Mycorrhiza

A mycorrhiza is a close, mutually helpful partnership between a fungus and the roots of a plant. The fungal hyphae spread far out into the soil, reaching water and mineral nutrients that the roots alone could not, and pass them to the plant. In return, the plant supplies the fungus with sugars made by photosynthesis. The great majority of vascular plant species form mycorrhizae, which makes this one of the most widespread symbioses on land.

There are two main kinds. In ectomycorrhizae, common on forest trees, the fungus wraps the root tip in a sheath and threads between the outer root cells without entering them. In endomycorrhizae, also called arbuscular mycorrhizae, the fungus grows into root cells and forms tiny branched structures called arbuscules, where nutrients are exchanged; these fungi all belong to the group Glomeromycota. Orchids rely on a fungal partner even to germinate, because their dust-like seeds carry almost no food.

Mycorrhizae are a classic example of mutualism in ecology and botany courses, and they help explain why healthy soil fungi matter in farming and forestry.