Mushrooms are fungi, full stop. They are not plants, they are not vegetables in any biological sense, and they are not their own separate category of life. A mushroom is the reproductive structure of a fungal organism, the way an apple is the fruit of a tree. The organism itself is a network of threadlike filaments called mycelium that lives hidden in soil, wood, or other material. What makes fungi their own kingdom, separate from both plants and animals, comes down to how their cells are built, how they eat, how they store energy, and where they sit on the tree of life. That last point holds a surprise: fungi are more closely related to animals than they are to plants.
What a Mushroom Actually Is
When you pick a mushroom, you are holding only a small fraction of the organism. The bulk of the fungus lives underground or inside whatever it is growing on, as a sprawling web of microscopic threads called hyphae. Collectively, those threads form the mycelium, which is the true body of the fungus. The mushroom itself is a fruiting body, produced when conditions are right so the organism can release spores and reproduce.
Research on the common button mushroom shows that these fruiting bodies draw nutrients from the mycelial network in a surprisingly uneven way. Individual mushrooms within the same flush feed from different layers of the substrate, suggesting each one taps into a different part of the underground network rather than drawing uniformly from the whole thing.1PubMed Central. Feeding of Agaricus bisporus Mushrooms is Impacted by Mycelium Colonization Time and Network Architecture As a mushroom begins to develop, the hyphal cords connecting it to the network physically thicken along the pathways leading to the growing fruiting body, almost like a road widening to carry more traffic.2Fungal Biology. The mycelial network organization adapts to facilitate new substrate colonization and fruiting in Agaricus bisporus – Section: Specific parts of the network on casing are consolidated during fruiting So the mushroom you see in a forest or a grocery store is essentially a temporary structure, pushed up by a much larger hidden organism to scatter its spores into the air.
Closer to Animals Than to Plants
For centuries, fungi were classified as plants. They grow in soil, they don’t move around, and many of them look vaguely plant-like. But molecular evidence demolished that grouping. When researchers compared protein sequences across animals, plants, fungi, and various single-celled organisms, all four proteins they examined placed animals and fungi together as a unified group to the exclusion of plants. No statistical support existed for grouping fungi with plants or animals with plants.3PubMed. Animals and fungi are each other’s closest relatives: congruent evidence from multiple proteins
This animal-fungal grouping, called the Opisthokonta, is now broadly accepted in biology. The evidence for it comes from multiple independent lines: a shared genetic insertion in one of the core protein-synthesis genes, plus consistent results from analyses of ribosomal RNA and several conserved proteins.4PubMed. A Search for the Origins of Animals and Fungi: Comparing and Combining Molecular Data In other words, the last common ancestor you share with a portobello mushroom lived more recently than the last common ancestor you share with an oak tree. Fungi and animals diverged from each other after both had already split from the lineage leading to plants. That is one of the most counterintuitive findings in biology, and it is rock-solid at this point.
Built Differently from Plants at Every Level
The reason fungi fooled scientists for so long is that superficial resemblance hides deep structural differences. Once you look at what fungal cells are actually made of, the distinction from plants becomes stark.
Plant cell walls are built from cellulose. Fungal cell walls are not. Instead, fungal walls are composed primarily of chitin, glucans, mannans, and glycoproteins.5PubMed. The structure and synthesis of the fungal cell wall Chitin is the same tough polymer found in insect exoskeletons and crustacean shells, which is another hint at the animal-fungal kinship. No plant makes chitin. This single difference in cell wall chemistry was one of the early clues that fungi were misclassified.
Cell membranes tell a similar story. Animal cell membranes use cholesterol as their main structural sterol. Plant membranes rely on a different set of sterols called phytosterols. Fungal membranes use ergosterol, a molecule critical for maintaining membrane structure and regulating cellular processes.6PubMed Central. Ergosterol distribution controls surface structure formation and fungal pathogenicity This is not just a biochemical curiosity. It is the reason many antifungal drugs work: they target ergosterol specifically, disrupting fungal membranes without harming human cells that run on cholesterol.7PubMed. Structural insights into the interaction of antifungal peptides and ergosterol containing fungal membrane
Energy storage rounds out the picture. Plants store energy as starch. Animals store it as glycogen. Fungi also store energy as glycogen, not starch, placing them on the animal side of this metabolic divide.8PubMed. Using molecular fine structure to identify optimal methods of extracting fungal glycogen Structurally, fungal glycogen sits somewhere between plant starch and animal glycogen: it has a high proportion of short molecular chains, which allows for rapid energy release.9PubMed. Structural evolution and functional adaptation of energy-storage polysaccharides in animals, plants and fungi
Eating from the Outside In
Perhaps the most fundamental way fungi differ from both plants and animals is how they get food. Plants photosynthesize: they make their own food from sunlight. Animals ingest food and digest it internally. Fungi do neither. They secrete enzymes into their environment and break down organic material externally, then absorb the dissolved nutrients through their cell walls. You could think of it as digesting your meal before you eat it, rather than after.
