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Chapter 26: Not a Plant, Not an Animal: The Secret Kingdom of Fungi

Quick. Picture where a mushroom grows.

You probably imagined one pushing up out of the dirt in a damp forest, or popping up overnight in the grass next to the flowers, or sitting in a little plastic box in the produce aisle right between the lettuce and the bell peppers. Mushrooms hang out with plants. They grow in soil like plants. They sit in the vegetable section like plants. So a mushroom is a plant, right?

Nope. Not even a little.

Back in Chapter 2, we sorted all of life into giant groups called kingdoms (remember Dear King David Came Over For Good Spaghetti?). We even mentioned, almost in passing, that when you classify a plant, one of the first things you check is that it is a plant and not an animal or a fungus. That little word, fungus, was a clue we breezed right past. Fungi are not plants. They are not animals either. They are their own separate kingdom, with their own bizarre way of living, and they have been hiding in plain sight your entire life.

This whole book has been about plants. But you cannot understand how plants actually live without understanding fungi, because the two are tangled together everywhere you look. So let’s pull back the curtain on the strangest kingdom of all. The science of studying fungi is called mycology (my-KOL-uh-jee), and a scientist who studies them is a mycologist. By the end of this chapter, you’ll be thinking like one.

What Makes a Fungus a Fungus?

Let’s start with what fungi are not, because that’s where it gets interesting.

A plant makes its own food. Way back when we studied leaves, you learned that plants use the green pigment chlorophyll to capture sunlight and build sugar out of thin air and water. That is the entire plant superpower. Fungi cannot do this. They have no chlorophyll, no green color, and no way to turn sunlight into food. A mushroom sitting in a sunny field is getting exactly zero nutrition from that sunshine.

If a fungus can’t make its own food, how does it eat? The same way you do, in one sense: it has to find food that already exists and take it in. That makes a fungus more like an animal than a plant when it comes to mealtime. But fungi don’t have mouths or stomachs, and that leads to one of the grossest and most clever feeding tricks in all of nature, which we’ll get to in a minute.

There’s one more big clue that fungi are their own thing. Remember in Chapter 3 when we learned that plant cells are wrapped in a tough cell wall made of cellulose, the same stuff that makes paper and cotton and the crunch in an apple? Fungi have cell walls too, but theirs are not made of cellulose. They are made of chitin (KY-tin), which is the exact same material that makes up the hard shell of a beetle, the crunchy exoskeleton of a crab, and the wings of a fly. Stop and think about that. The mushroom on your pizza is built out of the same stuff as an insect’s armor.

The Mushroom Is Just the Tip

Here is the single most important idea in this whole chapter, and almost nobody knows it. When you look at a mushroom, you are not looking at the fungus. You are looking at a tiny piece of it.

We actually got a sneak peek at this back in Chapter 25, when we talked about the underground partnership between plants and fungi. Remember how we said most of a fungus is a vast network of microscopic threads spreading through the soil, thinner than the finest hair? That was not an exaggeration. That hidden network is the real fungus.

Each of those individual threads is called a hypha (HY-fuh), and a whole tangled mass of them is called a mycelium (my-SEE-lee-um). The mycelium is the actual body of the fungus, and it can be enormous. It spreads silently through soil, through rotting logs, through fallen leaves, even through the bread in your kitchen, eating the whole time, and you usually never see it.

So what is the mushroom, then? The mushroom is just the reproductive part, the structure the fungus pushes up above ground when it’s time to make more fungi. Scientists call it a fruiting body.

The comparison isn’t perfect, but think of it a little like an apple on a tree. The apple is not the tree. It’s the part the tree makes to spread its seeds. In the same way, a mushroom is not the fungus. It’s the part the fungus makes to spread its spores. The real organism, the mycelium, is hidden underground, sometimes spread across an area bigger than you could walk across in a day.

Eating Without a Mouth (This Part Is Gross)

Okay, back to the question we left hanging. If a fungus can’t make food and doesn’t have a mouth or a stomach, how does it actually eat?

It digests its food on the outside of its body, and then drinks the results.

Here’s how it works. The threads of the mycelium ooze out powerful chemicals called enzymes directly into whatever they’re growing on, whether that’s a dead log, a pile of leaves, or last week’s leftovers shoved to the back of the fridge. Those enzymes break the food down into a kind of nutrient soup, right there in the environment. Then the fungus simply soaks that soup back up through its threads.

Read that again, because it’s wild. A fungus basically spits digestive juices onto its dinner, lets the food dissolve outside its body, and then absorbs the slop. Imagine if, instead of putting a sandwich in your mouth, you threw up on it, waited for it to turn to mush, and then soaked it back into your skin. That is, more or less, the daily life of a fungus. It’s revolting, and it’s also one of the most important jobs on the entire planet.

