Cladonia lichen (Cladonia diversa) growing on a drystone wall. A lichen is a symbiosis between a fungus and an alga. The fungus provides the structural support for the lichen partnership, while the alga provides food by photosynthesis. The bright colours appear only when the two organisms have joined together, and the red areas (apothecia) are where the fungal spores are made and released. This lichen has a fructose shape, as it has formed upright branches (podetia). Like all lichens, it grows very slowly: only a few millimetres in a year. Photographed in Kielder Forest, Northumberland, England.