Are Mushrooms Plants? Why Fungi Are Closer to Animals

They grow out of the soil, sit still and are sold next to the vegetables. Many of us learned at school that mushrooms are a kind of plant. But biologists put them in a completely different group, and the surprising truth is that a mushroom is more closely related to you than to the grass it grows in.

A kingdom of their own

For centuries, living things were simply divided into animals and plants, and fungi ended up with the plants. Only in the 20th century did scientists recognise fungi as a separate kingdom. The differences are fundamental:

• No photosynthesis. Plants are green because of chlorophyll, which lets them make sugar from sunlight. Fungi have no chlorophyll. They must get their food from other sources.
• Digesting outside the body. Fungi feed by releasing digestive enzymes into their surroundings, breaking down food outside their cells, and then absorbing the nutrients. Some recycle dead leaves and wood, some infect living hosts, and many form partnerships with plant roots.
• Chitin walls. Plant cell walls are made of cellulose. Fungal cell walls are made of chitin, the same tough material that forms the shells of insects and crabs.
• Different food stores. Plants store energy as starch. Fungi, like animals, store it as glycogen.

Closer to animals than to plants

The most surprising discovery came from genetics. In 1993, biologists Sandra Baldauf and Jeffrey Palmer compared the sequences of several proteins found in plants, animals and fungi. The evidence pointed clearly to one conclusion: animals and fungi are each other’s closest relatives. Later studies confirmed this. Fungi and animals belong to a larger group sometimes called the opisthokonts, which split from the ancestors of plants long ago.

So, in evolutionary terms, the button mushroom on your plate is a closer cousin of yours than the lettuce next to it.

The mushroom is just the fruit

What we call a mushroom is only a small part of the organism. The real body of the fungus is a vast, hidden web of thin threads called hyphae, which together form the mycelium. It spreads through soil, wood or leaf litter, feeding as it grows. When conditions are right, the mycelium produces a mushroom, a fruiting body whose job is to release spores, the fungal equivalent of seeds.

Mycelium networks can become enormous. A honey fungus in Oregon’s Blue Mountains forms a single network covering about 9 square kilometres, making it one of the largest organisms on Earth, perhaps thousands of years old.

Why fungi matter

Fungi are essential to life on land. They are the main recyclers of dead wood and leaves, returning nutrients to the soil. Most land plants form partnerships with fungi in their roots, called mycorrhizae, trading sugars for water and minerals. Fungi give us bread, beer, cheese and medicines such as penicillin. But some also cause diseases in crops, animals and people, and a few mushrooms, like the death cap, are deadly poisonous.

In short

• Mushrooms are not plants; they belong to the separate kingdom of fungi.
• Fungi have no chlorophyll and feed by absorbing nutrients after digesting food outside their cells.
• Their cell walls are made of chitin, like insect shells, not cellulose.
• Genetic studies show fungi are more closely related to animals than to plants.
• A mushroom is only the fruiting body of a hidden network called the mycelium.
• Fungi recycle nutrients, partner with plant roots and give us foods and medicines.