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Sexual Reproduction in Flowering Plants

NCERT Class 12 · Biology Based on NCERT Class 12 Biology textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explains the entire process of sexual reproduction in angiosperms, from flower structure to seed formation. It covers the development of male and female gametophytes, pollination mechanisms, double fertilization, and post-fertilization events. The one big idea a student must walk away with is that flowering plants have evolved a unique double fertilization process that produces a diploid zygote and a triploid endosperm, ensuring successful seed development and genetic variation.
Key Concepts (37)
Stamen
The male reproductive organ of a flower, consisting of a filament and an anther.
e.g. In Hibiscus, the stamen is a long stalk with a bilobed anther at the tip.
Anther
The terminal bilobed structure of a stamen that contains microsporangia and produces pollen grains.
e.g. The anther of a mustard flower is bilobed and dithecous.
Microsporangium
A structure within the anther where microspores (pollen grains) develop; typically four microsporangia are present in a typical anther.
e.g. In a transverse section of an anther, four microsporangia are seen at the corners.
Tapetum
The innermost wall layer of a microsporangium that nourishes developing pollen grains; cells are dense and often binucleate.
e.g. Tapetal cells have dense cytoplasm and provide nutrients to microspores.
Microsporogenesis
The process of formation of microspores from a pollen mother cell (PMC) through meiosis.
e.g. A PMC undergoes meiosis to form a microspore tetrad.
Pollen grain
The male gametophyte in angiosperms, with a two-layered wall (exine and intine) and containing a vegetative cell and a generative cell (or two male gametes).
e.g. Pollen grains of Hibiscus are yellowish and powdery.
Sporopollenin
The hard, resistant organic material that makes up the exine of pollen grains; it can withstand high temperatures, strong acids, and alkalis.
e.g. Pollen grains are preserved as fossils due to sporopollenin.
Germ pore
An aperture in the exine of a pollen grain where sporopollenin is absent, allowing the pollen tube to emerge.
e.g. The pollen tube emerges through a germ pore during germination.
Pistil
The female reproductive organ of a flower, consisting of stigma, style, and ovary.
e.g. In Hibiscus, the pistil has a sticky stigma and a long style.
Ovule
The megasporangium of a seed plant, containing the nucellus, integuments, and embryo sac; develops into a seed after fertilization.
e.g. In mango, the ovary contains a single ovule.
Megasporogenesis
The process of formation of megaspores from the megaspore mother cell (MMC) through meiosis.
e.g. The MMC undergoes meiosis to produce four megaspores.
Embryo sac
The female gametophyte in angiosperms, typically 7-celled and 8-nucleate, containing the egg apparatus, central cell, and antipodals.
e.g. The mature embryo sac has an egg cell, two synergids, three antipodals, and a central cell with two polar nuclei.
Pollination
The transfer of pollen grains from the anther to the stigma of a pistil.
e.g. Bees carry pollen from one flower to another, effecting pollination.
Autogamy
Pollination within the same flower, i.e., transfer of pollen from anther to stigma of the same flower.
e.g. Cleistogamous flowers like Viola are invariably autogamous.
Geitonogamy
Transfer of pollen grains from the anther to the stigma of another flower of the same plant.
e.g. In maize, pollen from one flower may pollinate another flower on the same plant.
Xenogamy
Transfer of pollen grains from the anther to the stigma of a different plant; the only type that brings genetically different pollen.
e.g. Cross-pollination in papaya involves xenogamy.
Double fertilisation
A unique phenomenon in angiosperms where two fusion events occur in the embryo sac: syngamy (fusion of one male gamete with egg) and triple fusion (fusion of the other male gamete with two polar nuclei).
e.g. In a typical angiosperm, one sperm fuses with the egg to form the zygote, and the other fuses with polar nuclei to form the primary endosperm nucleus.
Syngamy
The fusion of a male gamete with the egg cell to form a diploid zygote.
