Fungi: the hidden kingdom

Fungi were once grouped with plants, but they are a separate kingdom, and genetic evidence shows they are actually more closely related to animals than to plants. Unlike plants, fungi cannot photosynthesise. Like animals, they must get food from other organisms.

Fungi include mushrooms, moulds, yeasts and lichens, which are partnerships between fungi and algae. Scientists have described around 150,000 species, but estimates of the true number run into the millions.

A mushroom is only the fruiting body of a fungus, the part that produces spores, much as an apple is the fruit of a tree. Most of the fungus lives out of sight as mycelium, a network of microscopic threads called hyphae that spread through soil, wood or other food.

Fungi digest their food externally. Hyphae release enzymes that break down material around them and then absorb the nutrients. A single fungus can become enormous: one honey fungus in Oregon's Blue Mountains is estimated to cover almost 10 square kilometres, making it one of the largest living organisms known.

Fungi are among the most important decomposers on Earth. They are especially good at breaking down wood, including lignin, the tough material that stiffens plant cell walls, which few other organisms can digest.

Without decomposers, dead leaves, fallen trees and animal remains would pile up, and the nutrients locked inside them would never return to the soil for new plants to use.

Most land plants form partnerships with fungi called mycorrhizae. Fungal threads wrap around or grow into plant roots and extend far into the soil, gathering water, phosphorus and nitrogen for the plant. In return, the plant supplies the fungus with sugars made by photosynthesis.

Fossils suggest such partnerships helped the first plants colonise land hundreds of millions of years ago. Research has also shown that fungal networks can connect neighbouring trees and move carbon and nutrients between them, although how much this helps trees and how cooperative these networks really are is still debated among scientists.

• Yeast ferments sugars into alcohol and carbon dioxide, raising bread and brewing beer and wine.
• Moulds ripen cheeses such as Roquefort, Camembert and Brie.
• In 1928 Alexander Fleming noticed that Penicillium mould killed bacteria, leading to penicillin, the first widely used antibiotic.
• Fungi produce other medicines, including cyclosporine, which prevents organ transplant rejection.
• Some fungi cause disease, from athlete's foot to crop blights and the chytrid fungus devastating amphibians worldwide.

Many wild mushrooms are edible, but some are deadly and can closely resemble safe species. The death cap causes most fatal mushroom poisonings worldwide; its toxins damage the liver, and symptoms can be delayed for many hours, giving a false sense of safety.

Experienced foragers follow a firm rule: never eat a wild mushroom unless it has been identified with certainty by someone with expert knowledge.