Name the water fern that is an excellent biofertiliser for rice cultivation. What helps the fern to do so?

Explanation

**Step-by-Step Solution:**

1. **Identify the Water Fern**: The water fern that is recognized as an excellent biofertilizer for rice cultivation is called **Azolla**.

2. **Understand the Symbiotic Relationship**: Azolla has a symbiotic association with a type of blue-green algae known as **Anabaena**. This relationship is crucial for its effectiveness as a biofertilizer.

3. **Role of Anabaena**: Anabaena is capable of nitrogen fixation, which means it can convert atmospheric nitrogen into a form that is usable by plants. This process occurs within the leaf cavities of Azolla.

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