e.g. Syngamy results in the formation of the zygote, which develops into the embryo.
Triple fusion
The fusion of a male gamete with the two polar nuclei in the central cell to form a triploid primary endosperm nucleus (PEN).
e.g. Triple fusion produces the triploid endosperm that nourishes the developing embryo.
Endosperm
A triploid tissue formed from the primary endosperm cell after triple fusion; it provides nutrition to the developing embryo.
e.g. Coconut water is free-nuclear endosperm, and the white kernel is cellular endosperm.
Embryo
The structure that develops from the zygote, consisting of an embryonal axis and cotyledons; in dicots, it has two cotyledons, and in monocots, one.
e.g. A pea seed contains a dicot embryo with two cotyledons.
Seed
The fertilized ovule, consisting of seed coat(s), cotyledon(s), and an embryo axis; the final product of sexual reproduction in angiosperms.
e.g. A mango seed has a hard seed coat and a single embryo.
Fruit
The mature ovary, containing seeds; may be true (from ovary only) or false (from ovary and other floral parts).
e.g. Mango is a true fruit, while apple is a false fruit.
Apomixis
A form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilization.
e.g. Some species of Asteraceae and grasses produce seeds apomictically.
Polyembryony
The occurrence of more than one embryo in a seed.
e.g. Orange seeds often contain multiple embryos due to nucellar polyembryony.
Parthenocarpy
The development of fruit without fertilization, resulting in seedless fruits.
e.g. Banana is a parthenocarpic fruit.
Cleistogamous flowers
Flowers that do not open at all, ensuring autogamy as anthers and stigma remain close.
e.g. Viola, Oxalis, and Commelina produce cleistogamous flowers.
Chasmogamous flowers
Flowers with exposed anthers and stigma, typical of most species, allowing cross-pollination.
e.g. Hibiscus and rose have chasmogamous flowers.
Self-incompatibility
A genetic mechanism that prevents self-pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth.
e.g. In some species, self-pollen is rejected by the pistil.
Emasculation
The removal of anthers from a bisexual flower bud before they dehisce, to prevent self-pollination in artificial hybridization.
e.g. In a pea plant, anthers are removed from the female parent before they mature.
Bagging
Covering emasculated or female flowers with a bag (e.g., butter paper) to prevent contamination by unwanted pollen.
e.g. After emasculation, the flower is bagged to ensure only desired pollen is used.
Filiform apparatus
Special cellular thickenings at the micropylar tip of synergids that guide the pollen tube into the embryo sac.
e.g. The filiform apparatus helps direct the pollen tube to the synergid.
Nucellus
The mass of cells within the ovule that contains the embryo sac and provides nutrition; remnants may persist as perisperm.
e.g. In black pepper, the nucellus persists as perisperm in the seed.
Perisperm
The persistent nucellus in some seeds, providing nutrition.
e.g. Black pepper and beet seeds have perisperm.
Coleoptile
A hollow foliar structure enclosing the shoot apex and leaf primordia in grass embryos.
e.g. In a maize seed, the coleoptile protects the plumule.
Coleorrhiza
An undifferentiated sheath enclosing the radicle and root cap in grass embryos.
e.g. In wheat, the coleorrhiza protects the radicle.
Scutellum
The single cotyledon in grass embryos, situated laterally on the embryonal axis.
e.g. In a maize embryo, the scutellum is the cotyledon.
Formulas & Equations (5)
Pollen mother cell (2n) → Meiosis → Microspore tetrad (n, n, n, n)
Reads as: A diploid pollen mother cell undergoes meiosis to produce four haploid microspores.
Variables: 2n = diploid number; n = haploid number
Use when: Explaining microsporogenesis in an exam question about pollen formation.
⚠ Students often think microspores are formed by mitosis, not meiosis.
Megaspore mother cell (2n) → Meiosis → Megaspore tetrad (n, n, n, n) → One functional megaspore (n) → Mitosis → 2-nucleate → 4-nucleate → 8-nucleate embryo sac
Reads as: A diploid megaspore mother cell undergoes meiosis to produce four haploid megaspores; one functional megaspore undergoes three mitotic divisions to form an 8-nucleate embryo sac.
Variables: 2n = diploid; n = haploid
