What mycorrhiza is
Mycorrhiza is the symbiosis between a fungus and the root of a higher plant. The fungus receives carbohydrates from the tree, and in return helps it obtain water, phosphorus, nitrogen and other elements from the soil. Without mycorrhizal partners, most forest trees cannot survive in natural conditions.
Types of mycorrhiza
Researchers distinguish several types of mycorrhizal symbiosis. The most common are ectomycorrhiza (an outer sheath around roots, typical of conifers, birches, oaks) and arbuscular mycorrhiza (the fungus penetrates inside root cells, typical of grasses and many crop plants). Each type sets its own "exchange rules" between partners.
Why it matters
Mycorrhizal networks are often called the "wood wide web": through fungal hyphae, not only the roots of one tree but also neighboring plants — including different species — exchange signals and nutrients. These findings are confirmed in field experiments using isotope labeling, although the details of plant "conversations" remain debated.
More on this and related topics (mycoremediation, mycopesticides) in the adapted review of Stamets's book Mycelium Running.