write a short note on lichens.

Lichen is an organism that is formed by the symbiotic association of a fungus and an alga or cyanobacterium and occurs as crusty patches or bushy growths on tree trunks, bare ground, etc. . The fungus surrounds the algal cells, often enclosing them within complex fungal tissues unique to lichen associations.Both partners gain water and mineral nutrients mainly from the atmosphere, through rain and dust. The fungal partner protects the alga by retaining water.Lichens have also been used in making dyes and perfumes, as well as in traditional medicines.