From the beginning of mycorrhizal research, understanding of functioning was based on the morphological structure of the plant–fungus interface. The structure of the fungal hyphae within the root, which regulates the exchange of resources between plant and fungus, and the extramatrical hyphae, the extent and locations of which dictate the ultimate flows of resources between soil, hyphae, and root, characterizes a mycorrhiza, determines the type of mycorrhiza, and, to a large extent, determines the functioning of that mycorrhiza (255). Overall, knowing these two structural components, internal root colonization and extramatrical hyphae, allows us to make a number of predictions about the mechanisms and quantity of resource exchange between the two symbionts.