Lichen is a special association between a fungus and an alga.
The fungus forms the main body of the lichen, providing an upper surface that protects the alga underneath, while the alga manufactures food using the energy of sunlight (photosynthesis). Each lichen has its own distinct species of fungus, but all lichens share just a small number of algae species; in most cases this is a green alga.
These are amazing small plant like structures found on a range of surfaces ranging from rocks and boulders to gravestones and the bark of trees. They are renowned indicators of air quality, superb sources of dyes and can even tell us about the acidity or alkalinity of the habitat in which they are found.