By A.P. Mentzer
Updated Mar 24, 2022
Kingdom Fungi is divided into four primary phyla. Basidiomycota includes familiar mushrooms, toadstools, and puffballs. The visible portion above ground is merely the fruiting body of a vast underground mycelial network that constitutes the bulk of the organism.
Ascomycota is the largest group, encompassing everything from single‑cell yeasts to complex morels. Yeasts play essential roles in baking and fermentation, while some species cause common skin ailments like athlete’s foot and diaper rash. Many members are plant pathogens that damage crops; roughly 75% of all fungi belong to this phylum.
The phylum Zygomycota contains fewer than 1,000 species, including the familiar bread molds that appear as gray‑green fuzz on stale bread. Members of this group decompose dead plant and animal matter, and some act as parasites on living hosts.
Deuteromycota, often called “imperfect fungi,” reproduce exclusively by spore release. Unlike the other phyla, they lack a known sexual cycle. A notable example is Penicillium, the mold that yields the antibiotic penicillin.
Despite superficial plant‑like appearances, fungi are phylogenetically closer to animals. They lack chlorophyll and cannot photosynthesize. Fungi acquire nutrients by externally digesting organic matter with enzymes, then absorbing the resulting simple molecules—such as glucose—through their cell walls. This strategy allows them to break down tough plant fibers and, in the case of parasites, to digest living tissue before absorption.
The core of a fungus is a network of filamentous threads called hyphae. Hyphae are composed of stacked cells that facilitate nutrient flow across the organism, collectively forming the mycelium. Depending on the species, the mycelium can grow in soil, water, decaying plant or animal material, or even living tissue. New colonies may arise from hyphal fragments or from fruiting structures that release spores. Mushroom caps, for example, are such fruiting bodies.
Fungal cell walls differ from plant walls: they are built from chitin—the same polysaccharide that forms insect exoskeletons and crustacean shells—providing structural rigidity.
