The fixation of free nitrogen from the air in the nodules of roots of leguminous plants is done by symbiotic bacteria

Options

A. Azotobacter
B. Rhizobium
C. Bacillus
D. Micrococcus.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**:
The question asks about the bacteria responsible for the fixation of free nitrogen from the air in the nodules of roots of leguminous plants.

2. **Identifying the Symbiotic Relationship**:
In leguminous plants, certain bacteria form a symbiotic relationship with the plant roots. This means that both the bacteria and the plant benefit from each other.

3. **Recognizing the Role of Nitrogen-Fixing Bacteria**:
Nitrogen-fixing bacteria convert atmospheric nitrogen (N2) into a form that plants can use, such as ammonia (NH3). This process is crucial for plant growth, especially for legumes.

4. **Analyzing the Options**:
The options provided are:
- A) Azotobacter
- B) Rhizobium
- C) Bacillus
- D) Micrococcus

5. **Evaluating Each Option**:
- **Azotobacter**: This is a nitrogen-fixing bacterium, but it is free-living and does not form nodules with plant roots.
- **Rhizobium**: This is the correct answer as it specifically forms nodules in the roots of leguminous plants and is responsible for nitrogen fixation.
- **Bacillus**: This is a genus of bacteria that does not play a significant role in nitrogen fixation in legumes.
- **Micrococcus**: This bacterium is not involved in nitrogen fixation.

6. **Conclusion**:
Based on the analysis, the correct answer is **B) Rhizobium**. This bacterium forms a symbiotic relationship with leguminous plants, facilitating nitrogen fixation.