Use when: Describing megasporogenesis and female gametophyte development.
⚠ Students forget that the functional megaspore undergoes mitosis, not meiosis, to form the embryo sac.
Syngamy: n (male gamete) + n (egg) → 2n (zygote)
Reads as: A haploid male gamete fuses with a haploid egg to form a diploid zygote.
Variables: n = haploid number; 2n = diploid number
Use when: Explaining the first fertilization event in double fertilization.
⚠ Students confuse syngamy with triple fusion; remember syngamy involves the egg.
Triple fusion: n (male gamete) + n (polar nucleus) + n (polar nucleus) → 3n (primary endosperm nucleus)
Reads as: A haploid male gamete fuses with two haploid polar nuclei to form a triploid primary endosperm nucleus.
Variables: n = haploid; 3n = triploid
Use when: Explaining the second fertilization event in double fertilization.
⚠ Students often write that triple fusion produces a diploid endosperm; it is triploid.
Double fertilisation = Syngamy + Triple fusion
Reads as: Double fertilization is the sum of syngamy and triple fusion.
Variables: No variables
Use when: Defining double fertilization in an exam answer.
⚠ Students sometimes think double fertilization means two sperm fuse with the egg; it is one sperm with egg and one with polar nuclei.
How It Works
Microsporogenesis
  1. The sporogenous tissue in the microsporangium consists of compactly arranged cells.
  2. Each cell of the sporogenous tissue acts as a pollen mother cell (PMC).
  3. The PMC undergoes meiotic division to form a microspore tetrad (four haploid microspores).
  4. As the anther matures and dehydrates, the microspores dissociate from each other.
  5. Each microspore develops into a pollen grain (male gametophyte).
💡 Microsporogenesis involves meiosis, not mitosis. The microspore tetrad is haploid.
Megasporogenesis
  1. A single megaspore mother cell (MMC) differentiates in the nucellus near the micropylar region.
  2. The MMC undergoes meiotic division to produce four haploid megaspores.
  3. In monosporic development, three megaspores degenerate, and one functional megaspore remains.
  4. The functional megaspore develops into the female gametophyte (embryo sac).
💡 Only one megaspore is functional; the other three degenerate. The functional megaspore is haploid.
Development of female gametophyte (embryo sac)
  1. The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles (2-nucleate stage).
  2. Two more sequential mitotic nuclear divisions occur, forming 4-nucleate and then 8-nucleate stages.
  3. These mitotic divisions are free nuclear (no cell wall formation after each division).
  4. After the 8-nucleate stage, cell walls are laid down, organizing the embryo sac.
  5. Three cells at the micropylar end form the egg apparatus (two synergids and one egg cell).
  6. Three cells at the chalazal end form the antipodals.
  7. The two remaining nuclei (polar nuclei) are situated in the large central cell.
  8. The mature embryo sac is 7-celled and 8-nucleate.
💡 The embryo sac is 7-celled but 8-nucleate because the central cell has two nuclei (polar nuclei).
Pollination
  1. Pollen grains are shed from the anther.
  2. Pollen grains are transferred to the stigma of a pistil by abiotic (wind, water) or biotic (animals) agents.
  3. The pollen grain lands on the stigma and germinates if compatible.
  4. The pollen tube grows through the style and enters the ovule via the micropyle.
  5. The pollen tube enters one synergid through the filiform apparatus and releases two male gametes.
💡 Pollination is the transfer of pollen to the stigma, not fertilization. Fertilization occurs later.
Double fertilisation
  1. The pollen tube releases two male gametes into the cytoplasm of a synergid.
  2. One male gamete fuses with the egg cell (syngamy), forming a diploid zygote.
  3. The other male gamete fuses with the two polar nuclei in the central cell (triple fusion), forming a triploid primary endosperm nucleus (PEN).
  4. The central cell becomes the primary endosperm cell (PEC), which develops into endosperm.
  5. The zygote develops into the embryo.
💡 Double fertilization is unique to angiosperms. Two fusions occur: syngamy and triple fusion.
Endosperm development
