Q.Describe the process of double fertilization.

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

Working

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).

The question asks for one haploid, one diploid, and one triploid structure, in that order. Checking each option: (a) Antipodal, Synergid, PEN gives haploid, haploid, triploid -- two haploid structures, no diploid one, so it is wrong. (b) Synergid, Zygote, PEN gives haploid (n), diploid (2n), triploid (3n) -- exactly the required sequence. (c) Synergid, Polar nuclei, Zygote gives haploid, haploid (before fusion), diploid -- no triploid structure, so wrong. (d) Synergid, Antipodal, Polar nuclei gives haploid, haploid, haploid -- no diploid or triploid structure at all, so wrong. Only option (b) correctly matches haploid, diploid and triploid structures in order.

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.

This process, unique to flowering plants, is called double fertilisation because it involves two distinct fusion events using both male gametes delivered by the pollen tube. Options (a) and (c) are incorrect because a male gamete never fuses with a synergid or antipodal cell during normal fertilisation — synergids merely receive the pollen tube, and antipodals eventually degenerate.

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 fertilization process, it is called 'double fertilization'. This phenomenon, discovered by Nawaschin, is unique to flowering plants and is absent in bryophytes, pteridophytes, and gymnosperms (which lack the second, triple-fusion event forming a true triploid endosperm).

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