Lichen is an association of fungus and green alga or cyanobacterium. Lichen, therefore, is really a compound creature. This association is called “symbiosis”. The fungus is a network of small fungal root-hairs (called “hyphae”) that mechanically protects the algae which, in turn, produces carbohydrates, organic nitrogen and vitamins, thus providing the fungus with growth materials. In this manner, both organisms benefit. There are over 1,400 different species of lichens, so I will not make an attempt to see how many I can photograph.
Lichens eventually die and, when they do, their decomposition creates a weak solution of carbonic acid that eats into the rock. This “rock dust”, mixed with other organic materials makes a thin soil where mosses can grow. Mosses, in turn, provide a base for grasses and other small plants.