Reproduction in Plants and Humans

• A typical angiosperm anther.
• It is bilobed (dithecous) — two lobes.
• Microsporangia sit at the corners.

The number of microsporangia.

The standard description is bilobed and tetrasporangiate: two lobes, and two microsporangia in each lobe, giving four in all, one at each corner of a transverse section. The two words in that phrase count different things — “bi” counts the lobes, “tetra” counts the sporangia.

1. A transverse section of a typical anther shows two lobes joined by a connective.
2. Each lobe contains two microsporangia, sitting at the two corners of that lobe.
3. Total = 2 × 2 = 4.
4. Each microsporangium later develops into a pollen sac, so a mature anther has four pollen sacs.

• A functional megaspore — the one of four that survives.
• It divides and develops into a structure.

The type of division, and the structure formed.

Meiosis has already happened by this stage — it is what produced the four megaspores from the megaspore mother cell. The functional megaspore then undergoes three free-nuclear MITOTIC divisions, giving 2, then 4, then 8 nuclei. The 8-nucleate structure that results is the embryo sac, the female gametophyte.

1. The megaspore mother cell divides by meiosis to give four haploid megaspores. Three degenerate.
2. The surviving functional megaspore is already haploid, so no further meiosis is possible or needed.
3. It divides mitotically, three times, without cell wall formation — free nuclear divisions giving 2, 4 and then 8 nuclei.
4. The resulting structure is the embryo sac.
5. So A = mitosis, B = embryo sac.

• Four statements about embryo sac development.
• One of them is incorrect — that is the one to find.

Which statement is wrong.

The defining feature of embryo sac formation is that the three mitotic divisions are free-nuclear — the nucleus divides but no cell wall forms. Walls appear only after all eight nuclei are present. A statement claiming walls form immediately after each division reverses that sequence.

1. (a) The functional megaspore's nucleus divides mitotically three times to give eight nuclei. True.
2. (b) It claims each mitotic division is immediately followed by wall formation. But these are free-nuclear divisions with no wall formation at all. This is the incorrect statement.
3. (c) Walls are laid down after the 8-nucleate stage, organising the seven-celled female gametophyte. True — and it directly contradicts (b).
4. (d) Two polar nuclei sit in the large central cell, below the egg apparatus. True.

• One microspore mother cell (2n).
• It undergoes meiosis, then the products undergo mitosis.

The total number of male gametes produced.

Two multiplications in sequence, and the question is testing whether you carry both. Meiosis turns one mother cell into four microspores, each of which becomes a pollen grain. Then within each pollen grain the generative cell divides mitotically into two male gametes. Four pollen grains at two gametes each gives eight.

1. Meiosis of one microspore mother cell gives four haploid microspores.
2. Each microspore matures into one pollen grain.
3. In each pollen grain the generative cell divides mitotically into two male gametes.
4. Total = 4 × 2 = 8.

• Assertion about the stage at which pollen is shed.
• Reason about what the shed pollen contains.

The truth of each statement.

At the two-celled stage a pollen grain contains a vegetative cell and a generative cell — not two male gametes. The generative cell divides into the two male gametes only later, usually inside the pollen tube. So the reason describes the three-celled condition, which is the minority case (about 40% of species).

1. Assertion. In over half of angiosperms pollen is shed at the two-celled stage. True.
2. Reason. At the two-celled stage the generative cell has not yet divided, so there are no male gametes — only a cell that will later make them. False.
3. Notice that the reason actually contradicts the assertion: if the grain held two male gametes it would be three-celled, not two-celled.
4. So (A) is true and (R) is false.

• Statement I about sperm formation in old age.
• Statement II about when ovum formation stops.

The truth of each statement.

The two sexes differ sharply in timing, and both statements state that difference correctly. Spermatogenesis begins at puberty and continues, with gradually declining efficiency, throughout life. Oogenesis begins before birth, pauses, resumes at puberty, and stops altogether at menopause, around the age of fifty.

1. Statement I. The testes keep producing sperm well into old age. Correct.
2. Statement II. Ovulation ceases at menopause, which occurs around 50 years of age. Correct.
3. Both are true, and they are the standard contrast NCERT draws between male and female gamete production.
4. Note that a woman is born with all her primary oocytes already formed — no new ones are made after birth, which is ultimately why the supply runs out.

• A primary spermatocyte (2n) undergoing the first meiotic division.

The number, size and ploidy of the cells produced.

Two properties are being tested at once. Ploidy: meiosis I halves the chromosome number, so the products are haploid. Size: in the male the cytoplasm is divided equally, giving two cells of the same size. Unequal division belongs to oogenesis, where nearly all the cytoplasm is kept for the future egg and the leftover becomes a tiny polar body.

1. The primary spermatocyte is diploid, 2n.
2. Meiosis I separates the homologous chromosomes, so each product is haploid.
3. The cytoplasm divides equally — there is no reason for a male cell to hoard cytoplasm, since a sperm carries almost none.
4. So: two equal-sized haploid secondary spermatocytes.
5. Each then divides by meiosis II into two spermatids, giving four spermatids from one primary spermatocyte.