Sexual Reproduction in Flowering Plants Class 12 Notes पुष्पी पादपों में लैंगिक प्रजनन — कक्षा 12 जीव विज्ञान अध्याय 1

Revise these first (prerequisites)

• Basic parts of a flower — sepal, petal, stamen, pistil (Class 6–7 level)
• Mitosis vs meiosis: meiosis halves the chromosome number (2n → n)
• Meaning of haploid (n) and diploid (2n)
• Plant life cycle: seed → plant → flower → fruit → seed

1. Flower = the reproductive part of a plant

   A flower is not just for looking beautiful — it is the reproductive organ of the plant. Its male part is the stamen (पुंकेसर) and its female part is the pistil (स्त्रीकेसर). Just as the kitchen is the place in a home where food is made, the flower is the place where seeds are made.

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2. Pollen grain = the male 'packet'

   Pollen grains are formed in the upper part of the stamen, the anther (परागकोश). Male gametes are formed inside each pollen grain. Think of it as a sealed courier packet: a tough outer cover (exine) with the actual contents (male gametes) inside.

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3. The egg inside the ovule

   The swollen lower part of the pistil is called the ovary (अंडाशय). Inside the ovary are ovules (बीजांड), and inside each ovule is an embryo sac (भ्रूणकोश) containing the egg cell. This ovule later develops into the seed.

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4. Pollination = pollen reaching the stigma

   Pollen has to travel from the anther to the stigma (top of the pistil) — this is called pollination (परागण). This is done by wind, water, insects or birds. A flower cannot move on its own, so it needs 'delivery partners'.

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5. Fertilisation = fusion of male and female gametes

   After landing on the stigma, the pollen grain forms a pollen tube that grows to the ovule. There, a male gamete fuses with the egg; this is fertilisation (निषेचन). Flowering plants have two fusions, not one, so the process is called double fertilisation.

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6. What forms later?

   After fertilisation: ovule → seed (बीज), ovary → fruit (फल). The mangoes or peas you eat are the result of this process. Inside the seed, a tiny baby plant (embryo) stays peacefully 'asleep' until it gets the right season.

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Quick check ✔

Basics clear? Now move on to Learn in Detail 👆

As per NCERT 2026-27 — every point you need for the board exam.

1. Parts of a flower

   Stamen (पुंकेसर) = male part: anther + filament. Pistil / carpel (स्त्रीकेसर) = female part: stigma, style, ovary. Androecium = all the stamens, gynoecium = all the carpels.

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2. Structure of the anther

   A typical anther is bilobed and dithecous — 4 microsporangia. The wall has 4 layers: epidermis, endothecium, middle layers, tapetum (innermost; nourishes the developing pollen).

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3. Microsporogenesis

   Pollen mother cell (2n) → meiosis → microspore tetrad (n). Each microspore later develops into a pollen grain.

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4. Pollen grain

   Outer wall exine = sporopollenin (the most resistant organic material); sporopollenin is absent at the germ pores. Inner wall intine = cellulose + pectin. A mature pollen grain has a vegetative cell (large) + a generative cell (small). In over 60% of angiosperms, pollen is shed at the 2-celled stage.

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5. Ovule (बीजांड)

   Attached to the placenta by the funicle; hilum = where the funicle joins the ovule body. Integuments, micropyle, chalaza, nucellus. The embryo sac lies inside the nucellus.

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6. Megasporogenesis + embryo sac

   Megaspore mother cell (2n) → meiosis → 4 megaspores, only 1 functional. Monosporic development: 3 mitotic divisions → 7-celled, 8-nucleate embryo sac — egg apparatus (2 synergids + 1 egg), 3 antipodals, 2 polar nuclei in the central cell.

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7. Types of pollination

   Autogamy (same flower), geitonogamy (another flower of the same plant — genetically similar to autogamy), xenogamy (a different plant — genetically different pollen). Cleistogamous flowers (Viola, Oxalis, Commelina) never open → assured seed set.

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8. Agents of pollination

   Wind: light, non-sticky pollen, feathery stigma (grasses, maize). Water: Vallisneria, Hydrilla, Zostera (sea grass). Insects: coloured, scented, nectar-rich flowers. Amorphophallus has the largest flower; Yucca–moth mutualism.

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9. Outbreeding devices

   Ways to prevent self-pollination: pollen release and stigma receptivity at different times, anther and stigma at different positions, self-incompatibility, unisexual flowers (castor, maize = monoecious; papaya = dioecious).

