Lichens are an intimate combination of algae (phycoboint) and fungi (mycoboint), showing mutualism (symboisis). Algal partner may be a cyanobacterium or green alga and fungal partner is usually anascomycetes (ascolichen) or rarely is basidiomycetes (basidiolichen).

They are found in all types of habitats, growing on rocks(saxicolous), soil(terricolous), bark(corticolous), wood(lignocolous), etc., and occur in there growth forms- crutose, foliose and fruticose lichens. The thallus consists of algal cells, which synthesizes food and loosely interwoven fungal hyphae for food storing and water holding. Root like anchorage organ are called rhizine.

Vegetative reproduction is by fragmentation, rejuvenation and vegetables propagules like soredia, isidia, cephalodia, soralia, hormocytes, etc. Asexual reproduction occurs by oidia, conidia and pycniospores. In ascolichens, male sex organs are called spermogonia and female as ascogonia. The fruiting body is the apothecium bearing asci that contain ascospores.

Lichens are of great important to nature, humankind and animals. They are pioneers in a lithosores and soil formers. Some are air pollution indicators. Many lichens are used as food and fooder. Cladonia is called reindeer moss. Most edible linches are laxatives.

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Different lichens are used for tanning, brewing and preparing essential oils and perfumes. Many have antibiotic properties, used for diabetes and lung diseases. Usnic acid from Usnea is a broad spectrum antibiotic. Fungal partners of lichen secrets pigments used as dyes. Litmus is prepared from Rocella montaignei. Some are responsible for skin and respiratory diseases, allergy and forest fires.