Question

Even though each pollen grain has 2 male gametes, why atleast 20 pollen grains are

required to fertilize 20 ovules in a particular carpel?​

Answer 1

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because only one male gamete is involved in syngamy or the fusion of male gamete with the egg cell.

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Answer 2

During double fertilization, one of the male gamete of pollen grain fuses with egg cell, while other male gamete fuses with secondary nucleus. Thus to fertilize 20 ovules in a particular carpel, 20 pollen grains are required.