From among the situations given below, choose the one that prevents both autogamy and geitonogamy.

Options

A. Monoecious plant bearing unisexual flowers
B. Dioecious plant bearing only male or female flowers
C. Monoecious plant with bisexual flowers
D. Dioecious plant with bisexual flowers

Answer

To solve the question regarding which situation prevents both autogamy and geitonogamy, we need to understand the definitions of these terms and analyze the given options.

### Step-by-Step Solution:

1. **Understanding Autogamy and Geitonogamy**:
- **Autogamy**: This is a type of self-pollination where pollen from the anther of a flower fertilizes the ovule of the same flower. This occurs in bisexual flowers.
- **Geitonogamy**: This is also a form of self-pollination, but it occurs between different flowers of the same plant. Here, pollen from one flower fertilizes the ovule of another flower on the same plant.

2. **Identifying the Conditions**:
- We need to find a situation that prevents both types of self-pollination (autogamy and geitonogamy). This means we need a scenario where there are no bisexual flowers or no flowers of the same plant to facilitate self-pollination.

3. **Analyzing the Options**:
- **Option 1: Monoecious plant bearing unisexual flowers**:
- This means the plant has both male (staminate) and female (pistillate) flowers, but they are unisexual. Autogamy cannot occur because there are no bisexual flowers, but geitonogamy can occur since both types of flowers are on the same plant.
- **Conclusion**: This option does not prevent geitonogamy.

- **Option 2: Dioecious plant bearing only male and female flowers**:
- Dioecious plants have separate male and female plants. Since there are no bisexual flowers and no same plant flowers to cross-pollinate, neither autogamy nor geitonogamy can occur.
- **Conclusion**: This option prevents both autogamy and geitonogamy.

- **Option 3: Monoecious plants with bisexual flowers**:
- In this case, the plant has bisexual flowers, so autogamy can occur (self-pollination within the same flower) and geitonogamy can also occur (pollen from one flower fertilizing another flower on the same plant).
- **Conclusion**: This option does not prevent either type of pollination.

- **Option 4: Dioecious plant with bisexual flowers**:
- This is contradictory because dioecious plants cannot have bisexual flowers. Therefore, this option is invalid.
- **Conclusion**: This option is not applicable.

4. **Final Answer**:
- The situation that prevents both autogamy and geitonogamy is **Option 2: Dioecious plant bearing only male and female flowers**.