Which symbiotic nitrogen-fixing cyanobacterium lives in the roots of Cycas?

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks for the name of a symbiotic nitrogen-fixing cyanobacterium that resides in the roots of a specific plant, Cycas.

2. **Identifying Cycas**: Cycas is a genus of plants known for its unbranched, coral-like roots. These roots are apogeotropic, meaning they grow upwards, and they are adapted to harbor symbiotic organisms.

3. **Symbiotic Relationship**: In the roots of Cycas, there exists a symbiotic relationship with certain cyanobacteria. These cyanobacteria are capable of fixing nitrogen, which is crucial for the plant's nutrient supply.

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