Once thought to be rare and something of a biological oddity, it is now known that the association of fungi with the roots of plants, which is known as a mycorrhizal association, is very usual and takes place with most taxa of plants. This association of fungi with roots is an example of mutualism and is of special importance with certain edible fungi. There are a number of important edible fungi that are mycorrhizal fungi. Among these are the truffle (Tuber spp.), the matsutake (Tricholoma
matsutake), and the boletes (Boletus spp.). While the mycelium of mycorrhizal fungi can be grown saprophytically, the formation of fruiting bodies is a product of the interaction of the fungus with the roots of a particular plant or group of plants; until the details of these interactions are discovered, the ability to control the fruiting of these mycorrhizal fungi will elude us. It is interesting that the
edible species of mycorrhizal fungi include some of the most highly valued ones. These fungi belong to the type of mycorrhiza known as ectotrophic, in which the fungi form a sheath around the root with hyphae penetrating slightly into the root cortex. In ectotrophic mycorrhiza the hyphae grow between the cells of the root cortex, whereas in endotrophic mycorrhiza (also called vesicular arbuscular mycorrhiza) the hyphae form swollen or branched structures within the root cells. The endotrophic mycorrhizas do not form a sheath as is the case with ectotrophic mycorrhiza, and they are zygomycetes that are commonly associated with herbaceous plants. On the other hand, ectotrophic mycorrhizas are Ascomycetes or more frequently Basidiomycetes that are associated
with forest trees (either broad-leaved deciduous trees or conifers).