type of mycorrhizae that grow between the cortical or epidermal cells of absorbing roots. forming an outer sheath around the root (see mycorrhizae; contrast with endomycorrlzizae).

ectomycorrhizae (Wikipedia)
Ectomycorrhizal symbiosis, showing root tips with fungal mycelium from the genus Amanita

An ectomycorrhiza (from Greek ἐκτός ektos, "outside", μύκης mykes, "fungus", and ῥίζα rhiza, "root"; pl. ectomycorrhizas or ectomycorrhizae, abbreviated EcM) is a form of symbiotic relationship that occurs between a fungal symbiont, or mycobiont, and the roots of various plant species. The mycobiont is often from the phyla Basidiomycota and Ascomycota, and more rarely from the Zygomycota. Ectomycorrhizas form on the roots of around 2% of plant species, usually woody plants, including species from the birch, dipterocarp, myrtle, beech, willow, pine, Douglas fir and rose families. Research on ectomycorrhizas is increasingly important in areas such as ecosystem management and restoration, forestry and agriculture.

Unlike other mycorrhizal relationships, such as arbuscular mycorrhiza and ericoid mycorrhiza, ectomycorrhizal fungi do not penetrate their host's cell walls. Instead they form an entirely intercellular interface known as the Hartig net, consisting of highly branched hyphae forming a latticework between epidermal and cortical root cells.

Ectomycorrhizas are further differentiated from other mycorrhizas by the formation of a dense hyphal sheath, known as the mantle, surrounding the root surface. This sheathing mantle can be up to 40 µm thick, with hyphae extending up to several centimeters into the surrounding soil. The hyphal network helps the plant to take up nutrients including water and minerals, often helping the host plant to survive adverse conditions. In exchange, the fungal symbiont is provided with access to carbohydrates.

Well known EcM fungal fruiting bodies include the economically important and edible truffle (Tuber) and the deadly death caps and destroying angels (Amanita).

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