Algae have evolved a number of symbiotic partnerships with other organisms. The most famous is the plant-like lichen, which are each formed by a fungus with an alga. It is a highly successful life-form, and twenty thousand 'species' are known. In all cases the lichen are quite different in appearance and life-style from either constituent; it is possibly the most complete symbiosis known. Both constituents gain from their access to niches with low nutrient value, which is where lichen are found.