Explain why sexual reproduction in angiosperms is said to take place through double fertilization and triple fusion? Also,provide a labelled diagram of the embryo sac to explain the phenomenon.

(N/A) In angiosperms,the process of fertilization involves two distinct fusion events:
$1$. Syngamy (Fertilization): One of the two male gametes released by the pollen tube fuses with the egg cell to form a diploid zygote $(2n)$.
$2$. Triple Fusion: The second male gamete fuses with the two polar nuclei (or the secondary nucleus) located in the central cell to form a triploid Primary Endosperm Nucleus ($PEN$,$3n$).
Since these two fusion events occur simultaneously in the embryo sac,the process is collectively known as double fertilization. Following these events,the zygote develops into an embryo,and the $PEN$ develops into the endosperm,which provides nourishment to the developing embryo.