An arbuscular mycorrhizal fungus (plural mycorrhizae or mycorrhizas, a.k.a. endomycorrhiza, AM fungi, or AMF) is a type of mycorrhiza in which the fungus penetrates the cortical cells of the roots of a vascular plant. (Not to be confused with ectomycorrhiza or ericoid mycorrhiza.) Arbuscular mycorrhizas are characterized by the formation of unique structures, arbuscules and vesicles by fungi of the phylum Glomeromycota. 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.