Bread rises. Leaves fall. A tree stump goes soft and crumbly. None of that is a plant, and none of it is a bacterium. It is a fungus, and fungi are sorted into groups by looking at the spores they make and the walls their cells wear.
This post explains what makes a fungus a fungus, what the main groups are called, and how scientists decided to draw the lines.
What Makes Something a Fungus
A fungus is one of a few things at once. It has a nucleus, so it is a eukaryote. It builds its cell wall out of chitin, and no plant does that. It cannot make its own food, so it eats other material instead. And it grows as threads rather than as a solid body.
 |
| A hypha is one thread of a fungus. Many hyphae tangled together make a mycelium — Image: AHiggins12, CC BY-SA 3.0, via Wikimedia Commons |
The threads are called hyphae, and they are usually only 2 to 10 micrometres across. A micrometre is a thousandth of a millimeter, so a single thread is thinner than a hair. Each thread grows at its tip, and when threads branch and fuse they build a woven mass called a mycelium.
A mycelium is the real body of a fungus. The mushroom you pick up is the fruiting body, built to make and drop spores, not the living network that does the feeding. Most fungi are invisible for this reason. Their networks run through soil, dead wood, or the inside of a living root.
Chitin is the deciding ingredient
Chitin is a tough carbohydrate. Insects build their exoskeletons out of it, and so do fungi, but they build theirs differently and combine it with other materials. Fungal cell walls are built from a chitin and glucan complex, and they contain no cellulose at all.
 |
| Chitin sits inside a glucan layer. This is the wall that keeps a fungus from being classified as a plant — Image: Maya and Rike, CC BY 3.0, via Wikimedia Commons |
How a fungus eats
A fungus cannot photosynthesise, so it has to feed on what other organisms made. It does this by pumping enzymes out into the surroundings, chopping large molecules into small ones, and then absorbing the pieces. Biology calls this feeding style osmotrophy.
This is the opposite of an animal. An animal swallows its food and digests it inside. A fungus digests first, out in the open, and only then takes in the soluble soup.
Why Fungi Are Not Plants
Fungi were studied as a branch of botany for a long time, and the grouping survived in textbooks long after it stopped being right. Modern DNA evidence puts fungi closer to animals than to plants.
- No chloroplasts. A fungus has no photosynthesis machinery, and it never did.
- No cellulose. Plant walls are cellulose. Fungal walls are chitin and glucan.
- No tissue. Fungi do not form true roots, stems, or leaves. A mushroom is not a plant organ.
- Different reserve food. Plants store starch. Fungi store glycogen, which is the same carbohydrate animals store.
- Absorb, do not ingest. Fungi feed by absorption. Animals and slime moulds feed by taking food into the body.
The resemblance is a case of convergent evolution. Hyphae and spores are simple things that work, and separate lineages arrived at them separately.
How the Classification Works
For most of the history of mycology, classification was done with a microscope and no DNA. Scientists looked at spore color, spore shape, the size of fruiting bodies, and whether the hyphae had cross walls. The system worked, and it still works for field identification.
What DNA changed
When sequencing arrived, it sometimes contradicted the shape-based groupings. Some moulds that never produced a sexual spore turned out to be scattered across unrelated branches. The old catch-all group of "fungi imperfecti", which is defined by the absence of a sexual stage rather than by any shared feature, could not be placed at all. In current usage fungi are divided into one subkingdom, seven phyla, and ten subphyla.
The same fact may be recorded twice
About a third of all fungi reproduce in more than one way. A species may have a sexual stage and an asexual stage that look completely different, and the asexual one may even have been given a separate name. Old texts call these the teleomorph and the anamorph.
The Main Groups of Fungi
Five phyla carry almost all the species a textbook will ask you to name. Each is named for the spore-producing structure it builds.
Chytridiomycota: the ones that swim
Chytrids are the oldest living lineage of fungi, and the only ones that can move. They release motile zoospores with a single rear flagellum, which lets them swim through water films. Almost all of them live in soil or fresh water and eat other tiny organisms.
 |
| The swimming stage is what makes chytrids unusual. This one causes chytridiomycosis in amphibians — Image: M. Piepenbring, CC BY-SA 3.0, via Wikimedia Commons |
One chytrid species, Batrachochytrium dendrobatidis, has been blamed for a worldwide decline in amphibian populations. The same pattern of loss is being seen in salamanders, caused by a relative of the same species.
