Coralloid roots of Cycas are useful in

Options

A. absorption of water
B. Nitrogen fixation
C. absorption
D. respiration from air.

Answer

### Step-by-Step Solution:

1. **Understanding Coralloid Roots**:
- Coralloid roots are specialized structures found in certain plants, particularly in Cycas, which is a genus of cycads. These roots are typically found above the ground and have a unique morphology that resembles coral.

2. **Function of Coralloid Roots**:
- The primary function of coralloid roots in Cycas is related to nitrogen fixation. This means they play a crucial role in converting atmospheric nitrogen into a form that the plant can use for growth.

3. **Presence of Nitrogen-Fixing Bacteria**:
- Within the coralloid roots, there are symbiotic relationships with nitrogen-fixing bacteria, specifically cyanobacteria (also known as blue-green algae). These bacteria are capable of converting molecular nitrogen (N₂) from the atmosphere into ammonia (NH₃), nitrites (NO₂⁻), or nitrates (NO₃⁻), which are forms of nitrogen that plants can absorb.

4. **Importance of Nitrogen Fixation**:
- Nitrogen is an essential nutrient for plants, as it is a key component of amino acids, proteins, and nucleic acids. By fixing atmospheric nitrogen, coralloid roots help the plant to obtain this vital nutrient, especially in nitrogen-poor soils.

5. **Conclusion**:
- Therefore, the coralloid roots of Cycas are primarily useful for nitrogen fixation, allowing the plant to thrive in its environment by utilizing atmospheric nitrogen.

### Final Answer:
Coralloid roots of Cycas are useful in **nitrogen fixation**.

---