A bryophyte which harbours a nitrogen fixing blue-green alga in its thallus is

Options

A. Riccia
B. Anthoceros
C. Marchantia
D. Pogonatum.

Answer

### Step-by-Step Solution:

1. **Understanding Bryophytes**:
- Bryophytes are non-vascular plants that include mosses, liverworts, and hornworts. They are known for their simple structure and reliance on water for reproduction.

2. **Identifying Nitrogen-Fixing Algae**:
- The question mentions a "nitrogen-fixing blue-green alga." Blue-green algae are actually cyanobacteria, which are capable of fixing atmospheric nitrogen into a form that plants can use.

3. **Reviewing Options**:
- The options provided are:
a. Rixia
b. Enteroceros
c. Merchantia
d. Pogonatum

4. **Linking Algae to Bryophytes**:
- Among the bryophytes, certain species have a symbiotic relationship with cyanobacteria. This relationship allows the bryophyte to benefit from the nitrogen fixed by the algae.

5. **Identifying the Correct Bryophyte**:
- The bryophyte that is known to harbor nitrogen-fixing cyanobacteria (specifically Nostoc) in its thallus is **Anthoceros**. This was first reported by Hofmeister in 1862 and later confirmed by Leitecke.

6. **Conclusion**:
- Based on the information gathered, the correct answer to the question is **b. Enteroceros**.

### Final Answer:
The bryophyte which harbors a nitrogen-fixing blue-green alga in its thallus is **b. Enteroceros**.