A small branch covered with an unidentified, green, feathery lichen. Lichens are symbiotic organisms, consisting of an algal and a fungal partner. The algae live enclosed within a loose mass of fungal hyphae. The fungus benefits from the photosynthetic activity of the alga, whereas the advantage to the alga probably lies in protection from drought and intense light. Slow growing organisms, lichens live to immense ages, colonising tree trunks, soil and rock.