Two types of fusions, syngamy and triple fusion take place in embryo sac

Explanation

During fertilisation, one male gamete out of two male gametes of pollen tubes move towards the egg cell and fuses with its nucleus thus completing the syngamy. This result in the formation of a diploid cell, the zygote (2n). The other male gamete moves towards the two polar nuclei located in the central cell and fuses with them to produce a triploid Primary Endosperm Nucleus (PEN). As this involves the fusion of three haploid nucles it is termed as triple fusion. Since two types of fusions, syngamy and triple fusion take place in an embryo sac the phenomenon is termed double fertilisation an event unique to flowering plants.