Define/Explain: Double Fertilisation.

(N/A) $\rightarrow$ In angiosperms,out of the two male gametes,one male gamete fuses with the egg cell to form a diploid zygote (syngamy),while the other male gamete fuses with the two polar nuclei (or diploid secondary nucleus) to form a triploid primary endosperm nucleus $(PEN)$ (triple fusion).
$\rightarrow$ Since two types of fusion events occur in the embryo sac,this process is known as double fertilisation.
$\rightarrow$ This phenomenon is a unique characteristic of angiosperms.