Assertion: Cleistogamous flowers produce assured seed set in the absence of pollinators.
Reason: These flowers have exposed anthers and stigma .

Assertion is true but reason is false.

Cleistogamous flowers are flowers that do not open, so they are self-pollinated. This means that the pollen from the anthers is transferred to the stigma of the same flower, without the help of pollinators. This ensures that the flowers will produce seeds, even in the absence of pollinators.

The reason given is false because cleistogamous flowers do not have exposed anthers and stigma. The anthers and stigma are enclosed within the flower, so they are not exposed to the environment. This helps to protect the pollen and stigma from being damaged by insects or other factors.

Therefore, the correct reason for why cleistogamous flowers produce assured seed set in the absence of pollinators is that they are self-pollinated.