Nostoc is

Options

A. Green algae
B. Yellow-green algae
C. Blue- green algae
D. Red algae

Answer

To answer the question "Nostoc is," we can break down the information as follows:

### Step-by-Step Solution:

1. **Identify the Classification**:
- Nostoc is classified under the group known as cyanobacteria.

2. **Understanding Cyanobacteria**:
- Cyanobacteria are commonly referred to as blue-green algae (BGA). This term reflects their color and photosynthetic capabilities.

3. **Characteristics of Nostoc**:
- Nostoc, like other cyanobacteria, is prokaryotic. This means that it does not have a true nucleus or membrane-bound organelles. Instead, its genetic material is located in a region called the nucleoid.

4. **Body Structure**:
- The body of Nostoc is surrounded by a mucilaginous or gelatinous sheath. This sheath helps in protecting the cells and maintaining moisture.

5. **Specialized Cells**:
- Nostoc contains specialized cells known as heterocysts. These cells play a crucial role in nitrogen fixation, which is the process of converting atmospheric nitrogen into a form usable by living organisms.

6. **Conclusion**:
- Therefore, Nostoc can be defined as a type of blue-green algae (cyanobacteria) that is prokaryotic in nature, has a gelatinous sheath, and contains heterocysts for nitrogen fixation.

### Final Answer:
Nostoc is a type of blue-green algae (cyanobacteria).

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