Which of the following statements are true about heterocyst?
1. They are formed in blue - green algae.
2. Heterocysts are nitrogen - fixing cells.
3. Heterocyts are formed during nitrogen starvation .
4. Anabaena and Nostoc are examples of heterocyst forming bacteria.

Options

A. 1,3
B. 1,2,3
C. 2,3
D. 1,2,3,4

Answer

To determine which statements about heterocysts are true, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding Heterocysts**:
- Heterocysts are specialized cells found in certain filamentous cyanobacteria (also known as blue-green algae). They play a crucial role in nitrogen fixation, which is the process of converting atmospheric nitrogen into a form that can be used by living organisms.

2. **Evaluating Statement 1**:
- **Statement**: "They are formed in blue-green algae."
- **Analysis**: This statement is true because blue-green algae is another name for cyanobacteria, where heterocysts are indeed formed.

3. **Evaluating Statement 2**:
- **Statement**: "Heterocysts are nitrogen-fixing cells."
- **Analysis**: This statement is also true. Heterocysts are specifically adapted for nitrogen fixation, allowing them to convert atmospheric nitrogen into a usable form for the organism.

4. **Evaluating Statement 3**:
- **Statement**: "Heterocysts are formed during nitrogen starvation."
- **Analysis**: This statement is true as well. Heterocysts typically develop when the cyanobacteria experience nitrogen deficiency, prompting the need for nitrogen fixation.

5. **Evaluating Statement 4**:
- **Statement**: "Anabaena and Nostoc are examples of heterocyst forming bacteria."
- **Analysis**: This statement is true. Both Anabaena and Nostoc are well-known genera of cyanobacteria that form heterocysts.

### Conclusion:
All four statements about heterocysts are true. Therefore, the correct answer is that all statements are correct.

### Final Answer:
All four statements are true about heterocysts.

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