A lichen = A fungus + algae
Unlike moss, which often resembles grass, lichens seem more like part of the rock than living plants.
Crustose lichens grow closest to rock surfaces and resemble tiny pebbles. If you break off a fragment of this kind of lichen, you will find it dry.
Lichens are composite organisms composed of a fungus and one or more algae living together in symbiotic association, in which the algal partner produces essential nutrients for the fungal partner through photosynthesis, while the fungal partner provides mechanical support to the algae partner.
Lichens are often the very first life forms to colonize freshly exposed rock surfaces high in the mountains, and they immediately begin the very slow process of weathering minerals from the barren rock and incorporating them into their bodies.
Development and establishment of lichen on a substratum is achieved by fruiting bodies (apothecia) produced by the fungal partner, which must germinate and find an algal partner before they can form a new lichen thallus or may produce minute fragments (as finger-like outgrowths, isidia or sugar-like granules, soredia) containing both partners, which can disperse quickly and colonise available habitats.
Being pioneers on rock surface lichens are an important component of the ecosystem that establishes life on rock and barren disturbed sites. As lichens colonise rocks, they trap dust, silt and water which leads to biogeophysical and biogeochemical weathering of the rock surface leading to soil formation.
