(a) : Lichens are peculiar dual organisms produced by the intimate association of two organisms - a fungus and an alga. The association between the two organisms is called symbiosis. On the basis of fungal partner, lichens are of 2 types : (i) Ascolichens : In which ascomycetes member is the fungal partner. Further in ascolichens, algal partner is mostly member of green algae and rarely blue-green algae. (ii) Basidiolichens : Where basidiomycetes member is fungal partner. In them algal partner is generally blue-green algae. In 80% cases algal partner is member of green algae or chlorophyceae and in 20% cases, blue-green algae or myxophyceae. Important members of green algae found in lichens are: Trebauxia, Pleurococcus, Trentepohlia and Cladophora.