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 need to understand the definitions of autogamy and geitonogamy, and then analyze the given options.

### Step-by-Step Solution:

1. **Define Autogamy and Geitonogamy**:
- **Autogamy**: This is a form of self-pollination where fertilization occurs within the same flower. In this case, the pollen from the anther of a flower fertilizes the ovule of the same flower.
- **Geitonogamy**: This is a type of pollination where fertilization occurs between different flowers on the same plant. Although it is not true cross-pollination, it still involves different flowers of the same plant.

2. **Analyze the Options**:
- **Valisneria**: This plant is known for its water pollination (epihydrophily) and does not exhibit autogamy. However, it does not prevent geitonogamy since it can still have fertilization between different flowers on the same plant.
- **Papaya**: Papaya flowers are unisexual and primarily undergo xenogamy (cross-pollination). It does not prevent autogamy since it has separate male and female flowers, but it does not fit the criteria of the question.
- **Castor**: Castor plants prevent autogamy (self-pollination) because they have unisexual flowers (male and female) on different plants. However, they allow geitonogamy since fertilization can occur between different flowers on the same plant.
- **Cannabis**: Cannabis can undergo both autogamy and xenogamy, hence it does not fit the criteria of preventing autogamy.

3. **Conclusion**:
Based on the analysis, the plant that prevents autogamy but allows geitonogamy is **Castor**. Therefore, the correct answer is **Castor**.

### Final Answer:
**Castor**