Lace lichen (Ramalina menziesii) growing on the branches of a valley oak tree (Quercus lobata). Lichens are symbiotic organisms formed of a fungus and an alga that mutually benefit one another. The alga is contained within the hyphae (filaments) of the fungus. They are therefore protected from harsh conditions, and in particular from dessication. The alga supplies the fungus with nutrients that it makes by photosynthesis. Lichens are usually found growing on stones and the bark of trees.