Germination of pollen grain on the stigma is

Options

A. gemination in situ
B. germination in vivo
C. germination of vitro
D. autogamy

Explanation

**Step-by-Step Solution:**

1. **Understanding the Context**: The question asks about the process of pollen grain germination on the stigma. This is a crucial step in the fertilization process in flowering plants.

2. **Pollination Process**: When pollination occurs, pollen grains land on the stigma of a flower. This is the initial step that leads to germination.

3. **Chemical Interaction**: Upon landing on the stigma, a chemical dialogue occurs between the pollen grain and the stigma. This involves the release of specific chemicals that determine whether the pollen grain will be accepted or rejected by the stigma.

4. **Pollen Tube Formation**: If the pollen grain is accepted, it begins to germinate. This process involves the formation of a pollen tube, which emerges from one of the germ pores of the pollen grain.

5. **Pathway of the Pollen Tube**: The pollen tube then grows through the stigma and the style, eventually reaching the ovary. From the ovary, it moves towards the micropilar end of the ovule, where it enters the embryo sac.

6. **In Vivo Process**: Since the germination and growth of the pollen tube occur within the female reproductive structure (the carpel), this process is classified as "in vivo." This term refers to processes that occur within a living organism.

7. **Eliminating Other Options**:
- **In Vitro**: This term refers to processes that occur outside a living organism, typically in a lab setting. This is not applicable here.
- **In Situ**: This term means "in the original place." While it may seem relevant, the actual development does not occur on the stigma itself but within the ovule.
- **Autogamy**: This term refers to self-fertilization in bisexual flowers. While related to pollination, it does not describe the germination process on the stigma.

8. **Conclusion**: Therefore, the correct answer to the question is that the germination of the pollen grain on the stigma is an "in vivo" process.

---