Lichens

'Lichen', is such an association of an 'alga' and a 'fungus' in which two organisms remain so closely associated with each other that they appear to be a single plant.

The fungus usually a member of 'Ascomycotina', is less commonly of 'Basidiomycotina'. In alga, it mostly belongs to blue greens (cyanophyta) or green alga(chlorophyta). Theophrastus was the first person who introduced the word 'Lichen' (lie, ken) into Greek literature in about 300 BC. He used the word primarily to describe the outgrowths from the base of olive trees. Lichens occurs in all situation from the Arctic to Antartic. Structurally the lichen thallus is irregular, variously coloured and shows several morphological types. Depending on their external form four different kinds of lichens are recognised; they are crustose lichen, foliose lichen, fruiticose lichens and filamentous lichens. The photosynthetic partner (alga) prepare food while the fungus provides cover and obtains water and minerals. The lichens reproduced vegetatively (Fragmentation), asexually and by sexual method. Lichens are economically most important in the preparation of drugs, perfumes and food obtained from the lichens. Lichens are also indicators of Pollution. They can be used in monitoring pollution.