Answer:
A bisexual chasmogamous flower can evolvethe
following (three) strategies to preventself-pollination (autogamy).
(a)
Dichogamy In this mechanism, pollen release and stigma receptivity are notsynchronized.
In sunflower, the pollen is released before the stigma becomes
receptive(protandry).InDatura, Solarium, the stigma becomes receptive much
before therelease of pollen (protogyny) leads to cross-pollination.
(b)HerkogamyThe male and female
sex organs are placed at different positions or indifferent directions is
called Herkogamy. In these plants, the pollen cannot came incontact with the
stigma of the same flower. It has undergone cross pollinatione.g., Hibiscus
Gloriosa.
(c)Self-sterilityIt is a
genetic mechanism which prevents the self-pollen from fertilizingthe ovules by
inhibiting pollen germination or pollen tube growth in the pistil, e.g.,
Abotilon.
Note Another device to prevent self-pollination is the
production of unisexual flowers, but this is not beneficial as above mentioned
strategies. It prevents auto gamy but notgitonogamyin monoecious plants such as
castor and maize.
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