Assertion :- Cleistogamous flowers are invariably autogamous.
Reason :- These flowers do not open at all.

Options

A. A
B. B
C. C
D. D

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "Cleistogamous flowers are invariably autogamous." Here, "invariably" means that these flowers always undergo self-pollination (autogamy).

2. **Definition of Autogamous**: Autogamous flowers are those that primarily self-pollinate rather than cross-pollinate. This means that the pollen from the same flower fertilizes its ovules.

3. **Characteristics of Cleistogamous Flowers**: Cleistogamous flowers are typically small, bisexual, and possess reproductive organs that are closely situated. They do not produce nectar and are adapted to ensure self-pollination.

4. **Self-Pollination Mechanism**: The close proximity of the reproductive organs and the abundance of pollen grains in cleistogamous flowers facilitate self-pollination. This adaptation ensures that the same pollen grain fertilizes the ovule of the same flower.

5. **Understanding the Reason**: The reason states that "These flowers do not open at all." This is a characteristic of cleistogamous flowers. They may rarely open, and in many cases, they do not open even after fertilization.

6. **Evaluating the Assertion and Reason**: Both the assertion and the reason are correct. Cleistogamous flowers are indeed always autogamous, and the reason provided accurately describes their nature of not opening.

7. **Conclusion**: Therefore, both the assertion and reason are correct, and the reason given supports the assertion.

### Final Answer:
Both the assertion and the reason are correct, and the reason given for the assertion is also correct.

---