  1. The primary endosperm nucleus (PEN) undergoes successive nuclear divisions without cell wall formation, forming free-nuclear endosperm.
  2. Subsequently, cell walls form, making the endosperm cellular.
  3. The endosperm tissue is triploid and filled with reserve food materials.
  4. In some seeds (e.g., pea), the endosperm is completely consumed by the developing embryo (non-albuminous seeds).
  5. In other seeds (e.g., castor, coconut), the endosperm persists in the mature seed (albuminous seeds).
💡 Endosperm development precedes embryo development to provide nutrition. Coconut water is free-nuclear endosperm.
Embryo development (embryogeny) in dicots
  1. The zygote divides to form a proembryo.
  2. The proembryo develops into a globular stage.
  3. The globular stage becomes heart-shaped.
  4. The heart-shaped embryo matures into a mature dicot embryo with an embryonal axis and two cotyledons.
  5. The portion above the cotyledons is the epicotyl (terminates in plumule).
  6. The portion below the cotyledons is the hypocotyl (terminates in radicle).
💡 The zygote divides only after some endosperm is formed to ensure nutrition.
Embryo development in monocots (grasses)
  1. The embryo has a single cotyledon called the scutellum, situated laterally.
  2. The embryonal axis has the radicle and root cap enclosed in the coleorrhiza.
  3. The epicotyl has a shoot apex and leaf primordia enclosed in the coleoptile.
💡 Grass embryos have a scutellum (cotyledon), coleoptile (protects plumule), and coleorrhiza (protects radicle).
Seed and fruit development
  1. After fertilization, the ovule develops into a seed.
  2. The integuments harden to form the seed coat.
  3. The micropyle remains as a small pore for water and oxygen entry during germination.
  4. The ovary develops into a fruit; the ovary wall becomes the pericarp.
  5. In some plants (e.g., apple, strawberry), the thalamus also contributes to fruit formation (false fruits).
  6. In most plants, other floral parts degenerate and fall off.
💡 A seed is a fertilized ovule; a fruit is a mature ovary. False fruits involve other floral parts.
Apomixis
  1. In some species (e.g., Asteraceae, grasses), seeds are produced without fertilization.
  2. In some cases, the diploid egg cell forms without reduction division and develops into an embryo.
  3. In other cases (e.g., Citrus, mango), nucellar cells divide and protrude into the embryo sac, forming embryos (polyembryony).
💡 Apomixis is asexual reproduction that mimics sexual reproduction. It produces clones.
Exam Traps — Don't Make These Mistakes
⚠ Confusing microsporogenesis and megasporogenesis
✗ Microsporogenesis produces megaspores, and megasporogenesis produces microspores.
→ Microsporogenesis produces microspores (pollen grains) in the anther; megasporogenesis produces megaspores in the ovule.
✓ Microsporogenesis is the formation of microspores from a pollen mother cell via meiosis. Megasporogenesis is the formation of megaspores from a megaspore mother cell via meiosis.
⚠ Mistaking the ploidy of endosperm
✗ The endosperm is diploid (2n).
→ The endosperm is formed by triple fusion (one male gamete n + two polar nuclei n + n = 3n), so it is triploid.
✓ The endosperm is triploid (3n) because it results from the fusion of a haploid male gamete with two haploid polar nuclei.
⚠ Confusing autogamy and geitonogamy
✗ Geitonogamy is pollination within the same flower.
→ Autogamy is pollination within the same flower. Geitonogamy is pollination between different flowers of the same plant.
✓ Autogamy: pollen from the same flower lands on its own stigma. Geitonogamy: pollen from one flower lands on the stigma of another flower on the same plant.
⚠ Thinking the embryo sac is 8-celled
✗ The mature embryo sac is 8-celled.
→ The embryo sac is 7-celled because the central cell contains two polar nuclei but is a single cell. It is 8-nucleate.
✓ The mature embryo sac is 7-celled and 8-nucleate: 3 cells in the egg apparatus, 3 antipodals, and 1 central cell with 2 nuclei.
⚠ Confusing true fruit and false fruit
✗ Apple is a true fruit because it develops from the ovary.
→ Apple is a false fruit because it develops from the thalamus as well as the ovary.
✓ True fruits develop only from the ovary (e.g., mango). False fruits develop from the ovary and other floral parts like the thalamus (e.g., apple, strawberry).