Zygomycota: the moulds on bread and fruit
The old phylum name comes from the zygospore, a thick walled resting spore that forms when two mating hyphae fuse. Members of this group are saprotrophs, feeding on dead plant material, and they include the black bread mould Rhizopus.
 |
| Zygomycetes build sporangia on tall stalks. The spore case sits above the food supply — Image: Jon Houseman, CC BY-SA 3.0, via Wikimedia Commons |
One important note for older textbooks. This group has been rebuilt. Most of what used to be called Zygomycota is now placed in Mucoromycota, and only a small part keeps the old name.
The life cycle itself is unchanged. A plus strain and a minus strain meet, and their tips fuse into gametangia. The nuclei fuse to form a zygote, and the zygote hardens into a zygospore. Finally the zygospore germinates, undergoes meiosis, and releases spores. Each spore grows into a new plus or minus mycelium, and the cycle begins again.
Glomeromycota: the root partners
This phylum contains only arbuscular mycorrhizal fungi. They cannot grow on their own, and they cannot reproduce in the way the other fungi do. Their defining structure is the arbuscule, a branched tube that grows inside a plant root cell. The fungus takes sugars from the plant. The plant hands back phosphate and nitrate that its own roots could not reach.
 |
| The arbuscule is the exchange point. One side feeds the fungus, the other feeds the plant — Image: Msturmel, public domain, via Wikimedia Commons |
Ascomycota: the sac fungi
Ascomycota is the largest group. Around 30,000 species have been described in it, against roughly 600 in the old zygomycete group. Its name comes from the ascus, a little sac in which the sexual spores are made. Many ascomycetes also push their asci up into a cup-shaped fruiting body called an apothecium, so the group includes morels, truffles, and many of the cup fungi.
 |
| The largest group, and the one behind yeast, moulds, morels and truffles — Image: David Midgley, CC BY-SA 2.5, via Wikimedia Commons |
Two very different fungi sit in this group. Saccharomyces cerevisiae is a single-celled yeast that leavens bread and ferments beer. Penicillium roqueforti is a mould that gives blue cheese its color and its flavour.
Basidiomycota: the mushroom makers
Basidiomycota holds the familiar mushrooms, bracket fungi, rusts, and smuts. Their sexual spores are made on tiny club-shaped cells called basidia, usually on the gills under a cap. Basidiomycetes also have a special cross wall, the dolipore septum, which has a pore that lets cytoplasm and sometimes nuclei pass through.
 |
| Gills hold the spore-bearing surface. They pack a huge spore-producing area into a small cap — Image: Ashkananisi7, CC BY-SA 4.0, via Wikimedia Commons |
| Group | Named for | Where it lives | Seen as |
| Chytridiomycota | a flagellated swimming spore | soil and fresh water | often only a microscope slide |
| Zygomycota (now Mucoromycota) | the zygospore | soil, dead plants, bread and fruit | black or white fluffy mould |
| Glomeromycota | the arbuscule | inside living plant roots | nothing above ground |
| Ascomycota | the ascus sac | almost everywhere | yeasts, moulds, morels, truffles |
| Basidiomycota | the basidium club | soil, wood, and living plants | mushrooms and brackets |
How Many Fungi Are There
Almost certainly far more than anyone has counted. Around 148,000 species had been described by 2020, while a 2017 estimate put the real total between 2.2 million and 3.8 million species. More than 90 percent of fungi are still unnamed. The rate of discovery is not slowing: it went from about 1,000 new species a year to more than 2,000, and 2020 set a record with 2,905.
One colony older than farming
A single individual of the honey fungus Armillaria solidipes in Oregon covers more than 900 hectares. That is about 3.5 square miles, and its age is estimated at nearly 9,000 years. One organism has been spreading there for longer than farming has existed.
Fungi also form rhizomorphs, hardened rope-like strands that carry food and water across soil in the way plant roots do.
Things That Look Like Fungi but Are Not
Two groups cause endless confusion, because textbooks and the general public both call them fungi.
- Water moulds, the oomycetes. Their cell walls contain cellulose and lack chitin, so they are not true fungi. Downy mildews and white rust of crucifers are oomycetes. The old version of this blog post filed them with the zygomycetes, which was wrong.
- Slime moulds, the myxomycetes. They have no cell wall during the feeding stage, and they swallow food particles instead of absorbing them. Amoebae are their relatives.
 |
| This looks like a small mushroom and behaves like one, but it is an amoeba. The fruiting body is only the spore stage — Image: zituba, CC BY-SA 3.0, via Wikimedia Commons |
Mycology, the study of fungi, still has a place for all of these. Many oomycetes cause serious plant disease, so plant pathologists work on them constantly.
Lichens Are a Partnership, Not a Group
A lichen looks like one organism with its own name. It is actually two. A fungus, usually an ascomycete and sometimes a basidiomycete, has taken up with a photosynthetic partner. In lichen terminology the partner is the photobiont and can be a green alga or a cyanobacterium.
