Ploidy - the Master Trap

More NEET marks are lost on ploidy than on any other part of this chapter. Track every structure back to its origin:

Diploid (2n) - somatic/sporophytic tissue and the products of fertilisation of the egg:

  • Microspore (pollen) mother cell and megaspore mother cell - the last 2n cells before meiosis.
  • Nucellus, integuments, and the seed coat (testa/tegmen) that forms from them.
  • Perisperm (persistent nucellus) and the pericarp (from the ovary wall).
  • The zygote and the embryo (egg n + male gamete n).

Haploid (n) - everything in the gametophytes (products of meiosis):

  • Microspore, pollen grain, vegetative cell, generative cell and the two male gametes.
  • Megaspore and functional megaspore.
  • Inside the embryo sac: the egg, the two synergids, the three antipodals, and each polar nucleus are all n.

Triploid (3n):

  • The primary endosperm nucleus (PEN) and the endosperm (one male gamete n + two polar nuclei n + n = 3n).

Traps: the central cell contains two n polar nuclei (so 'n + n'), but after triple fusion it gives a 3n endosperm. Perisperm is 2n (nucellus), while endosperm is 3n - do not confuse them. A common NEET item: 'the haploid, diploid and triploid structures of a fertilised embryo sac are, respectively, synergid, zygote and primary endosperm nucleus.'

Divisions, Sequences and Counts

  • From one megaspore mother cell to the mature female gametophyte: 1 meiosis (-> 4 megaspores) followed by 3 free-nuclear mitoses of the functional megaspore (2 -> 4 -> 8 nuclei). So 1 meiosis + 3 mitoses, and the sac is monosporic (one functional megaspore).
  • From one microspore mother cell: 1 meiosis gives a tetrad of 4 microspores; each microspore then forms a pollen grain, whose generative cell divides mitotically to give 2 male gametes.
  • Microsporogenesis sequence: sporogenous tissue -> microspore (pollen) mother cell -> meiosis -> microspore tetrad -> pollen grains.
  • Female-gametophyte sequence: MMC -> meiosis -> 4 megaspores -> 1 functional megaspore -> 2-nucleate -> 4-nucleate -> 8-nucleate -> 7-celled embryo sac.
  • Embryo-sac count: 8 nuclei, 7 cells (egg + 2 synergids + 3 antipodals = 6 nuclei in 6 cells; + 2 polar nuclei in 1 central cell).

Fine Distinctions Worth Memorising

  • Geitonogamy is functionally cross-pollination (needs an agent) but genetically autogamy (same plant). Only xenogamy brings genetically different pollen.
  • Cleistogamous flowers never open and are invariably autogamous - no cross-pollination is possible, so they show no benefit of genetic recombination but always set seed.
  • Monoecious (castor, maize) prevents autogamy only; dioecious (papaya) prevents both autogamy and geitonogamy.
  • Free-nuclear divisions occur in both the endosperm (common) and the early embryo sac - divisions of the nucleus without immediate wall formation.
  • Endosperm develops before the embryo. Tender coconut water = free-nuclear endosperm; the white kernel = cellular endosperm.
  • False fruit = a fleshy part other than the ovary contributes (apple = thalamus); true fruit = ovary only. Parthenocarpy = fruit without fertilisation (banana), hence seedless.
  • Apomixis = seed without fertilisation; useful because it fixes hybrid vigour across generations. Polyembryony = more than one embryo per seed (citrus, mango).

Assertion-Reason watch-outs: wind- and water-pollinated flowers are not colourful because they do not need to attract animals (true, and R explains A). Tapetum has dense cytoplasm and is often multinucleate (true) but it does nourish pollen (so 'tapetum does not nourish' is false).