Q.Why is double fertilisation considered unique to flowering plants?

The correct haploid, diploid, triploid sequence in the fertilised embryo sac is Synergid (n), Zygote (2n), and PEN (3n).

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In a mature angiosperm embryo sac (typically the Polygonum-type, 7-celled/8-nucleate), the structures present, before and after double fertilisation, have different ploidy levels:

• Synergids (2 of them, forming the egg apparatus along with the egg cell) remain haploid (n) throughout -- they are never fertilised and eventually degenerate after pollen tube entry.
• Antipodal cells (3, at the chalazal end) are also haploid (n) and are not involved in fertilisation.
• Egg cell, on fusion with one male gamete (n + n), becomes the zygote, which is diploid (2n).
• Polar nuclei (2, in the central cell) are each individually haploid (n); when they fuse with each other they form the diploid secondary nucleus, and when this secondary nucleus then fuses with the second male gamete (n) during double fertilisation, it forms the Primary Endosperm Nucleus (PEN), which is triploid (3n). …

Double fertilisation in angiosperms = syngamy (male gamete + egg gives zygote) plus triple fusion (male gamete + secondary/polar nuclei gives endosperm).

In angiosperms, the pollen tube delivers two non-motile male gametes into the embryo sac. Double fertilisation involves two simultaneous fusion events:

1. Syngamy: One male gamete fuses with the egg cell (n) to form the diploid (2n) zygote.
2. Triple fusion: The other male gamete fuses with the diploid secondary nucleus (formed by the fusion of the two polar nuclei) to form the triploid (3n) primary endosperm nucleus, which develops into the nutritive endosperm tissue. …

Double fertilization -- fusion events forming both the zygote and the nutritive endosperm -- occurs exclusively in angiosperms.

In angiosperms, the pollen tube delivers two non-motile male gametes into the embryo sac. One male gamete fuses with the egg cell (syngamy) to form the diploid zygote, which develops into the embryo. Simultaneously, the second male gamete fuses with the two polar nuclei (or the secondary nucleus formed by their fusion) to form the triploid primary endosperm nucleus (triple fusion), which develops into the endosperm -- the nutritive tissue for the developing embryo. Because BOTH of these fusion events occur together within the same fertiliz …

Double fertilization: gamete 1 + egg = zygote; gamete 2 + secondary nucleus = endosperm.