SEM of Reindeer Lichen (Cladonia rangiferina). 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. Cladonia lichen are classified as fruticose due to their tall erect structures. Like all lichens, the ones in this photograph grow very slowly: only a few millimeters in a year. The calibration bar is 100 um and the magnification was .52 x