⚠ Assuming all aquatic plants use water for pollination
✗ All aquatic plants are pollinated by water.
→ Many aquatic plants like water hyacinth and water lily have flowers that emerge above water and are pollinated by insects or wind.
✓ Only some aquatic plants (e.g., Vallisneria, Hydrilla, Zostera) use water for pollination. Others like water hyacinth and water lily are pollinated by insects or wind.
Compare & Contrast
Type of pollination
Autogamy
Geitonogamy
Autogamy is pollination within the same flower; geitonogamy is pollination between different flowers of the same plant.
Origin of fruit
True fruit
False fruit
True fruit develops only from the ovary (e.g., mango); false fruit develops from the ovary and other floral parts like the thalamus (e.g., apple).
Presence of endosperm in mature seed
Albuminous seed
Non-albuminous seed
Albuminous seeds retain endosperm (e.g., castor, coconut); non-albuminous seeds have no residual endosperm as it is consumed during embryo development (e.g., pea, groundnut).
Flower opening and pollination
Chasmogamous flowers
Cleistogamous flowers
Chasmogamous flowers open and expose anthers and stigma, allowing cross-pollination; cleistogamous flowers do not open and are invariably autogamous.
Exam Focus
Marks pattern: This chapter typically appears in board exams with 2-3 MCQs (1 mark each), 1-2 short answer questions (2-3 marks), and 1 long answer question (5 marks). Total: approximately 10-12 marks.
Most asked: Double fertilization and its significance · Structure of anther and microsporangium · Structure of ovule and embryo sac · Types of pollination (autogamy, geitonogamy, xenogamy) · Agents of pollination (wind, water, animals) · Post-fertilization events: endosperm and embryo development · Apomixis and polyembryony
Don't skip: Describe the process of double fertilization with a labeled diagram.; Explain the structure of a typical angiosperm ovule and the development of the embryo sac.; Differentiate between microsporogenesis and megasporogenesis.; Describe the different types of pollination and their significance.; Explain post-fertilization events: endosperm and embryo development.
Safe to skip: The detailed structure of the coleoptile and coleorrhiza in grass embryos is low-weightage and can be de-prioritized under time pressure.
One-Line Revision (read aloud before the exam)
  1. The anther is bilobed, dithecous, and tetrasporangiate, containing four microsporangia.
  2. Microsporogenesis is the formation of microspores from a pollen mother cell via meiosis.
  3. The pollen grain has a two-layered wall: exine (sporopollenin) and intine (cellulose and pectin).
  4. Sporopollenin is the most resistant organic material and preserves pollen grains as fossils.
  5. The mature embryo sac is 7-celled and 8-nucleate: egg apparatus (2 synergids + 1 egg), 3 antipodals, and 1 central cell with 2 polar nuclei.
  6. Pollination is the transfer of pollen from anther to stigma.
  7. Autogamy is self-pollination within the same flower; geitonogamy is between flowers of the same plant; xenogamy is between different plants.
  8. Cleistogamous flowers are invariably autogamous and produce assured seed-set even without pollinators.
  9. Double fertilization is unique to angiosperms: syngamy (n + n = 2n zygote) and triple fusion (n + n + n = 3n endosperm).
  10. Endosperm development precedes embryo development to provide nutrition.
  11. A true fruit develops only from the ovary; a false fruit develops from the ovary and other floral parts like the thalamus.
  12. Apomixis is asexual reproduction that produces seeds without fertilization, resulting in clones.
  13. Polyembryony is the occurrence of more than one embryo in a seed, common in Citrus and mango.
  14. Parthenocarpy is fruit development without fertilization, producing seedless fruits like banana.
  15. Emasculation is the removal of anthers from a bisexual flower before they dehisce in artificial hybridization.
  16. The filiform apparatus in synergids guides the pollen tube into the embryo sac.
  17. In grasses, the cotyledon is called the scutellum, the plumule is enclosed in the coleoptile, and the radicle is enclosed in the coleorrhiza.

