Sexual Reproduction in Flowering Plants - Double Fertilization

Double fertilization is a unique characteristic of angiosperms, first discovered by Nawaschin in $Lillium$ and $Fritillaria$.

It involves two types of nuclear fusions: Syngamy and Triple Fusion.

Syngamy: One male gamete ($n$) fuses with the egg cell ($n$) to form a diploid zygote ($2n$), which eventually develops into the embryo.

Triple Fusion: The second male gamete ($n$) fuses with the two polar nuclei ($n+n$) or the secondary nucleus ($2n$) in the central cell to produce a triploid Primary Endosperm Nucleus ($3n$ or $PEN$).

The pollen tube enters the embryo sac through the micropylar end, guided by the filiform apparatus of the synergids.

The central cell after triple fusion becomes the Primary Endosperm Cell ($PEC$) and develops into the endosperm, which provides nutrition to the developing embryo.

The total number of nuclei involved in double fertilization is five (two male gametes, one egg cell, and two polar nuclei).