Lichens commonly grow on rocks, trees, fence posts, and similar objects. Its typical greenish gray color is due to the combination of the chlorophyll from the photosynthetic organism with the colorless fungi, although sometimes the thallus may be red, orange, or brown.
Lichens require no food source other than light, air, and minerals.
They depend heavily on rainwater for their minerals and are sensitive to rain-borne pollutants. Lichens usually reproduce by the breaking off of a segment that contains both components. The fungal component of lichens produces acids that disintegrate rock, giving the lichen a better hold and aiding weathering processes, which eventually turn rock into soil. Lichens can withstand great extremes of temperature and are found in arctic, antarctic, and tropical regions. They are often the pioneer forms of life-as in parts of Iceland and Greenland, where they are the predominant vegetation.