This external digestion is powered by an impressive toolkit. Fungi produce a wide variety of extracellular enzymes, secreted in large quantities.10PubMed Central. A Comprehensive Insight into Fungal Enzymes: Structure, Classification, and Their Role in Mankind’s Challenges Decomposer fungi break down dead organic matter that consists mainly of tough plant polymers like cellulose, hemicellulose, lignin, and pectin, along with proteins and other biological materials. Those polymers are too large to pass through a cell wall, so hydrolytic enzymes chop them into smaller pieces outside the cell first.11British Mycological Society Symposia Series. Enzymes of saprotrophic basidiomycetes
This feeding strategy is the reason fungi cannot be plants. A plant makes its own carbon from carbon dioxide using sunlight. A fungus has no chlorophyll and no photosynthetic machinery. It must obtain carbon from organic sources already in the environment, which makes it heterotrophic, like animals. The difference is in the method: animals engulf and internally digest; fungi externally dissolve and absorb.
How Fungi Scatter Their Spores
The mushroom exists to spread spores, and fungi have evolved some dramatic ways to accomplish that. Spore release in many mushroom-forming species is passive: spores drop from gills or pores and are carried by air currents. But some fungi launch spores with astonishing force. High-speed video work on fungal spore discharge has revealed that certain species eject spores at extreme velocities, with the projectiles sometimes held together by mucilage and fluid from the spore-bearing structure.12PLoS ONE. The Fastest Flights in Nature: High-Speed Spore Discharge Mechanisms among Fungi In some species, surface tension reshapes the ejected mass into a sphere mid-flight for better aerodynamics. Other fungi rely on bursting mechanisms, where internal pressure ruptures a cell wall and flings the spore away.
Spores themselves are remarkably durable. They can survive drought, UV radiation, and temperature extremes that would kill the parent mycelium. A single mushroom can release billions of spores over its short lifespan, and spore clouds from large fungal communities have been detected high in the atmosphere. Reproduction by spores is one of the defining traits that sets fungi apart from animals, which generally reproduce through eggs, live birth, or cell division. Plants also produce spores in certain life stages, but the mechanism and structure differ significantly from what fungi do.
The Recyclers of the Natural World
Fungi are the primary decomposers of organic material in land-based ecosystems, making them essential to the global carbon cycle.13PubMed Central. A trait-based understanding of wood decomposition by fungi Without them, dead trees, fallen leaves, and animal remains would pile up without being broken down, and the nutrients locked inside that material would never return to the soil for other organisms to use.
Wood decomposition illustrates this role well. Two broad strategies exist among wood-decay fungi. White rot fungi break down both lignin and cellulose, eventually leaving behind a bleached, fibrous residue. Brown rot fungi target cellulose and hemicellulose but leave most of the lignin behind. That residual lignin from brown rot can contribute up to roughly 30% of forest soil carbon, meaning these fungi shape the very chemistry of forest soils.14PubMed. The plant cell wall-decomposing machinery underlies the functional diversity of forest fungi Brown rot appears to have evolved from white rot ancestors, a shift that changed the composition of entire ecosystems.
Underground Partnerships with Plants
Fungi do not only eat dead things. Many form intimate partnerships with living plants through structures called mycorrhizae, where fungal hyphae colonize plant roots. Plants invest photosynthetic sugars to feed their fungal partners, and in return the fungi provide mineral nutrients that their far-reaching hyphal networks can forage from soil far beyond the plant’s own root zone.15PubMed Central. Mycorrhizal Networks: Common Goods of Plants Shared under Unequal Terms of Trade The arrangement resembles a marketplace. Phosphorus is a particularly valuable currency in these exchanges, and research shows that fungi adjust how much phosphorus they hand over depending on how much the plant is offering in return.16PubMed Central. Mycorrhizal fungi control phosphorus value in trade symbiosis with host roots when exposed to abrupt ‘crashes’ and ‘booms’ of resource availability
The scale of these networks is enormous. A single mycorrhizal network can link dozens of individual trees, creating what some ecologists describe informally as a “wood wide web.” Through these connections, carbon and nutrients can flow between plants, sometimes subsidizing seedlings growing in the shade of a canopy. The vast majority of land plants form some kind of mycorrhizal association, suggesting this partnership is not a special case but a default condition of plant life on Earth.
