FIGURE 20.7 Lichens. Lichens, such as this species named “British soldiers” (Cladonia cristatella), are composed of a fungus and an alga that are tightly linked together in a mutualistic interaction. The fungus provides nutrients to the alga, and the alga provides carbohydrates for the fungus via photosynthesis.
(Ed Reschke/Peter Arnold Inc./Getty Images)