Specialised cells for fixing atmospheric nitrogen in Nostoc are

Options

A. Akinetes
B. Heterocysts
C. Hormogonia
D. Nodules

Answer

To answer the question about the specialized cells for fixing atmospheric nitrogen in Nostoc, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Organism**: Recognize that Nostoc is a type of cyanobacteria, also known as blue-green algae. This classification is important as it helps us understand the biological context of the question.

2. **Understand the Structure**: Familiarize yourself with the structure of Nostoc. It consists of filaments made up of different types of cells, including vegetative cells, which are responsible for photosynthesis.

3. **Recognize Specialized Cells**: In Nostoc, there are specialized cells that play a crucial role in nitrogen fixation. These cells are distinct from the vegetative cells.

4. **Identify the Specialized Cells**: The specialized cells responsible for fixing atmospheric nitrogen in Nostoc are called **heterocysts**. These cells are thick-walled and are specifically adapted for nitrogen fixation.

5. **Eliminate Incorrect Options**: The question provides other options (Echinacea, Etrosis, Hormogonia, Medusa). By knowing that heterocysts are the correct answer, we can eliminate the other options as they do not relate to nitrogen fixation in Nostoc.

6. **Conclude the Answer**: Therefore, the specialized cells for fixing atmospheric nitrogen in Nostoc are heterocysts.

### Final Answer:
The specialized cells for fixing atmospheric nitrogen in Nostoc are called **heterocysts**.