An arbuscular mycorrhiza (AM) (plural mycorrhizae, a.k.a. endomycorrhiza) is a type of mycorrhiza in which the symbiont fungus (AM fungi, or AMF) penetrates the cortical cells of the roots of a vascular plant forming arbuscules. (Not to be confused with ectomycorrhiza or ericoid mycorrhiza.) Arbuscular mycorrhizae are characterized by the formation of unique structures, arbuscules and vesicles, by Glomeromycota and Mucoromycota, sister clades of the more well-known and diverse dikaryan fungi (all three are together called "symbiomycota"). AM fungi help plants to capture nutrients such as phosphorus, sulfur, nitrogen and micronutrients from the soil. It is believed that the development of the arbuscular mycorrhizal symbiosis played a crucial role in the initial colonisation of land by plants and in the evolution of the vascular plants. It has been said that it is quicker to list the plants that do not form endomycorrhizae than those that do. This symbiosis is a highly evolved mutualistic relationship found between fungi and plants, the most prevalent plant symbiosis known, and AMF is found in 80% of vascular plant families in existence today. Advances in research on mycorrhizal physiology and ecology since the 1970s have led to a greater understanding of the multiple roles of AMF in the ecosystem. An example is the important contribution of the glue-like protein glomalin to soil structure (see below). This knowledge is applicable to human endeavors of ecosystem management, ecosystem restoration, and agriculture. Evolution of mycorrhizal symbiosis Paleobiology Both paleobiological and molecular evidence indicate that AM is an ancient symbiosis that originated at least 460 million years ago. AM symbiosis is ubiquitous among land plants, which suggests that mycorrhizas were present in the early ancestors of extant land plants. This positive association with plants may have facilitated the development of land plants. The Rhynie chert of the lower Devonian has yielded fossils of the earliest land plants in which AM fungi have been observed. The fossilized plants containing mycorrhizal fungi were preserved in silica. The Early Devonian saw the development of terrestrial flora. Plants of the Rhynie chert from the Lower Devonian (400 m.yrs ago) were found to contain structures resembling vesicles and spores of present Glomus species. Colonized fossil roots have been observed in Aglaophyton major and Rhynia, which are ancient plants possessing characteristics of vascular plants and bryophytes with primitive protostelic rhizomes. Intraradical mycelium was observed in root intracellular spaces, and arbuscules were observed in the layer