Symbiotic Relationship:
Symbiotic relationship refers to a close and long-term interaction between two different species that benefits at least one of the species involved.

Example of Symbiotic Relationship:
i) Lichen:
Lichen is an example of a symbiotic relationship between a fungus and an alga. The fungus provides a protective environment and absorbs water and nutrients from the air, while the alga produces food through photosynthesis.

ii) Rhizobium and leguminous plants:
Rhizobium is a type of bacteria that lives in the root nodules of leguminous plants such as peas, beans, and lentils. The bacteria take nitrogen from the air and convert it into a form that the plant can use for growth. In return, the plant provides the bacteria with carbohydrates and other nutrients.

Explanation:
The relationship between Rhizobium and leguminous plants is an example of mutualism, where both the species involved benefit from each other. The bacteria provide the plant with nitrogen, an essential nutrient for growth, while the plant provides the bacteria with carbohydrates and other nutrients. This relationship is important for the growth and development of leguminous plants and is also beneficial for the environment. The conversion of nitrogen gas into a usable form by Rhizobium reduces the need for nitrogen fertilizers, which can be harmful to the environment if overused.

In conclusion, symbiotic relationships are important for the survival and growth of different species. They provide mutual benefits and help maintain the balance of ecosystems.