Lichens are peculiar organisms in which algae and fungi live together. They are usually flat and crust-like, with no roots and often grow on roofs, rocks or tree branches, and are frequently brightly coloured. The main structure of a lichen is the fungal part, but it also contains algae cells which contribute food through photosynthesis. Reindeer moss is a form of lichen that is very common throughout the Arctic. It forms the main diet of the caribou and other grazing animals. Lichens grow very slowly, but can eventually cover very large areas. Some individual ones are extremely old, and some lichens growing in rocks in Antarctica are thought to be 10,000 years old — they are the oldest living organisms.
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