Which of these is a condition that makes flowers invariably autogamous?

Options

A. Dioecy
B. Self incompatibility
C. Cleistogamy
D. Xenogamy

Explanation

To determine which condition makes flowers invariably autogamous (self-pollinating), we need to analyze the given options one by one.

### Step-by-Step Solution:

1. **Understand Autogamy**:
- Autogamy refers to self-pollination, where pollen from the same flower fertilizes its ovules.

2. **Evaluate Option 1: Dioecy**:
- Dioecy refers to plants that have male and female reproductive organs on separate individuals.
- This condition promotes cross-pollination because a male plant cannot fertilize its own female flowers.
- **Conclusion**: Incorrect.

3. **Evaluate Option 2: Self-Incompatibility**:
- Self-incompatibility is a mechanism where a plant's pollen cannot fertilize its own ovules.
- This condition also promotes cross-pollination, as it prevents self-fertilization.
- **Conclusion**: Incorrect.

4. **Evaluate Option 3: Clistogamy**:
- Clistogamy refers to flowers that remain closed and do not open.
- In this condition, the pollen from the anthers of the same flower fertilizes the ovules, leading to self-pollination.
- **Conclusion**: Correct.

5. **Evaluate Option 4: Xenogamy**:
- Xenogamy refers to cross-pollination between different flowers, typically on different plants.
- This condition promotes genetic diversity and does not lead to self-pollination.
- **Conclusion**: Incorrect.

### Final Answer:
The condition that makes flowers invariably autogamous is **C. Clistogamy**.

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