Many lichens grow in inhospitable terrain; for instance, in the rocky highlands and cold Arctic. They also grow on the walls of the houses in big cities. The growth rate of lichens is very slow—a few species grow less than one millimetre in a year. If you step on them, you may destroy in seconds a plant that took hundreds of years to grow. Since lichens are very sensitive to air pollution, they are often used as pollution indicators. The cleaner the air the more lichen will grow.
