Cladonia Lichens (Cladonia diversa). 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 brown fruiting bodies (apothecia) appear only when the two organisms have joined together. It is where the fungal spores are made and released. The vertical parts of the lichens are called podetia. Like all lichens, the ones in this photograph grow very slowly: only a few millimeters in a year.