Macrophotograph of a lichen Xanthoria aureola, growing on a gravestone in a Norfolk, UK, churchyard. The dark orange discs are the apothecia or spore producing bodies. The yellowish area is the crustose (crustlike) thallus or plant body. Symbiotic organisms, lichens consist of an algal & a fungal partner. The fungus benefits from the photosynthetic activity of the alga. The advantage to the alga remains unclear, but may lie in protection from drought & intense light. Slow growing organisms (1-2mm per year), they live to immense ages, colonising tree trunks, soil, rock, & mortar. Lichens are sensitive to sulphur dioxide & hence used to monitor air pollution.