MCQs — 10 Questions with Answers

Q1. Which of the following correctly describes the ploidy of cells in the microspore tetrad formed during microsporogenesis?

  • A. All four cells are diploid (2n)
  • B. All four cells are haploid (n) ✓
  • C. Two cells are diploid and two are haploid
  • D. One cell is haploid and three are polyploid

Answer: B — During microsporogenesis, the pollen mother cell (diploid) undergoes meiosis I and II to produce four haploid microspores in a tetrad.

Q2. In a transverse section of an anther, the layer that provides nutrition to developing pollen grains is the:

  • A. Epidermis
  • B. Endothecium
  • C. Tapetum ✓
  • D. Middle layers

Answer: C — The tapetum is the innermost nutritive wall layer with dense cytoplasm and multiple nuclei that nourishes developing pollen grains.

Q3. The mature embryo sac of most angiosperms is characterised by having how many cells and nuclei respectively?

  • A. 6 cells, 7 nuclei
  • B. 7 cells, 8 nuclei ✓
  • C. 8 cells, 8 nuclei
  • D. 7 cells, 7 nuclei

Answer: B — The typical mature embryo sac is 7-celled with 8 nuclei: 3 antipodal cells (1 nucleus each), egg apparatus (egg + 2 synergids, 3 nuclei), 2 polar nuclei (1 nucleus each).

Q4. Consider the process of double fertilisation in flowering plants. Which of the following statements is correct?

  • A. Both syngamy and triple fusion produce diploid nuclei
  • B. Syngamy produces a 2n zygote and triple fusion produces a 3n central cell nucleus ✓
  • C. Triple fusion produces the embryo and syngamy produces the endosperm
  • D. Both fertilisation events occur at different times, separated by mitotic divisions

Answer: B — In double fertilisation, syngamy (one sperm + egg nucleus) produces a 2n zygote that develops into the embryo, while triple fusion (second sperm + 2 polar nuclei) produces a 3n nucleus that develops into the endosperm.

Q5. Pollen-pistil incompatibility in some plants is controlled by the S-locus. In such a self-incompatibility system, what happens when a plant is pollinated with its own pollen?

  • A. Pollen tube grows normally through the style to the ovule
  • B. Pollen tube growth is blocked due to matching S-locus alleles between pollen and stigma ✓
  • C. Pollen germinates but fails to produce viable gametes
  • D. Fertilisation occurs but the zygote aborts due to genetic imbalance

Answer: B — Self-incompatibility systems prevent self-fertilisation when the S-locus alleles of the pollen match those of the pistil, blocking pollen tube growth before it reaches the ovule.

Q6. A student observes a pollen grain under the microscope and sees two nuclei in its cytoplasm. What does this indicate about the developmental stage of the pollen?

  • A. It is a haploid microspore that has not yet undergone mitosis
  • B. It is a mature pollen grain with a vegetative nucleus and a generative nucleus ✓
  • C. It is undergoing meiosis II and is therefore unstable
  • D. It has completed fertilisation and contains both male and female nuclei

Answer: B — A two-celled pollen grain (bicellular stage) contains a larger vegetative nucleus and a smaller generative nucleus; the generative nucleus will later divide to form two sperm cells.

Q7. After pollination, the pollen tube grows through the style and enters the ovule via the micropyle. Which of the following best explains why this specific pathway ensures successful fertilisation?

  • A. The micropyle is the only opening in the ovule, making it the direct entry point for the pollen tube
  • B. The stigma guides pollen tubes chemically, and the micropyle is where synergids release attractants ✓
  • C. The style acts as a selective barrier that allows only compatible pollen to reach the ovule
  • D. The micropyle contains the egg cell, which is the primary target of the sperm nucleus

Answer: B — Chemical signals from the stigma and style guide pollen tube growth, and filiform apparatus in the synergids release attractants that guide the pollen tube through the micropyle into the embryo sac.

Q8. Which of the following is NOT a correct statement about apomixis?

  • A. Apomixis is the formation of seeds without meiosis or fertilisation
  • B. Apomictic seeds always produce genetically identical offspring to the parent plant
  • C. Apomixis requires pollination to trigger seed development but no fusion occurs
  • D. Apomixis increases genetic diversity in populations faster than sexual reproduction ✓

Answer: D — Apomixis produces clones with no genetic variation, whereas sexual reproduction generates diversity; apomixis actually reduces genetic diversity compared to sexual reproduction.

Q9. A researcher studying seed coat formation observes that integuments of an ovule thicken and harden after fertilisation. Based on the post-fertilisation developmental pathway, what is the embryological origin and function of the seed coat?

