CDS (II) 2023 General Knowledge · Question 49 of 120

Correct — A, Nucellus. The ovule is a part of the mother plant, and the mother plant of a flowering plant is the diploid sporophyte. The nucellus is that mother tissue — the mass of parenchyma inside the integuments that nourishes and encloses the embryo sac — so every one of its cells carries the full 2n chromosome set. The other three names all belong to the embryo sac, which is a different generation altogether. Inside the nucellus a single cell enlarges into the megaspore mother cell and divides by meiosis into four haploid megaspores; three degenerate and the surviving one divides mitotically three times to give the seven-celled, eight-nucleate female gametophyte. That gametophyte holds three antipodal cells at the chalazal end, two synergids and one egg at the micropylar end, and a large central cell with two polar nuclei. Because all of them descend by mitosis from one haploid megaspore, the synergid, the egg and the antipodal are haploid; the nucellus around them never went through meiosis and stays diploid.

• (b)Synergid — A synergid is one of the two cells flanking the egg at the micropylar end of the embryo sac, complete with the filiform apparatus that guides the pollen tube in. It is a gametophyte cell descended from the functional megaspore, so it is haploid.
• (c)Egg — The egg is the female gamete. A gamete is haploid by definition — that is the whole point of the meiosis that produced the megaspore it came from.
• (d)Antipodal — The three antipodal cells sit at the chalazal end of the embryo sac and usually degenerate. They are produced by the same mitotic divisions as the egg and synergids, so they too are haploid.

A flowering plant alternates between a large diploid sporophyte and a tiny haploid gametophyte hidden inside it. The ovule belongs to the sporophyte: funicle, integuments, nucellus and chalaza are all 2n. Meiosis in one nucellar cell — the megaspore mother cell — produces four haploid megaspores, and the single functional one grows by three mitotic divisions into the female gametophyte, the seven-celled eight-nucleate embryo sac. Every cell of that embryo sac is n.

The item rewards one habit — asking, for each name offered, which generation it belongs to. Three of the four options are cells you can point to on a labelled embryo sac diagram; the fourth is the tissue the embryo sac is buried in. Once you place the meiotic step at the megaspore mother cell, everything upstream of it is diploid and everything downstream is haploid, and the answer falls out without any memorising. One printed looseness is worth noting honestly: the stem asks which is not a haploid cell, but the nucellus is a tissue of many cells rather than a single cell. That does not affect the answer, since it is the only diploid item on the list.

• The ovule's funicle, integuments, nucellus and chalaza are sporophytic and therefore diploid.
• Meiosis of the megaspore mother cell inside the nucellus yields four haploid megaspores, of which normally one survives.
• Three free-nuclear mitotic divisions of the functional megaspore give the eight nuclei that are organised into seven cells — three antipodals, two synergids, one egg and one central cell with two polar nuclei.
• This 7-celled, 8-nucleate pattern is called the monosporic or Polygonum type, and it is the commonest embryo sac in angiosperms.
• After double fertilisation the zygote is diploid (2n) and the primary endosperm nucleus is triploid (3n), because one male gamete fuses with the egg and the other with the two polar nuclei.

• Reading the ovule as gametophytic. Only the embryo sac inside it is the gametophyte; the rest of the ovule is the mother plant.
• Forgetting that the antipodals, though they usually degenerate, are still haploid embryo-sac cells.
• Mixing up the nucellus with the endosperm — the nucellus is diploid mother tissue, the endosperm is the triploid product of fertilisation.

Usually as a ploidy question — which structure is haploid, diploid or triploid — or as a labelled embryo-sac diagram with one part to be named.

• practice — not a real PYQ

  In a typical angiosperm embryo sac, how many cells and how many nuclei are present at maturity?

  • (a)Eight cells and eight nuclei
  • (b)Seven cells and eight nuclei
  • (c)Six cells and seven nuclei
  • (d)Seven cells and seven nuclei

  Answer(b) Seven cells and eight nuclei — three antipodals, two synergids, one egg and one central cell that holds two polar nuclei.

• practice — not a real PYQ

  Which structure in the seed of black pepper represents the persistent remains of the nucellus?

  • (a)Endosperm
  • (b)Perisperm
  • (c)Aril
  • (d)Caruncle

  Answer(b) Perisperm — the diploid nucellar tissue that survives into the mature seed instead of being used up.