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 |