  • A. The seed coat originates from the ovary wall and provides physical protection
  • B. The seed coat originates from the integuments and provides protection and regulates water entry ✓
  • C. The seed coat originates from the nucellus and stores nutrients for germination
  • D. The seed coat originates from the endosperm and aids in seed dispersal

Answer: B — Integuments differentiate into the seed coat (testa and tegmen layers) after fertilisation, providing mechanical protection, preventing mechanical injury, and regulating water absorption during germination.

Q10. Polyembryony in citrus plants can occur through multiple mechanisms. If a citrus ovule produces multiple embryos, one of which arises from the fusion of sperm and egg (zygotic embryo) while others develop from nucellar tissue, what is the immediate consequence for genetic variation in the resulting seeds?

  • A. All embryos will be genetically identical because they come from the same ovule
  • B. The zygotic embryo will be genetically unique while the adventive (nucellar) embryos will be clones of the parent ✓
  • C. Genetic variation increases because multiple independent fertilisation events occur
  • D. Genetic variation decreases because all embryos share the same maternal cytoplasm

Answer: B — In polyembryony, the zygotic embryo (from syngamy) is genetically unique and represents sexual reproduction, while adventive embryos from nucellar cells are clones of the maternal parent, reducing overall genetic diversity.

Flashcards

What is a pollen mother cell (PMC), and what does it undergo during microsporogenesis?

A PMC is a diploid sporogenous cell that undergoes meiosis to produce a tetrad of four haploid microspores.

Define the tapetum and state its function in the anther.

The tapetum is the innermost nutritive layer of the microsporangium that provides nutrients to developing pollen grains.

What are the three main parts of a typical pollen grain structure?

A pollen grain has the exine (outer wall), intine (inner wall), and cytoplasm containing the generative and vegetative nuclei.

How many cells and nuclei does a mature embryo sac (7-celled, 8-nucleate type) contain?

A mature embryo sac has 7 cells (3 antipodal cells, 2 synergids with egg cell, and 2 polar nuclei) and 8 nuclei.

What is double fertilisation, and what are its two events in flowering plants?

Double fertilisation consists of syngamy (fusion of sperm with egg to form 2n embryo) and triple fusion (fusion of second sperm with polar nuclei to form 3n endosperm).

Distinguish between pollen-pistil compatibility and incompatibility with one example.

Compatibility allows pollen tube growth and fertilisation; incompatibility prevents it due to biochemical barriers (e.g., S-locus alleles in self-incompatibility systems).

What is apomixis, and how does it differ from sexual reproduction?

Apomixis is asexual reproduction through seeds without meiosis or fusion, producing genetically identical offspring unlike sexual reproduction.

Define polyembryony and name one plant in which it occurs naturally.

Polyembryony is the development of multiple embryos from a single ovule, occurring naturally in plants like Citrus (orange and lemon).

What happens to the integument and nucellus after fertilisation during seed formation?

Integuments harden to form the seed coat; the nucellus may be absorbed or persist as perisperm to store nutrients.

Why is seed dispersal important for plant species survival in evolutionary terms?

Seed dispersal reduces competition with parent plants, colonises new habitats, and increases genetic diversity and population fitness.

Important Board Questions

Define microsporogenesis and state the ploidy of microspores produced. Draw a labelled diagram showing the microspore tetrad and its development into a mature pollen grain. [2 marks]

Microsporogenesis = meiosis in PMC; microspores are haploid (n). Diagram must show tetrad arrangement, mitotic division of microspore nucleus into vegetative and generative nuclei, and mature 2-celled pollen with exine and intine.

Explain the structure and developmental stages of the mature embryo sac (7-celled, 8-nucleate type). Identify the role of each region in double fertilisation and subsequent seed development. [5 marks]

Describe: egg apparatus (egg + 2 synergids with filiform apparatus), 2 polar nuclei in central cell, 3 antipodal cells. Explain: synergids guide pollen tube; egg fuses with sperm (syngamy → zygote/embryo); polar nuclei fuse with second sperm (triple fusion → 3n endosperm); antipodals degenerate. Link each to fertilisation outcome.

