A wide array of interactions among plants, animals, and microorganisms occurs in nature. Some of these relationships are characterized by a close physical association among species that continues for a large period of the life cycle. In 1879 German botanist Heinrich Anton de Bary coined the term "symbiosis" to describe these relationships, meaning the living together of different species of organisms. Many people associate symbiosis with mutualism, interactions that are beneficial to the growth, survival, and/or reproduction of both interacting species. But symbiotic interactions also include commensalism (one species receives benefit from the association and the other is unaffected), amensalism (one species is harmed, with no effect on the other), and parasitism. An example of commensalism is found in the anemone fish, which gains protection from living among the poisonous tentacles of the sea anemone, but offers no known benefit to its host.
A common and widespread symbiosis occurs between terrestrial plants and fungi that colonize their roots; this is called "mycorrhizae.” Unlike pathogenic fungi that cause disease, mycorrhizal fungi benefit the plant in several ways. These fungi germinate from spores in the soil to form thin threadlike structures called hyphae, which grow into the roots of plants. Once the roots are colonized, the fungal hyphae grow out from the root to explore the soil beyond the reach of the roots, gathering essential mineral nutrients and transporting them into the plant, increasing its growth. In return, the plant provides carbohydrates as a food source for the fungus. Mycorrhizal symbiosis occurs in about 80 percent of all plant species. It is essential to many plants in low-nutrient environments because their roots alone are incapable of absorbing enough of some essential minerals such as phosphorus. The symbiosis is essential to the fungus because,