Some yeasts reproduce sexually, by forming asci or basidia, and are classified as Ascomycota or Basidiomycota. Others are placed in the imperfect fungi because no sexual stages are known. Humans have used yeasts to raise bread and ferment alcoholic beverages for thousands of years. A lichen is a symbiotic association of unicellular or filamentous green algae or cyanobacteria that are held in a mesh of fungal hyphae. The alga provides the fungus with food.
Cyanobacteria fix nitrogen and provide organic nitrogen.
The fungus provides the alga or the cyanobacteria with a suitable physical environment for growth. The fungal component is most commonly an ascomycete, but several basidiomycete lichens are known. Mycorrhizae are mutualistic associations of plant roots and fungi. The extensions of the fungal mycelium from the hyphae that form the mycorrhizae greatly increase the absorptive surface of the plant root. The partners exchange minerals accumulated from the soil by the fungus for organic nutrients synthesized by the plant.