Fungi

For most of history, fungi were filed under plants. They grow in soil and do not move, after all. But a mushroom has no leaves, no roots and no chlorophyll. It cannot make food from sunlight; it has to find it, as animals do, and it digests that food outside its body before absorbing it. Fungi are a kingdom of their own, more closely related to us than to oak trees, and without them the living world would choke on its own dead.

The hidden body

The mushroom you see is not the fungus. It is the fruit, pushed up briefly to release spores, like an apple on a tree. The organism itself is almost always hidden: a network of fine white threads, each thinner than a hair, spreading through soil, wood or whatever it is feeding on. This network, the mycelium, is the fungus's body. It grows from its tips, branching and fusing, and can be vast. A single honey fungus in the Blue Mountains of Oregon extends across nearly ten square kilometres and is probably thousands of years old, making it one of the largest and oldest living things known.

Fungi feed by secreting enzymes into their surroundings that break down whatever is there, wood, leaves, flesh, bread, then soaking up the dissolved results through the walls of their threads. Because they eat from the outside, they can digest things almost nothing else can, including the tough lignin that stiffens wood.

Spores

Fungi spread by spores, microscopic particles produced in staggering numbers: a single large mushroom can release billions in a day, and the air you breathe contains thousands in every cubic metre. Some are fired from their mushrooms with accelerations far higher than a rocket's; puffballs release clouds when struck by raindrops; stinkhorns smear theirs in a foul-smelling slime that flies carry away. Where a spore lands on something edible and damp, it sprouts a thread, and a new mycelium begins.

The recyclers

Every autumn forests drop billions of tonnes of leaves and, over time, whole trees. Fungi are the main reason those materials do not simply pile up. They break dead wood and leaves into their components and return the carbon, nitrogen and minerals to the soil, where plants take them up again. The soil itself, dark and crumbly, is largely the product of fungal digestion. Without fungi, nutrients would stay locked in dead matter and the cycle of life would stall.

The same ability makes them troublesome. Fungi rot timber, spoil food, mildew fabric and cause rust and blight in crops. Potato blight, a fungus-like organism, caused the Irish famine of the 1840s. Fungal diseases are currently devastating amphibians and bats around the world, and wheat rusts remain a major threat to the global food supply.

Partners of plants

About 90 percent of plant species live in partnership with fungi at their roots. The fungal threads wrap around or penetrate the root and spread far into the soil, gathering water and minerals, especially phosphorus, which the plant's own roots are poor at finding. In return the plant pays in sugar from its leaves. These mycorrhizal networks, which link the roots of different trees across a forest floor, may be the oldest partnership on land; fossils show them in the earliest land plants, 400 million years ago, and plants probably could not have colonised land without them.

Lichens are an even closer alliance: a fungus and a photosynthetic alga or bacterium growing as a single organism, the alga feeding the fungus and the fungus sheltering the alga. Lichens can live on bare rock, in deserts and in Antarctica, and are often the first life to colonise new ground.

Fungi and people

Yeasts, single-celled fungi, turn sugar into alcohol and carbon dioxide, which is why bread rises and why beer and wine exist; humans have been exploiting them for at least 10,000 years. Moulds ripen blue cheese and soy sauce. In 1928 a mould contaminating a laboratory dish turned out to produce penicillin, the first antibiotic, and fungi have since yielded drugs for cholesterol, organ transplants and cancer. Edible mushrooms are farmed in millions of tonnes, while some wild species are among the deadliest poisons known; a single death cap can kill. And mycelium is now being grown into leather substitutes, packaging and building insulation.

What we still don't know

Fungi are among the least explored of the major groups of life. Only about 150,000 species have been described, and estimates of the real total range from two to four million, most of them microscopic and most of them unnamed. How mycorrhizal networks function across whole forests, whether they truly carry signals between trees, and how far fungal diversity shapes the soil beneath every field are questions that researchers are only beginning to answer.