Lichens are plant-like organisms that do not have roots, leaves or flowers. Lichens are colourful and their colour ranges from pale grey to bright green in moist conditions. There are about 15,000 species of lichens exist on the Earth. They grow in waste places, on bare rocks, bare soil, dead wood and tree bark. They can bear heat, cold, dampness or dryness. However, they can be destroyed by smoke, dust and coal gases.