The Cleanup Crew the World Can’t Live Without

That disgusting eating style makes fungi the greatest recyclers in nature. They are nature’s cleanup crew, and we call living things with this job decomposers.

Think about it. Every autumn, trees drop tons of leaves. Plants die. Branches fall. Animals die. If nothing broke all of that down, dead material would pile up higher and higher, year after year, until the whole world was buried under a mountain of leaves, logs, and worse. The nutrients locked inside all that dead stuff would be trapped forever, and new plants would have nothing to grow from.

Fungi prevent that. They move into dead material and slowly digest it away, releasing the trapped nutrients back into the soil where living plants can use them all over again. In fact, fungi are the main organisms on Earth that can break down wood. Wood is full of a tough, stubborn material called lignin that almost nothing else can digest. Without wood-rotting fungi, fallen trees would essentially last forever. The forest floor would be a junkyard of ancient logs.

So the next time you see a mushroom growing on a dead stump, you’re not looking at something creepy. You’re watching the planet’s recycling system at work, turning death back into life. Quietly, constantly, and for free.

Making More Fungi: Spores, Spores, and More Spores

Fungi don’t make seeds. They make spores, the same tiny, dust-like specks of life we met back in Chapter 22 when we looked at ferns and their relatives. A spore is far simpler than a seed. There’s no baby plant and no packed lunch inside, just a single cell or a few cells wrapped in a tough coat, ready to grow into a new fungus if it lands somewhere good.

And fungi do not make spores in small numbers. They make them in mind-melting amounts. A single ordinary mushroom can release billions of spores. A giant puffball, which is a round white fungus that can grow as big as a beach ball, may hold around seven trillion spores inside it. If even a sliver of one percent of those ever grew, the planet would be wall-to-wall puffballs. Almost all of them fail, drifting off to land somewhere too dry or too bare. But when you launch trillions of lottery tickets, a few are going to win.

Want to see spores for yourself? You can make something called a spore print. If you place a mushroom cap, gills down, on a piece of paper for a few hours, the falling spores leave a delicate dust pattern on the page that traces the exact shape of the gills. Mycologists use these prints to help figure out which species they’re looking at, because spore color is a big clue.

Meet the Family

The fungus kingdom is huge. Scientists have described well over 100,000 kinds of fungi, and they think there may be millions more we haven’t even found yet. They don’t all look like the classic mushroom either. Here are the main characters you’ll run into.

Type	What It’s Like	Where You’ve Met It
Mushroom-formers	The classic umbrella-shaped fungi with caps and gills. The part you see is the fruiting body sitting on top of a hidden mycelium.	Button mushrooms on a pizza, toadstools in the yard, shelf fungi growing off a dead tree.
Molds	Fuzzy, fast-spreading fungi that grow in flat patches and love damp surfaces and old food.	The blue-green fuzz on forgotten bread, the spots on an old orange, mildew in a damp bathroom.
Yeasts	Tiny single-celled fungi, so small you need a microscope to see one. They reproduce by budding, basically growing a little copy of themselves that breaks off.	The yeast that makes bread dough rise by puffing it full of carbon dioxide gas.
Sac fungi	A massive group that includes cup-shaped fungi, wrinkly morels, and the prized, pricey truffles that pigs and dogs sniff out underground. The picture to the left are white and black truffles.	Morels hunted by foragers in spring, and truffles shaved over fancy meals.

Yes, you read that right in the chart above. The yeast that makes your bread rise is a living fungus. It eats the sugars in the dough and burps out carbon dioxide gas, and those tiny gas bubbles get trapped in the stretchy dough, puffing it up into something soft and fluffy. Every slice of fluffy bread you’ve ever eaten was inflated by the burps of millions of hungry little fungi. You’re welcome.

Fungi Stories: Let’s Meet Some of the Famous Types

Fungi have saved more human lives than almost anything else in history, and it started with a messy lab and a stroke of luck.

In 1928, a scientist named Alexander Fleming came back from vacation to find that one of his lab dishes had gotten contaminated. A bit of mold had drifted in and started growing on a plate where he was raising bacteria. Most people would have just tossed the ruined dish in the trash. But Fleming noticed something strange. In a clear ring all around the mold, the bacteria had died off. The mold was producing something that killed bacteria.

That mold was a fungus called Penicillium, and the bacteria-killing substance it made got the name penicillin. It became the first true antibiotic, a medicine that wipes out harmful bacteria inside the body, and it has saved an almost uncountable number of lives from infections that used to be deadly. All of it traces back to a tidy little fungus, an untidy lab, and a scientist who paid attention to a mistake instead of throwing it away.

