What is meant by double fertilization? Explain.

(N/A) $\rightarrow$ After pollination,the pollen grain germinates on the stigma,and the pollen tube grows through the style to reach the ovule.
$\rightarrow$ The pollen tube enters the embryo sac and releases two male gametes.
$\rightarrow$ One male gamete fuses with the egg cell to form a diploid zygote,a process known as $Syngamy$.
$\rightarrow$ The second male gamete fuses with the two polar nuclei (or the diploid secondary nucleus) to form a triploid primary endosperm nucleus $(PEN)$,a process known as $Triple$ $Fusion$.
$\rightarrow$ Since two types of fusions occur within the embryo sac,this phenomenon is called $Double$ $Fertilization$,which is a unique characteristic of angiosperms.