Nitrogen fixation by Nostoc /Anabaena takes place in
(a) Heterocysts
(b) Akinetes
(c) Hormogones
(d) Vegetative cells.

Options

A. Heterocysts
B. Akinetes
C. Hormogones
D. Vegetative cells.

Answer

To solve the question regarding where nitrogen fixation occurs in Nostoc or Anabaena, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Organisms**: Recognize that Nostoc and Anabaena are types of cyanobacteria, also known as blue-green algae.

2. **Understand Nitrogen Fixation**: Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into a form usable by living organisms, typically ammonia (NH3). This process is crucial for the growth of plants and is often carried out by certain bacteria.

3. **Specialized Cells for Nitrogen Fixation**: In Nostoc and Anabaena, nitrogen fixation occurs specifically in specialized cells.

4. **Recognize Heterocysts**: These specialized cells are called heterocysts. Heterocysts provide an anaerobic environment, which is essential for the nitrogen-fixing enzyme, nitrogenase, to function effectively. Nitrogenase becomes inactive in the presence of oxygen, hence the need for an anaerobic environment.

5. **Evaluate Other Cell Types**:
- **Akinetes**: These are dormant cells that help in survival during unfavorable conditions but are not involved in nitrogen fixation.
- **Hormogones**: These are motile filaments that aid in the movement of the cyanobacteria but do not participate in nitrogen fixation.
- **Vegetative Cells**: These are the normal cells of the cyanobacteria that carry out photosynthesis and other metabolic processes but do not fix nitrogen.

6. **Conclusion**: Based on the above information, the correct answer to the question is (a) Heterocysts, as they are the cells where nitrogen fixation occurs in Nostoc and Anabaena.

### Final Answer:
(a) Heterocysts

---