Most plant species form a symbiosis (mutually advantageous living arrangement) with beneficial fungi. The roots are colonized by the fungus, which also ramifies through the soil. The combination of root and fungus is called mycorrhiza. Mycorrhiza is considered such a fundamental part of the plant that most species could not survive in nature without it. The few plants that do not need mycorrhiza (mostly weeds) are considered to be departures from the normal state of the plant kingdom.