How do plant roots and fungi help each other?

1. A tree needs water and minerals from the soil. But roots are thick and grow slowly, so they can only reach the soil close by.
2. A fungus lives in the soil too. Its body is a web of threads far thinner than a hair. They spread a long way and slip into gaps too small for any root.
3. The threads wrap around the root tips like a sock and push in between the root's cells. This joined-up root and fungus is called a mycorrhiza.
4. Now they trade. The tree's leaves make sugar and send some down to the fungus. The fungus sends back water and minerals that it gathered far away.
5. When the fungus is well fed, it grows mushrooms to spread its spores. The same web of threads can join onto many plants at once.
6. With the fungus's help the tree gets more water and minerals and grows faster. With the tree's sugar the fungus grows too. Both win.

Tap each part to see what it does in the partnership.

• Leaves. The leaves use sunlight to make sugar. Some of that sugar travels down to the roots to pay the fungus.
• Roots. Roots anchor the tree and take in water and minerals. They are thick, so they can only explore the soil close to the tree.
• Where they join. Fungus threads wrap around the root tip and squeeze between its cells. This is the trading place: sugar goes out, and water and minerals come in.
• Fungus threads. These threads are the real body of the fungus. They are far thinner than a hair, so they can reach far and squeeze into the tiniest gaps.
• Mushroom. A mushroom is like the fruit of the fungus. It grows up into the air to spread tiny spores, which can start new fungi. The rest of the fungus stays hidden below.
• Water. Water clings in the small spaces between soil grains. The fungus threads reach water that is too far away, or in gaps too small, for roots.
• Minerals. Minerals are the nutrients a plant needs besides sugar. The fungus is very good at finding them and carries them back to the root.

Join the fungus to the root, then change how much sugar the tree shares. Tap the sugar, the water or the minerals to follow where each one goes.

Experiment: What happens if the tree stops sharing its sugar?

1. Tap the glowing thread to join the fungus to the root.
2. Slide the sugar share to None and watch the threads and the tree.
3. Slide it back up and watch again.

What we found: With no sugar, the fungus cannot grow. Its threads shrink back, so less water and fewer minerals reach the tree, and the tree does worse. Sharing some sugar pays the tree back.

Turn on JavaScript to try it yourself.

The tree needs water and minerals. It has 4 sugar to pay for fungus threads, and longer threads cost more. Tap what to reach, then press Trade.

Under almost every forest, field and garden there is a hidden partnership. Plant roots and soil fungi join together and help each other. The joined-up root and fungus is called a mycorrhiza, which means fungus-root.

Each partner has something the other needs. A plant can make sugar from sunlight, air and water, but its roots are thick and can only reach the soil close by. A fungus cannot make sugar at all. But its body is a huge web of threads, far thinner than a hair, that spread a long way and squeeze into the tiniest gaps.

So they trade. The fungus wraps its threads around the root tips. The plant sends sugar down to the fungus. The fungus collects water and minerals from far away and passes them to the plant. With this help, most plants grow better, and they cope better when the soil is dry.

When the fungus has plenty of food, it grows mushrooms above the ground to spread its spores. The same fungus can join onto many plants at once, linking them in one web. Scientists are still finding out how much plants share through it. Our lunch-swap comparison breaks down here: friends agree to trade, but a tree and a fungus make no agreement. Each simply grows better when it does its part.

Key words

Mycorrhiza

A plant root and a fungus joined together and helping each other. The word means fungus-root.

Hyphae

The very thin threads that make up the body of a fungus.

Mutualism

A partnership between two living things that helps both of them.

Minerals

Nutrients from the soil, such as phosphorus and nitrogen, that plants need to grow.

Fruiting body

The part of a fungus, such as a mushroom, that makes and spreads its spores.

Check your thinking

1. What does the tree give to the fungus?

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   Sugar made in its leaves. A fungus cannot make its own sugar, so the tree pays it with sugar.

2. Why can a fungus reach water and minerals that roots cannot?

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   Its threads are very thin and spread very far. Threads thinner than a hair can grow a long way and fit into gaps too small for roots.

3. A tree stops sending sugar to its fungus. What happens next?

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   The fungus shrinks, and the tree gets less water and minerals. Without sugar the fungus cannot keep its threads, so the tree loses its helper.

4. What is a mushroom for?

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   Spreading the fungus's spores. A mushroom releases spores into the air. The main body of the fungus is the web of threads below.

5. Why is a mycorrhiza called a mutualism?

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   Both partners gain more than they give up. The plant gains minerals and water worth more to it than the sugar it pays, and the fungus gains sugar it could not make.