10. Artificial hybridisation

    Emasculation (removal of anthers from a bisexual flower) + bagging (butter paper bag) → dusting of desired pollen → rebagging.

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11. Double fertilisation (दोहरा निषेचन)

    The pollen tube releases 2 male gametes through a synergid. 1st male gamete + egg = syngamy → zygote (2n). 2nd male gamete + 2 polar nuclei = triple fusion → PEN (3n). Found only in angiosperms.

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12. Endosperm, embryo, seed

    Endosperm forms first (it provides nutrition to the embryo). Coconut water = free-nuclear endosperm, white kernel = cellular endosperm. Dicot embryo: 2 cotyledons, epicotyl, hypocotyl. Monocot: 1 cotyledon = scutellum, coleoptile, coleorhiza. Perisperm = residual nucellus (black pepper, beet).

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13. Fruit

    Ovary → fruit, ovule → seed. False fruit: the thalamus also contributes to fruit formation (apple, strawberry, cashew). Parthenocarpy = fruit without fertilisation (banana) — seedless.

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14. Apomixis & polyembryony

    Apomixis = formation of seeds without fertilisation (Asteraceae, grasses) — useful for hybrid seeds. Polyembryony = more than one embryo in a seed (citrus, mango). Seed viability records: Lupinus arcticus ~10,000 years; date palm ~2000 years.

The concept in depth — the why and the how. Everything stays within the NCERT syllabus.

1. Why is the exine so tough?

   The exine is made of sporopollenin — one of the most resistant organic materials known; it withstands high temperatures and strong acids/alkalis, and no enzyme can degrade it. This is why pollen is preserved even as fossils. Sporopollenin is absent at the germ pores — that is where the pollen tube emerges.

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2. Geitonogamy: functionally cross, genetically self

   In geitonogamy, pollen goes to the stigma of another flower of the same plant. A pollinator is needed, so functionally it is like cross-pollination; but the pollen is from the same plant, so genetically it is the same as autogamy — no new genetic variation arises. Hence maize (monoecious) prevents autogamy but not geitonogamy; papaya (dioecious) prevents both.

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3. Complete ploidy calculation

   Megaspore mother cell 2n → meiosis → megaspore n → 3 free-nuclear mitotic divisions → all cells of the embryo sac are n. Egg (n) + male gamete (n) = zygote 2n. Two polar nuclei (n+n) + male gamete (n) = PEN 3n. Nucellus and integuments are maternal tissues, so they are 2n. If you remember this table, you can solve all ploidy questions.

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4. Why does the endosperm form first?

   Endosperm development begins before embryo development, because the endosperm provides food to the developing embryo — the lunchbox is packed first, then the baby grows. Coconut water = free-nuclear endosperm, and the white kernel = cellular endosperm. In non-albuminous seeds like pea and groundnut, the embryo uses up the endosperm; in wheat, maize and castor it persists (albuminous).

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5. Use of apomixis in agriculture

   Apomixis = formation of seeds without fertilisation (Asteraceae, grasses). Hybrid seeds must be bought afresh every year because characters segregate in their progeny. If hybrids are made apomictic, hybrid characters will not segregate and farmers can use the same seeds year after year — that is why scientists are working to understand apomixis genes. This concept is directly linked to Chapter 4 (segregation).

6. Seed dormancy and viability: a survival strategy

   Seed dormancy protects the plant from unfavourable weather: the embryo stays inactive until suitable conditions are available. NCERT examples: a Lupinus arcticus seed germinated after ~10,000 years, and so did a ~2000-year-old date palm (Phoenix dactylifera) seed. This dormancy is also what makes seeds suitable for storage and transport.

Common confusions — cleared up

1. Syngamy vs Triple fusion

   Syngamy = male gamete + egg → zygote (2n). Triple fusion = male gamete + 2 polar nuclei → primary endosperm nucleus (3n). Together they form double fertilisation.

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2. Pollination vs Fertilisation

   Pollination = pollen reaching the stigma (transfer). Fertilisation = fusion of gametes, which happens inside the ovule after pollination.

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3. Perisperm vs Endosperm

   Endosperm is formed by fertilisation (3n). Perisperm is the remaining nucellus (2n), as in black pepper and beet.

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4. Parthenocarpy vs Apomixis

   Parthenocarpy = fruit without fertilisation (seedless, e.g. banana). Apomixis = seed without fertilisation.

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