Lichens

These primitive organisms are the result of an association between a fungus and algae. Some can be spotted due to the yellow colour they give to rocks.
Lichens can develop thanks only to nutritious mineral substances in the air and in rainwater. They are good indicators of air pollution as the absorb miniscule quantities of pollutants and store them up.
In the picture below: Rhizocarpon gr. geographicum (specific identification only after examination of spores). Photo: Franck Guigo