Explanation:

 Justification: A lichen is not a single organism. Rather, it is a symbiosis between different organisms - a fungus and an alga or cyanobacterium.
• The lichen fungi (kingdom Fungi) cultivate partners that manufacture food by photosynthesis. Sometimes the partners are algae (kingdom Protista), other times cyanobacteria (kingdom Monera), formerly called blue-green algae.
• Lichens can be found growing in almost all parts of the terrestrial world, from the ice-free polar areas to the tropics, from tropical rainforests to those desert areas free of mobile sand dunes. While generally terrestrial a few aquatic lichens are known. Learning: Most lichens grow very, very slowly, often less than a millimeter per year, and some lichens are thought to be among the oldest living things on Earth. Lichens with known, slow growth rates, like Rhizocarpon geographicum, have been used to estimate the dates of geological events such as the retreat of glaciers.