Sexual Reproduction in Flowering Plants
Sexual Reproduction in Flowering Plants carries a large share of II PUC marks: labelled structure questions, the sequence from sporogenous tissue to embryo sac, double fertilisation, and numerical questions on meiotic divisions.
Core rules
- Anther wall layers: epidermis, endothecium, middle layers, tapetum. The tapetum is diploid and nourishes developing pollen.
- Microsporogenesis: sporogenous cells → microspore mother cells → meiosis → microspore tetrads → 2-celled pollen (large vegetative cell + small generative cell). About 60% of angiosperms shed 2-celled pollen; the rest shed 3-celled pollen.
- Ovule parts: funicle, hilum, integuments, micropyle, chalaza, nucellus, embryo sac. Monosporic (Polygonum) development: one megaspore mother cell → meiosis → four megaspores → three degenerate → one functional megaspore → three mitotic divisions → 8 nuclei, 7 cells: 3 antipodals, 2 synergids (filiform apparatus), 1 egg, 1 central cell with 2 polar nuclei.
- Double fertilisation: syngamy gives the 2n zygote; triple fusion gives the 3n primary endosperm nucleus.
- Apomixis (seed without fertilisation), polyembryony (more than one embryo), emasculation and bagging for controlled crosses.
Worked example
How many meiotic divisions produce 50 seeds? Each seed needs one embryo sac, and one megaspore mother cell meiosis yields one functional megaspore, so 50 meioses in ovules. Each seed needs one pollen grain, and one microspore mother cell meiosis yields four pollen grains, so 50 ÷ 4 = 12.5 → 13 meioses in anthers.
Traps
The tapetum is not haploid. The filiform apparatus belongs to synergids, not antipodals. Coconut water is free-nuclear endosperm; the white kernel is cellular endosperm. Apple flesh develops from the thalamus, not the ovary wall.