The assertion is correct, and the reasoning provided is also correct.

Cleistogamous flowers are a type of self-pollinating flowers that do not open fully or remain closed during their entire reproductive period. These flowers typically have modified petals that do not expose the reproductive organs to the external environment. As a result, they do not rely on external pollinators for pollination.

The reasoning is accurate in stating that cleistogamous flowers do not open at all. Because these flowers remain closed, they are protected from external influences and have a higher probability of achieving pollination and seed set even in the absence of pollinators. The reproductive organs of cleistogamous flowers are in close proximity, facilitating the transfer of pollen from the stamens to the stigma within the same flower, ensuring successful pollination and seed production.

So, both the assertion and the reasoning are correct. Cleistogamous flowers produce assured seed set in the absence of pollinators because they do not rely on them for pollination, and they do not open, ensuring self-pollination within the closed flower structure.