Step 1: Understanding the Concept:
The question asks for the defining characteristic of cleistogamous flowers. The term itself provides a clue: 'cleisto' means closed, and 'gamy' refers to marriage or fertilization.
Step 2: Detailed Explanation:
Cleistogamous flowers are flowers that are modified to remain closed and bud-like throughout their life. They never blossom or expose their reproductive organs to the outside environment.
Since the flower never opens, pollination by external agents like wind (anemophily) or insects (entomophily) is impossible.
Pollination must occur within the closed flower. The anthers dehisce to release pollen directly onto the stigma of the same flower. This ensures autogamy or self-pollination.
Cleistogamy is an adaptation that guarantees seed set even in the absence of pollinators. Plants like {Viola} (Pansy) and {Commelina} often produce both open, cross-pollinating (chasmogamous) flowers and closed, self-pollinating (cleistogamous) flowers.
Analyzing the options:
(A) and (C) are incorrect as external pollinators cannot access the closed flower.
(D) describes chasmogamous flowers, the opposite of cleistogamous ones.
(B) correctly states that they never open and are consequently self-pollinated.
Step 3: Final Answer:
The defining characteristic of cleistogamous flowers is that they remain closed and undergo self-pollination.