The photobiont makes sugars by photosynthesis. The fungus supplies minerals and water, and holds the partnership on bare rock, bark, or sand where neither partner could manage alone. The two are so entangled that the result behaves like a single organism and looks nothing like its two parts.
Lichens occur on every continent, and they help form soil and start ecological succession on bare ground. About 27 percent of known fungi are lichenized, which is more than 19,400 species.
Why the Classification Matters
Knowing which group a fungus belongs to is not just a quiz. It tells you what it can infect, what it can be used for, and how dangerous it might be.
 |
| This family gave us the first widely used antibiotic — Image: Y_tambe, CC BY-SA 3.0, via Wikimedia Commons |
- Farming. Over 90 percent of plant species form mycorrhizal partnerships with fungi. Fungi also include the worst crop pathogens, such as the rice blast fungus and the cause of Dutch elm disease.
- Food. Yeast makes bread and beer. Aspergillus oryzae makes soy sauce and sake. Rhizopus makes tempeh. Penicillium roqueforti makes blue cheese, and Fusarium venenatum makes the meat substitute Quorn.
- Medicine. Penicillins from Penicillium moulds still treat bacterial infections. Fungal drugs also include ciclosporin, used in transplant surgery, and the statins, which came from fungi before anyone found the same effect elsewhere.
- Dangers. Some fungi make toxins. Aspergillus species on grain and nuts can make aflatoxins, which damage the liver. The death cap, Amanita phalloides, makes amatoxins and causes more deadly mushroom poisonings than any other species.
- Research. Neurospora crassa is a bread mould that made the one gene-one enzyme hypothesis possible. Yeast is a standard model for studying the cell cycle.
 |
| The single most common cause of deadly mushroom poisoning. The symptoms wait days to appear — Image: Archenzo, CC BY-SA 3.0, via Wikimedia Commons |
Key Takeaways
- Fungi are their own kingdom, closer to animals than to plants, and chitin in the cell wall is the clearest single sign of it.
- A fungus grows as threads called hyphae; tangled together they form a mycelium, which is the real body of the organism.
- Fungi feed by absorbing nutrients after digesting them outside the body.
- Modern classification has one subkingdom, seven phyla, and ten subphyla, based on DNA rather than only on spore shape.
- The five big groups are Chytridiomycota, Zygomycota (now largely Mucoromycota), Glomeromycota, Ascomycota and Basidiomycota.
- Each group is named for its spore structure: a flagellated zoospore, a zygospore, an arbuscule, an ascus, or a basidium.
- About 148,000 fungal species were described by 2020 out of an estimated 2.2 to 3.8 million, so most fungi are still unknown.
- Oomycetes and slime moulds look like fungi but are not. Downy mildews and white rust belong to the first group.
- A lichen is a fungus living with a photosynthetic partner, and about 27 percent of known fungi are lichenized.
Frequently Asked Questions
How many groups of fungi are there?
Classically, four. Today the accepted answer is one subkingdom, seven phyla and ten subphyla. Most school courses still teach five named groups, because those are the ones with familiar examples.
Why is a mushroom not a plant?
A mushroom is the fruiting body of a fungus, and fungi have no chloroplasts. They build their walls from chitin rather than cellulose, and they absorb nutrients instead of making their own food. DNA evidence puts them nearer to animals.
What is the difference between a mould and a mushroom?
Neither is a formal group. A mould is a fungus growing as a visible fuzzy layer on a surface. A mushroom is a fungus that has built a fruiting body large enough to notice. The same species can do either.
Are all fungi dangerous?
No, and most are harmless or helpful. A few hundred fungal species can infect humans, and more than 8,000 are known plant pests. Many thousands more decompose dead material or live in harmless partnerships with plant roots.
What does zygospore mean?
It is a thick-walled resting spore, formed when two compatible hyphae fuse and their nuclei join. It survives bad conditions and germinates when conditions improve. The name zygomycota comes from it, and the group has since been split up.
Why are there so few named fungi if there are millions?
Most fungi are small, hidden, and hard to grow in a laboratory, and the ones that are large and visible were collected first. Naming a species usually means describing how it reproduces, which most hidden species have never been seen doing.
Sources: Wikipedia articles on fungus, mycology, chytridiomycota, zygomycota, glomeromycota, ascomycota, basidiomycota, mushroom, yeast, lichen, mold, spore, Rhizopus, Penicillium, Saccharomyces cerevisiae, Amanita muscaria, Armillaria and Neurospora crassa. Images: Wikimedia Commons, with authors and licenses noted in each caption.
