Lichens are unique living beings, as they are formed by the symbiotic relationship of a fungus and an alga. Their ability to cling to rocks and their tolerance for dry weather enable them to colonise environments which are impossible for other plants.
The fungus provides support, protection, a source of minerals, store of moisture and sexual reproduction, while the alga provides nutrients through photosynthesis.
They feed from the atmosphere, and they also dissolve minerals from the substrate by secreting organic acids.
Some of their principal characteristics are very slow growth (a few mm per year), strong resistance to drying out (water content of 2%) and the ability to survive large temperature variations (from -70 °C to +70 °C). They purify the air of minerals and pollen and are a good indicator of the absence of contamination.
They have uses as food, medicine, dyes and in perfumery.