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A Fungus Eats Outside Its Own Body

About 11 minutes

A fungus cannot make its own food out of sunlight, so it takes its food from other things. That sentence hides a real problem.

Take it from them how? A fungus has no mouth. It has no teeth, no throat and no stomach. It cannot bite a log, and it cannot swallow one. Yet a fallen log in a wood really does get eaten, and fungi really are the things eating it.

First, find the actual animal

A mushroom is the fruiting body — the apple, not the tree. So where is the tree?

Underground, and it does not look like a tree at all. It is a mass of threads called hyphae, each one far thinner than a hair, spreading out through soil or through the wood itself. Together they make a net, and the net is called the mycelium.

That net is the fungus. Everything else is a temporary attachment.

A single teaspoon of healthy soil can hold kilometres of hyphae, wound through it too fine to see without pulling the soil apart. When you find a mushroom in the grass you have found the tip of something that was there long before it and will still be there after it has gone.

Now the eating

The net does not carry food anywhere. It goes to the food and settles on it, and then it does something no animal can do.

It squirts out chemicals that digest, straight into the wood.

Those chemicals are the same sort of thing that works inside your stomach: they take something solid and break it into pieces small enough to soak up. The difference is where they do it. Yours stay locked inside you and the food comes to them. A fungus sends its outside and leaves them there.

The wood dissolves where it lies. The threads then soak up the liquid straight through their walls, the way a paper towel takes up a spill.

Three small mushrooms above ground and the huge thread network below them A ground line runs right across the picture near the top. Above it stand three small mushrooms, well apart from each other, each with a short stalk and a broad low cap that is much wider than it is tall. Below the ground line, filling the whole rest of the picture and running off both edges, is a wide branching network of fine threads. The threads spread out sideways and downwards from under each mushroom, cross over one another and join up, so the three mushrooms are shown to be parts of one single connected thing. The mushrooms take up a tiny corner of the picture and the network takes up nearly all of it, which is the point: the mushroom is the small part you can see and the network is the actual organism.
  1. A hypha, a single thread, grows along until it reaches a piece of dead wood. It presses against it. Nothing is bitten and nothing is picked up.
  2. The thread releases digesting chemicals out through its own wall and into the wood. They are now outside the fungus and in the food.
  3. The wood breaks down where it sits. Solid timber turns into a soup of pieces small enough to pass through a wall — and the log has not moved a millimetre.
  4. The thread soaks that soup back in along its whole length. Then it grows a little further into the softened wood and starts again.

Notice what the fungus never does. It never moves its food, and it never takes anything solid inside itself. It is standing in its own dinner, and it has already been digested by the time any of it goes in.

Why this lets a fungus get enormous

The biggest living thing on Earth is a fungus, in a forest in Oregon, spread over more than two thousand acres. That fact is usually told as a curiosity. It is really a consequence.

Think about what limits the size of an animal. An animal has one stomach, and everything it eats has to be carried to that stomach and then carried away again from there. Get too big and the journey defeats you. The body has to be built to hold itself up and to move food around inside it, and building costs food.

A net has no such problem. There is no centre to carry anything to. Each thread digests where it is and absorbs where it is, so a fungus a mile across works exactly like a fungus the size of a coin — it is just the same arrangement, repeated further. Nothing has to travel and nothing has to be carried.

That is also why fungi are the great decomposers. They do not have to be stronger than a log or bigger than a log. They only have to get inside it and stay there, and almost nothing else on Earth can digest wood at all.

Digesting outside your body sounds risky. What is the real cost of it?

The cost is worth naming, because it explains a lot of fungal behaviour. Food dissolved out in the open is available to anybody. Bacteria drink it, other fungi drink it, and a fungus that cannot defend its patch loses the meal it paid for. A great deal of what fungi do to each other underground is an argument over a soup that one of them made.

Next time you pass a rotting log, look at how it is going: soft, crumbling, giving way. Nothing chewed it. Something grew into it, turned it to liquid where it lay, and drank it.

Look, and leave it. The rule from before still stands, and it matters most with fungi: never taste a wild one.