Lichens have traditionally been referred to as a prime example of a symbiotic relationship. Each lichen consists of an intimate association between a fungus and a species of algae. The algae within the lichen photosynthesize, providing food for both symbionts. The fungus protects the alga from harmful light intensities, produces a substance that accelerates photosynthesis in the algae, and absorbs and retains water and minerals for both organisms. There is physiological and ultrastructural evidence that suggests the fungus parasitizes the algae in a controlled fashion and, in some instances, actually destroys the algal cells. There are about 25,000 species of lichens known and they are capable of living in environmental conditions that kill most other forms of life.
Although lichens can reproduce sexually, they are predominantly asexual reproducers. In the latter case, small powdery clusters of hyphae and algae, called soredia are formed and cut off from the thallus as it grows. These soredia are dispersed by wind or water and take up residence elsewhere. Sexual reproduction occurs when the fungal ascomata produce spores which germinate and parasitize independently living algae upon contact with them. Lichen algae reproduce by mitosis and simple cell division.