Name the organism that fixes nitrogen in symbiotic association with a water fern. Where does it live in such plants?

Explanation

**Step-by-Step Solution:**

1. **Identify the Water Fern**: The organism in question is a water fern known as Azolla. This is a small aquatic fern that grows in freshwater bodies.

2. **Determine the Nitrogen-Fixing Organism**: The nitrogen-fixing organism that has a symbiotic relationship with Azolla is a type of blue-green algae called Anabaena.

3. **Understand the Symbiotic Relationship**: In this symbiotic association, Anabaena lives within the leaf cavities of the Azolla plant. This relationship is beneficial for both organisms.

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