lichen

A lichen is a symbiotic association between an alga and a fungus - in other words, each lichen is not one organism but two. Lichens grow very slowly, usually as encrustations on rocks, walls, or wood. They are found throughout the world but are unable to survive where the atmosphere is polluted, so they are good indicators of clean air.

Any organism of a unique group that consists of associations of a specific fungus and a specific photosynthetic organism (an alga or a cyanobacterium) living together in a mutually beneficial relationship (symbiosis). Found as coloured patches or spongelike masses on trees, rocks, and other surfaces, lichens flourish in harsh conditions, for example on exposed rock faces in Antarctica. (Group Lichenes.)

Some lichens are edible, for example, reindeer moss and Iceland moss; others are a source of colour dyes, such as litmus, or are used in medicine. They are sensitive to pollution in the air (see indicator species).