A lichen is essentially a stable self-supporting association of a fungus (mycobiont) and an alga or cyanobacterium (photobiont). A lichen is an ecologically obligate, stable mutualism between an exhabitant fungal partner and an inhabitant population of extracellularly located unicellular or filamentous algal or cyanobacterial cells.
(Hawksworth & Honegger in Williams (ed.) Plant galls: 77, 1994)
Institute of Ecology and Botany, Centre for Ecological Reserach, Hungarian Academy of Sciences, H-2163 Vácrátót, Hungary