A mycorrhiza (typically seen in the plural form mycorrhizae meaning "fungus roots") is a specialized form of hyphae that extends from the mycelium of a fungus into the roots of a plant.
Mycorrhizae form a mutualistic symbiotic relationship with the roots of 95% of all vascular plants. This association provides the fungus with a renewable source of food, and allows the plant to make use of the mycelium's tremendous surface area to absorb nutrients from the soil. Some of the earliest fossil plants show evidence of mycorrhizae associated with them.
Some mycorrhizal hyphae enter within the plant cell's wall and grow to envelop the cell. However, most mycorrhizae have a more advanced structure, in which the hypha lives inside an extensive invagination (inpocketing) of the cell membrane.