Question

Even through each pollen grain has 2 male gamate why atleast 20 pollen grains are required to fertilization 20 ovules in particular carpel

Answer 1

Answer: because out of the two Male gametes produced from one pollen grain, only one of them fuses with ovum and participates in fertilization.

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.