Fungi feed us in other ways too. The tang in blue cheese? That’s a Penicillium mold growing right through it on purpose, and those blue-green veins are fungus. Many other cheeses, sauces, and foods around the world get their flavor from carefully chosen fungi. We have turned the cleanup crew into the kitchen staff.

The Troublemakers

Not every fungus is your friend. Remember, fungi eat by digesting things, and sometimes the thing they decide to digest is alive, including plants, and including you.

Because this is a plant book, the plant villains matter most. Fungi cause some of the most devastating plant diseases in history. A fungus called chestnut blight swept across North America in the early 1900s and killed billions of American chestnut trees, wiping out a tree that had once dominated entire forests. Dutch elm disease, also caused by a fungus and spread by beetles, has destroyed huge numbers of elm trees that once lined city streets. Farmers wage constant battle against fungal diseases like rusts and smuts that attack wheat, corn, and other crops we depend on for food.

Fungi can bother people directly too, though usually in much milder ways. Athlete’s foot is a fungus growing on the skin of your feet, thriving in the warm, damp inside of a sweaty shoe.

Ringworm is another one, and here’s a fun fact to gross out your friends: ringworm is not a worm at all. There’s no worm involved anywhere. It’s a fungus that grows in a ring-shaped patch on the skin, and somebody named it badly a long time ago and it stuck.

The Largest Living Thing on Earth Is a Fungus

Quiz time. What’s the biggest living organism on the planet? A blue whale? A giant redwood tree?

Not even close. The largest known living thing on Earth is a fungus.

In the Malheur National Forest in Oregon, there is a single honey fungus whose mycelium spreads through the soil across an area of roughly four square miles. That’s around 2,400 acres of one connected organism, mostly hidden underground as a giant web of threads, occasionally pushing up clusters of mushrooms here and there. Scientists estimate it could be thousands of years old. It has been quietly growing and eating its way through that forest since long before the United States existed.

And it all comes back to what we learned earlier. The fungus you can see is never the whole story. The real organism is the mycelium, the hidden web, and a single web can grow until it is the heavyweight champion of all living things.

The Team Players

We can’t leave fungi without circling back to their partnerships, because fungi are some of the best team players in all of nature. In Chapter 25 we sorted relationships between living things into mutualism, commensalism, and parasitism. Fungi are stars of the mutualism category, where both partners win.

You already met the biggest example. Mycorrhizae, the underground deal where a fungus trades hard-to-reach water and nutrients to a plant in exchange for sugar, is happening under the vast majority of plants on Earth. Now that you know the fungus body is a giant web of threads, you can picture what’s really going on: those threads are wrapping around and reaching into plant roots, linking up with thousands of plants at once. Some scientists nickname this huge underground fungal network the wood wide web, because it connects whole forests together beneath the soil. (And remember the ghostly white Indian pipe from Chapter 25, the plant that cheats by stealing sugar out of that fungal network? Now you understand exactly what it’s tapping into.)

Then there are lichens (LY-kuns), one of the strangest partnerships in nature. A lichen looks like a single crusty, papery, or leafy patch growing on a rock, a tree trunk, or an old gravestone.

But a lichen is not one organism at all. It’s a fungus and a tiny green photosynthetic partner, usually an alga, living together as a team. The alga makes food using sunlight, since it has the chlorophyll the fungus lacks, and the fungus provides shelter, structure, and protection. Together they can survive in brutal places where neither could make it alone, clinging to bare rock in scorching deserts and freezing mountaintops. Lichens are basically a fungus and an alga deciding to become roommates and conquer the harshest real estate on the planet.

Note: The following video mentions 10,000 years:

So, Are Mushrooms Plants?

You know the answer now, and you can explain exactly why.

A fungus has no chlorophyll and cannot make its own food, so it isn’t a plant. Its cell walls are built from chitin, the stuff of insect shells, not the cellulose of plants. Its body is a hidden web of threads called a mycelium, and the mushroom you see is only its fruiting body, the spore-launcher poking above the surface. It eats by digesting its food on the outside and soaking it back in. It reproduces by flinging out billions of spores. It serves as the planet’s recycling crew, partners up with plants underground, teams with algae to form lichens, and occasionally grows large enough to become the biggest living thing on Earth.

Fungi were never plants. They were always something else, an entire kingdom living right alongside the plants we’ve spent this whole book getting to know, often hidden, often underfoot, and absolutely essential. The motionless plant standing in the soil and the silent web of fungus woven through that same soil have been partners the entire time. You just finally know how to see it.