Question

Why is cross pollination completely absent in cleistogamous flowers? class 12?

Answer 1

☸CONDITION FOR CROSS FERTILISATION:-
the anther of 1 plant and stigma of another plant (/ another flower of same species) must be well exposed for the pollinating agent to transfer the pollen grains from male flower(♂) to female flower(♀) ..
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➡BUT...
in cleistogamous flowers there is no cross fertilisation because the flower (contains anther and stigma) are ALWAYS CLOSED.
=> anther and stigma is never exposed for cross pollination

so... these cleistogamous flowers reproduce through only self fertilisation... here... cross fertilisation is absent completely.
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☸EXAMPLES:-

▶Especially well known in peanuts, peas, and pansy.

▶This is most widespread in the grass family.

▶However, the largest genus of cleistogamous plants is Viola.
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HOPE IT HELPS YOU
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Answer 2

cleistogamous flowers is those flowers which do not open at all.

the anthers and stigma lie close to each other.
when anthers dehinsce in the flower buds,pollen grains come in contact
with stigma to effect pollination.
thus,cleistogamous flowers are autogamous (pollination within same flower)
as there is no chance of cross pollen landing on stigma.

so, these are the reason behind completely absent cross pollination in cleistogamous flowers.