Fungi Living Inside Plants
Beyond the root zone, fungi also live inside plant tissues as endophytes. These organisms colonize stems, leaves, and other parts without causing visible disease symptoms, often conferring benefits to their host.17PubMed Central. Endophytic fungi: a tool for plant growth promotion and sustainable agriculture Endophytic fungi are found in most, if not all, plant species on the planet. Their benefits to the host can be direct, like helping the plant acquire nutrients and produce growth hormones, or indirect, like triggering the plant’s immune defenses, producing antibiotic compounds, or shielding the plant from environmental stress.18PubMed Central. Exploring the potentialities of beneficial endophytes for improved plant growth
Not all fungal inhabitants are so generous. Parasitic fungi form specialized feeding structures called haustoria inside host cells to siphon off nutrients and suppress the plant’s defenses.19PubMed Central. Hidden robbers: the role of fungal haustoria in parasitism of plants Crop diseases caused by parasitic fungi, including rusts, smuts, and blights, are among the most economically damaging threats to agriculture worldwide. The line between endophyte and parasite is not always clean: some fungi shift from harmless to harmful depending on the host’s health or environmental conditions.
Lichens and the Fungi in Disguise
Lichens look like crusty patches on rocks or papery growths dangling from tree branches, and most people would not guess they are fungi at all. But the structural body of a lichen is built by a fungus. The fungal partner houses algae or photosynthetic bacteria within its tissues, creating a composite organism where the photosynthetic partner provides sugars and the fungus provides shelter and mineral acquisition.20PubMed. Lichenized Fungi and the Evolution of Symbiotic Organization These symbioses can involve two, three, four, or even more distinct organisms living together as what appears to be a single life form.21Botanical Journal of the Linnean Society. The variety of fungal-algal symbioses, their evolutionary significance, and the nature of lichens
Taxonomically, lichens are classified as fungi. The species name of a lichen refers to the fungal partner, not the algal one. This convention underscores the point: the organism’s structural identity and evolutionary lineage are fungal. Lichen-forming fungi have evolved this symbiotic lifestyle independently many times across different fungal lineages, suggesting that the strategy is highly successful even if it makes lichens look nothing like a typical mushroom.
The Fungal Chemistry Cabinet
Fungi produce an enormous range of chemical compounds beyond the basic enzymes they use for digestion. These secondary metabolites fall into several broad classes, including alkaloids, terpenes, polyketides, and non-ribosomal peptides, many of which are synthesized from common starting materials in the cell’s metabolism.22PubMed Central. A Comprehensive Review of the Diversity of Fungal Secondary Metabolites and Their Emerging Applications in Healthcare and Environment Penicillin, the first widely used antibiotic, came from a fungus. So did the cholesterol-lowering statins, the immunosuppressant cyclosporine, and a variety of other pharmaceuticals. Psilocybin, the psychoactive compound in “magic mushrooms,” is another fungal secondary metabolite.
On the darker side, some of these compounds are dangerously toxic. Aflatoxins produced by certain molds are potent carcinogens that contaminate grain and nut crops. Amatoxins from death cap mushrooms shut down protein synthesis in human liver and kidney cells. Ergot alkaloids from a grain-infecting fungus caused widespread poisoning in medieval Europe. This chemical versatility is one reason fungi are so ecologically successful, and so medically relevant, in both positive and negative ways.
Puzzles in the Fossil Record
The deep history of fungi is harder to trace than that of animals or plants because fungal tissues rarely fossilize well. Soft mycelium decomposes quickly, and even mushrooms are mostly water. The oldest widely accepted fungal fossils date back hundreds of millions of years, but the exact timing of key evolutionary events remains debated.
One famous case illustrates the difficulty. Prototaxites was a towering, trunk-like organism that lived around 407 million years ago, and for decades it was assumed to be a giant fungus. Recent analysis of exceptionally well-preserved Prototaxites fossils from the Rhynie chert in Scotland found that these organisms were chemically distinct from fungi living at the same time and structurally unlike any known fungus, living or extinct. The researchers concluded that Prototaxites likely belongs to an entirely extinct lineage of life rather than to the fungal kingdom.23Science Advances. Prototaxites fossils are structurally and chemically distinct from extinct and extant Fungi That finding is a reminder that the fungal kingdom’s boundaries are still being refined, even at the level of major fossil organisms.
Growing Buildings Out of Mycelium
The properties that make fungi effective decomposers, a self-assembling network of tough, chitin-reinforced filaments, also make mycelium an intriguing raw material. Mycelium-based composites are created by growing fungal mycelium through an organic substrate like agricultural waste, then drying or heat-treating the result to kill the fungus and lock the structure in place. The mycelium acts as a natural adhesive, binding the substrate particles into a solid material.24PubMed Central. Mycelium-Based Composite: The Future Sustainable Biomaterial
These composites are already being used for packaging, insulation panels, and experimental architectural elements. They are lightweight, biodegradable, produce low emissions during manufacturing, and can be grown on waste streams that would otherwise go to landfill. Several companies sell mycelium-based packaging as a replacement for expanded polystyrene foam. Fashion designers have experimented with mycelium leather, and at least one company has produced mycelium-based panels for interior construction. The technology is still young, but the underlying principle is simple: fungi have been building complex three-dimensional structures from organic waste for hundreds of millions of years, and engineering is just beginning to harness that capacity.