capped-mushrooms are shaped so that the spores drop out from the cap (some from gills, some from pores) and are carried on the wind or on animals or fall into the soil around the mushroom. Other mushrooms attract animals with odors and then those animals unknowingly scatter the spores (the most famous examples being culinary truffles with their famous pheromones that attract pigs; the most infamous, stinkhorns with their stench of carrion that attract flies).
Spores have several extraordinary properties. One is their sheer number. It is estimated that mushrooms produce millions and even billions of spores in their lifetimes. And once they’ve found their way into the world, spores are remarkably durable. They can survive for long periods of dormancy, only to germinate when conditions are hospitable. Once a spore finds such an environment, it develops into a hypha and begins the process over again. Fungal sexuality is highly complex; some species are known to have tens of thousands of mating types or “sexes.” There are also species that produce asexually, the “fungi imperfecti.”
All mushroom-producing fungi are subdivided into basidiomycota (singular: basidiomycete) and ascomycota (singular: ascomycete) based on morphological distinctions in their spore production and dispersal. Although fungi are well-known as mushrooms, only a small percentage of the fungal kingdom produce mushrooms. (In other words, all mushrooms are fungi, but not all fungi are mushrooms.) The same sub-kingdom that includes mushroom-producing fungi (named Dikarya) also includes molds (like the kind that grow on old food), rusts (like the kind that decimate crops), and yeasts (like the ones that are used to make alcohol and bread). Along with fungi that parasitize plants (and a few that parasitize humans), there are also fungi that live symbiotically with plants, both inside and outside the plants’ cells.
This range of lifestyles is another means by which fungal life is categorized. Fungi that derive nutrients from a living source, gradually killing it, are called parasitic fungi; those that derive nutrients through partnerships with plants (through incorporating themselves into the plants’ root structures) are called mycorrhizal fungi; and those that derive nutrients from dead matter, like the kind that grow on decaying wood, are called saprobic fungi. To make matters more complicated (as they usual are with fungi), there are species that move between these modes throughout their lifecycle.
Fungi absorb nutrients by secreting enzymes that break down material in their environments. These enzymes are quite similar to the enzymes that animals secrete in their digestive tracks. This is one way that (as I have often been told) fungi are more similar to “us” (that is, humans, and animals more broadly) than they are to plants. Mycologists Moore, Robson, and Trinci define the essential differences between these three kingdoms this way:
Animals engulf; Plants photosynthesize; Fungi absorb externally digested nutrients.
You might have to read that a few times to get it. Animals derive nutrients by putting other things into their bodies (or alternately, putting their bodies around other things) to digest them — “engulfing” them. We’re all familiar with photosynthesis — but let’s just take a moment to ponder how wild it is that plants eat sunshine. And then fungi exude digestive enzymes into their environment, breaking down the material that surrounds them, and absorbing it as nutrients. It is a kind of external digestion.
In that both animals and fungi digest their nutrients (although one internally, the other externally), the fungal and animal kingdoms are considered closer to each other in the phylogenetic tree of life. Among the chemical compounds that fungi produce, some defend them against bacteria, others against other fungi, and others have unclear evolutionary benefit. Those that are considered medicinal, psychoactive, or toxic to humans are a small sample of the chemical productivity of fungi.
Among mycologists of all stripes, academic and amateur, fungi are appreciated for their unexpected and wonderful attributes. Once newly enthused mycophiles move past their fascination with the charismatic microfungi like A. muscaria, the psychoactive Psilocybe, and the medicinal (and beautiful) Ganoderma, they begin to learn about the idiosyncratic qualities of more obscure species. For example, Coprinus comatus (common name: inky cap mushroom) that breaks through concrete and then melts in a puddle of what looks like black ink; or the fact that “the fastest accelerating organism on Earth” is said to be a fungus (Pilobolus crystallinus); or simply their amazing adaptability and omniscience — there will be at least a few species of fungus in almost every ecosystem on earth.
