An association between two organisms in which both are mutually benefitted. It is a close physical & an obligatory relationship. Organisms provide resources to each other.
- Lichens – an association between fungus and algae.
- Mycorrhizae – an association between fungi and roots of higher plants.
- Plant-animal relationships – animals help in pollination and seed dispersal.
- Rhizobium grows in the root nodules of leguminous plants.
- Coralloid roots in Cycas.
- Ruminant mammals have microorganisms in their intestine that help in cellulose digestion.