What is double fertilization? Describe fertilized embryo sac with a neat labelled diagram.

Explanation

One of the male gamete moves towards the egg cell and fuses with the nucleus thus completing the syngamy. This leads to zygote formation which further develops into an embryo.
The other male gamete move towards the two polar nuclei located in the central cell and fuses with them to produce triploid primary endosperm nucleus (PEN). Since two types of fusions, syngamy and triple fusion takes place in an embryosac the phenomenon is called as double fertilization.

Fertilized embryosac exhibit degenerating synergids towards micropylar end and a fertilized egg cell called diploid zygote.
Antipodals towards the chalazal end also degenerate. Central cell becomes primary endosperm cell with a triploid primary endosperm nucleus. Primary endosperm cell develops in to the endospetm by repeated mitotic division. The cells of endosperm are filled with food reserves and thereby nourish the developing embryo.