As strange as fungi are, it is perhaps not surprising that mycology, the study of fungi, is an unusual field in several ways. As a discipline, mycology is relatively recent, its formation tied to the early technical innovations of (what would become) modern science. The term itself was first used in 1836, deriving from the Greek múkēs (μύκης), which was used to refer to culinary mushrooms; the term fungus is from Latin and dates back to Roman times, when it also seems to have referred solely to culinary species. While mushrooms have been observed, speculated upon, feared and enjoyed for as long as recorded history, early natural philosophers could not decisively determine their generative mechanisms or basic biology. Spores are minimally visible to the unaided human eye — they can be discerned en masse (as in spore prints) but not individually — and so for millennia, mushrooms were explained as the effect of weather (dampness; lightening), metaphysical beings (fairies; witches), and well into the early modern period, as the product of “spontaneous generation.” They were often grouped with plants and considered a part of botany but their identity as living things was contested. Even up into the 1770s, the question of whether or not fungi were animals or vegetables was a topic of lively debate among natural philosophers.
Spores could only be properly observed with the invention of the compound microscope, invented in the late 1500s and improved upon for wider use in the mid- to late-1600s. It was not until the 1720s that Pier Antonio Micheli (1679–1737) coupled observations using the compound microscope with a systematic inquiry into fungal morphology. His results were published in the Nova planetarum genera (1729), the first publication on fungi (in the model of the medieval “herbals”) that included descriptions and depictions of the spores of all the fungi listed. In his Introduction to the History of Mycology, Ainsworth cites this publication as the foundation of the discipline.
The other development that made modern mycology possible was the emergence of germ theory from the work of Pasteur and Tyndall in the mid-1800s, which overtook the theory of spontaneous generation. Although Giambattista della Porta (1535–1615), the 16th century Italian scholar and polymath, observed and documented spores in his book Phytognomonica (1588), the belief that fungi resulted from spontaneous generation (from the “putrefying” material itself), as opposed to “seeds,” was widely held well into the 19th century. The rise of sterilization techniques (i.e., pure culture) in experimental practices opened up a new arena of inquiry as scientists were now able to isolate and grow saprobic fungi in a lab. Pure culture, compound microscopes, and a host of simple yet specialized tools like flasks and petri dishes, along with the observational practices of the experimental method, allowed early mycologists to definitively trace fungal procreation to spores and to begin to understand the fungal life cycle and the nutrient needs of fungi. Heinrich Anton de Bary (1831–1888), a German surgeon, botanist, microbiologist, and mycologist, disproved the spontaneous generation of fungi through his work on potato pathogens in the 1860s. By this time, it had been established that fungi were a distinct group that was akin to plants in their generation from spore but were distinct in most other ways.
As can be inferred from all this history, the taxonomic placement of fungi was similarly ambiguous and contested until relatively recently. After fungal morphology was properly described, they were still included in Kingdom Plantae in the foundational Three Kingdom system (Plantae, Animalia, and Protista) that Ernst Haeckel created in 1866. It was only in 1969 that fungi became their own kingdom in the Five Kingdom system proposed by ecologist Robert Whittaker that has since become commonly accepted. Fungi are now recognized as one of the three major eukaryotic Kingdoms: Eumycota (fungi), Plantae (green plants), and Animalia (multicellular animals).
Reflecting this taxonomic ambiguity, in the early organization of modern universities, mycology was subsumed within botany in academic departments. Today, mycologists find their academic homes in a range of related fields like ecology and microbiology. Scientists who work exclusively on fungi are still relatively rare. Rather, mycology is often a sub-specialization for biologists or ecologists whose research is broader in scope. Yet there is much unknown about fungi, especially considering the diversity of the kingdom. Even in the realm of species identification, the field is considered wide open: it’s estimated that only five percent (or less) of the Earth’s fungi have been described by modern science, leaving around 1.4 million species unknown. Many aspects of fungal life, evolution, and ecological behavior are still mysterious to scientists today. All of this makes mycology especially fruitful for curious amateurs.