4. what is the significance of double fertilization in angiosperms?

Double fertilization is a significant feature in angiosperms or flowering plants.

Explanation:

The pollen grains carrying the sperm cells from a flower land on the stigma which is a component of the female reproductive part of a flower. The pollen grain contains the sperms and once the pollen lands on the stigma it grows pollen tub into the ovary through the style which is a tube connecting stigma to ovary.

Ovary contains ovule which contains egg cell as well as two polar nuclei and all these are haploid.  The sperm in the pollen is also haploid. On the release of two sperm cells, one sperm fuses with the egg cell and the other sperm fuses with the two polar nuclei which results in the formation of a diploid zygote which is diploid and a triploid cell.

The diploid zygote develops into the embryo after cell division and the triploid cell develops into endosperm that provides nourishment to the embryo.  Thus double fertilization becomes significant in flowering plants.