Popular name for various chronic skin diseases, e.g. eczema, psoriasis vulgaris, pityriasis versicolor, lichen planus and others.
Botanically, lichens are composed of the symbiosis of fungus and algae: while the fungus forms hyphae adhering to the soil and absorbs water and nutritive salts, the alga is responsible for photosynthesis. Lichens grow in cool, humid places, but are also able to tolerate drought and extreme temperatures.