Prothallus of fern in haploid / diploid .

Answer

### Step-by-Step Solution:

1. **Understanding Prothallus**:
- Prothallus is a small, heart-shaped structure found in ferns. It is the gametophyte stage of the fern life cycle.

2. **Identifying the Ploidy Level**:
- In plant biology, structures can be either haploid (n) or diploid (2n).
- Haploid structures contain one set of chromosomes, while diploid structures contain two sets.

3. **Gametophyte vs. Sporophyte**:
- The gametophyte stage (which includes the prothallus) is haploid.
- The sporophyte stage, which develops after fertilization, is diploid.

4. **Function of Prothallus**:
- The prothallus produces gametes (sperm and eggs) and is essential for sexual reproduction in ferns.

5. **Conclusion**:
- Since the prothallus is involved in the gametophytic stage and produces gametes, it is classified as a haploid structure.

### Final Answer:
The prothallus of a fern is haploid.

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