Lichen is a combination of an alga and a fungus and rhey help each other by forming a symbiotic relationship. Symbiosis i slichens is a symbiotic relationship of green or blue - green algae which is living among the filamentous of fungus. Because, the blue-green alage( cyanobacteria) produces food by photosynthesis , fungus gets benifited out of it. Also , fungus filaments provide protection to the alage from outside enviroment.