Umbilicaria mammulata growing on a boulder along the trail near Linville Falls. These lichens are common in the Southern Appalachians and grow on granitic and acid rock, sandstone, and quartz boulders and steep rock walls, often in a forest setting. They can grow quite big where conditions are right and they are left undisturbed. The thallus is typically a few centimeters in diameter up to dinner plate-size for the larger ones, but specimens up to 60 cm (2 feet) in diameter have been reported in the Smoky Mountains. They are among the largest lichens in the world.They are typically a lighter brown or grayish-brown on the upper side and dark brown to black on the underside and cling to the rock at a single point of their bowl or plate-like thalli (umbilicate attachment). When exposed to water, such as during rain, the brown layer on top turns green, facilitating photosynthesis by the algal partner of the symbiosis. These here had recently been exposed to rain, so their center is still green from the photosynthetic algae, but they are in the process of drying up again. They tend to turn brown when dry and then re-green when moistened again.