(1) Lichens represent an unique example of mutualism between algae and fungi. In these plants algae (phycobionts ) and fungi (mycobionts) live together in such a way that they appear as one organism. On the basis of type of thallus the lichens are divided into three type–

(i) Crustose or crustaceous: They occur as thin or thick crust over rocks, soil, tree barks etc.

(ii) Foliose or foliaceous: These are leaf like lichens, which attach to the substratum by means of special rhizoid like organs developed from lower side of thallus.

(iii) Fruticose or filamentous: Cylindrical, flat or ribbon like, branched structures attached to the substratum by their basal ends. They are shrubby lichens.