Lichens are the composite or dual organisms representing a symbiotic association between a fungus and alga. The fungal partner is called mycobiont whereas algal partner is called phycobiont. Algae prepares food for fungi and fungi provides shelter and absorbs mineral nutrient and water for its partner. Lichens grow on barren rocks, cooled volcanic lava and icy tundra soils. They can tolerate extreme dessication but not the polluted air. Depending upon the shape, the three morphological forms of lichens are :
(a) Crustose lichens : They are thin membranous forms of lichens attached to the substratum in the form of a crust, e.g., Khizocarpon, Haematoma etc.
(b) Foliose lichens : They have flat, lobed and leaf like body attached to the substratum by fungal hyphae called rhizines. e.g., Parmelia.
(c) Fruticose lichens : They have branched and small bushy bodies, which is attached to the substratum by discs, e.g., Usnea and Cladonia etc.
Importance of lichens :
(a) Litmus used as pH indicator was prepared from lichen Rocella tinctorea.
(b) Scented incense or perfumes are obtained, from Ramalina and Evernia.
(c) Lichens acts as pollution indicators.
(d) Certain lichens are source of food for grazing animals.