Which of the following plants prevent autogamy but not geitonogamy?

Options

A. Vallisneria
B. papaya
C. castor
D. cannabis

Answer

To solve the question "Which of the following plants prevent autogamy but not geitonogamy?", we will analyze the definitions of autogamy and geitonogamy, and then evaluate the options provided.

### Step-by-Step Solution:

1. **Understand Autogamy and Geitonogamy**:
- **Autogamy**: This is a type of self-pollination where pollen is transferred from the anther of a flower to the stigma of the same flower. Therefore, it occurs within the same flower.
- **Geitonogamy**: This is also a form of self-pollination, but it involves the transfer of pollen from the anther of one flower to the stigma of another flower on the same plant.

2. **Evaluate the Options**:
- **Valisneria**: This is an aquatic plant that is unisexual and primarily undergoes cross-pollination. Therefore, it does not prevent either autogamy or geitonogamy.
- **Papaya**: Papaya flowers are bisexual, meaning they contain both male and female reproductive organs. Thus, autogamy cannot be prevented in papaya.
- **Castor**: Castor plants have unisexual flowers. This means that one flower will have either male or female reproductive organs, preventing autogamy. However, since the plant has both male and female flowers, geitonogamy can still occur as pollen can be transferred between different flowers on the same plant.
- **Cannabis**: Cannabis also has bisexual flowers, so it cannot prevent autogamy.

3. **Conclusion**:
- The only plant that prevents autogamy but not geitonogamy is **Castor**. This is because it has unisexual flowers, which means that the pollen cannot fertilize the stigma of the same flower (preventing autogamy), but can still fertilize the stigma of another flower on the same plant (allowing geitonogamy).

### Final Answer:
The correct answer is **Castor**.