A mycorrhiza (from Greek words mykes, "fungus" and rhiza, "root"; pl. mycorrhizae, or mycorrhizas is a symbiotic association between a fungus and a plant. The term mycorrhiza refers to the role of the fungus in the plant's rhizosphere, its root system. Mycorrhizae play important roles in plant nutrition, soil biology and soil chemistry. In a mycorrhizal association, the fungus colonizes the host plant's root tissues, either intracellularly as in arbuscular mycorrhizal fungi (AMF or AM) or extracellularly as in ectomycorrhizal fungi. The association is sometimes mutualistic. In particular species or in particular circumstances mycorrhizae may have a parasitic association with host plants. The associations of fungi with the roots of plants have been known since at least the mid-19th century. However early observers simply recorded the fact without investigating the relationships between the two organisms. This symbiosis was studied and described by Franciszek Kamieński in 1879-1882. Further research was carried out by Albert Bernhard Frank, who introduced the term mycorrhiza in 1885.