"Crustose lichen is the only plant that will grow on a bare rock. This is accomplished by gathering small amounts of water and then in winter when the water freezes, it cracks the rock surface, providing the lichen with minerals and organic materials. Over time this process forms a tiny bit of soil on the rock. These lichen tend to lay flat on their host, looking much like paint splatters, making them hard to remove. They range in color from black, gray, brown, orange, yellow and green. About 75 percent of all lichen in the world are crustose lichen. When dating stone walls and gravestones, scientists measure the radius of this lichen growth since it grows so slowly and lives for centuries."
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