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. **Statement 1: They are formed in blue-green algae.**
- **Analysis:** Heterocysts are indeed formed in blue-green algae, which are also known as cyanobacteria. This statement is true.

2. **Statement 2: Heterocysts are nitrogen-fixing cells.**
- **Analysis:** Heterocysts are specialized cells that are responsible for nitrogen fixation. They convert atmospheric nitrogen into a form that can be used by plants. This statement is true.

3. **Statement 3: Heterocysts are formed during nitrogen starvation.**
- **Analysis:** Heterocysts are formed in response to nitrogen starvation in the environment. When nitrogen levels are low, these specialized cells develop to facilitate nitrogen fixation. This statement is true.

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

### Conclusion:
All four statements about heterocysts are true.

### Final Answer:
All statements (1, 2, 3, and 4) are true about heterocysts.

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