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How does a bisexual flowering plant ensures cross pollination? Explain.

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In bisexual flowering plant, cross pollination is favored by number of methods: 

• Dichogamy: In such cases, the male and female parts of a bisexual flower mature at different times and xenogamy is favored. If the stamens mature before the pistil, the condition is known as protandry (e.g., sunflower, Salvia) while if the carpels mature before stamens, the condition is known as protogyny (e.g., banyan, Plantago, Mirabilis jalapa). 

• Self-sterility or self-incompatibility: In some plants, the pollen grains of a flower are not effective on the stigma of the same flower or that of flower of the same plant (e.g., tea, tobacco, etc.). In such cases, there is genetic incompatibility and xenogamy is favored. 

• Heterostyly: In this, length of filaments and styles in a flower are different. If the styles are of different lengths, it is called heterostyly while if the stamens are of different lengths, it is called heteroanthy. 

• Herkogamy: It is a condition in which a physical barrier is present between the anther and the style. They are arranged in such manner that the pollen grain does not fall on stigma of the same flower. For example, in orchids, pollen grains are present in special structures called pollinia and insects are required for pollination. In members of the family Caryophyllaceae, the stigma is present at a higher level than the anther.

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