Generally the fungal threads create the body of the lichen with the algal cells living within them. This gives the lichen strength and prevents the alga from drying out. The fungi also manufacture a type of acid which serves to eat away at underlying rock and provide the lichen with a foothold. Lichens therefore erode rock, very slowly. It has been estimated that they produce a centimetre of topsoil in two thousand years.
Many lichens produce small powdery granules called soredia on their surface.
These contain both algal cells and fungal threads. When washed or blown away, or carried by insects, they grow into new lichens. Sometimes the fungal partners make their own fruiting bodies. These vary in appearance, resembling small volcanoes, cups or jam tarts. When these burst, the fungal spores are dispersed but if they do not pair up with the correct algal cells they cannot develop.