A lichen is a symbiotic association between a fungus and an alga, or cyanobacterium. Although the fungus body normally completely surrounds the alga, the algal cells aren’t actually inside the fungal cells. Most lichens reproduce asexually; when conditions are good they will simply expand across the surface of the rock or tree. In dry conditions they become crumbly and small pieces will break off and are dispersed by the wind. The fungal component of many lichens will also sometimes reproduce sexually to produce spores. These spores must meet up with an algal partner in order to form a new lichen.