Double fertilisation is a defining feature of flowering plants. Explain the mechanisms of syngamy and triple fusion, showing why both events are essential for seed viability and endosperm development. Use a labelled diagram of the embryo sac during double fertilisation. [6 marks]

Show pollen tube entry at micropyle; trace both sperm nuclei movement; syngamy (sperm + egg → 2n zygote developing into embryo) must be justified (diploid for growth stability); triple fusion (sperm + 2 polar nuclei → 3n endosperm) must explain why 3n is essential (polyploidy provides metabolic vigour, stores nutrients). Diagram: label micropyle, synergids, egg, polar nuclei, pollen tube, both fusions occurring simultaneously. Conclude: embryo + endosperm balance ensures successful seed maturation and germination.

Assertion-Reason Questions for Sexual Reproduction in Flowering Plants

Q1. Assertion (A):

The androecium and gynoecium differentiate and develop within the flower during floral development.

Reason (R):

Hormonal and structural changes are initiated before the flower becomes visible on the plant, leading to differentiation of floral primordia.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The reason correctly explains why the assertion is accurate—developmental changes occur early at the hormonal and structural level, before visible flower appearance. This is the causal relationship between the two statements.

Q2. Assertion (A):

Tapetal cells are multinucleate with dense cytoplasm.

Reason (R):

Multinucleate tapetal cells synthesize and transport large quantities of nutrients required for pollen wall formation and development.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true, but the reason does not fully explain why tapetal cells are multinucleate. While nutrient synthesis is a function of tapetal cells, multiple nuclei primarily enable high metabolic activity. The reason focuses on function rather than explaining the structural adaptation itself.

Q3. Assertion (A):

Sporogenous tissue in the young anther occupies the centre of the microsporangium.

Reason (R):

The sporogenous tissue is replaced by pollen grains as the anther matures and the cells of the sporogenous tissue undergo meiosis.

Answer: A — Both A and R are true, and R is the correct explanation of A — The assertion is true—sporogenous tissue is a group of compactly arranged cells at the center of the microsporangium. The reason correctly explains this localization; as development proceeds, these cells undergo meiosis to form microspores, which eventually mature into pollen grains that fill the space.

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Case-Based Questions (CBQ) for Sexual Reproduction in Flowering Plants

Case Study:

A floriculture research institute in Bangalore is studying the development of anthers in hybrid rose varieties. They collected stamens from buds of different developmental stages and performed histological sections. In Stage 1 buds (2 cm length), they observed that the sporogenous tissue occupied the entire central region of the microsporangium, consisting of densely packed homogenous cells with no signs of meiotic division. In Stage 3 buds (5 cm length), they observed microspore tetrads arranged in clusters, with the tapetum significantly depleted of cytoplasm but still multinucleate. In Stage 5 buds (8 cm length), individual pollen grains were observed with distinct wall layers, and the tapetum had almost completely degenerated. The researchers noted that at each stage, the anther maintained its dithecous structure with four distinct microsporangia.

Q1. At Stage 1, the sporogenous tissue consists of cells that are best described as

  • A. Haploid microspores ready for pollen wall formation
  • B. Diploid pollen mother cells capable of undergoing meiosis ✓
  • C. Multinucleate tapetal cells supporting pollen development
  • D. Wall layer cells involved in anther protection

Answer: B

Q2. The significant depletion of tapetal cytoplasm observed in Stage 3 most likely indicates

  • A. Premature tapetal degeneration due to disease or stress
  • B. Active transport and synthesis of nutrients being transferred to developing microspores ✓
  • C. Failure of microspores to properly receive nourishment
  • D. Completion of the anther's developmental program with no further function

Answer: B

Q3. If the researchers observed 16 microsporangia across all four anthers in a single flower, and each microsporangium contained approximately 5,000 pollen grains at Stage 5, how many total pollen grains would be produced by the flower?

  • A. 20,000 pollen grains
  • B. 40,000 pollen grains
  • C. 80,000 pollen grains ✓
  • D. 160,000 pollen grains

Answer: C

Q4. At Stage 5, the complete degeneration of the tapetum and the presence of mature pollen with distinct wall layers suggest that the anther is

  • A. Still in its developmental phase and not yet mature
  • B. Unable to undergo proper dehiscence due to tapetal loss
  • C. Experiencing abnormal development due to nutrient deficiency
  • D. Functionally mature and ready for dehiscence and pollen release ✓

Answer: D

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Previous Year Questions (PYQ) for Sexual Reproduction in Flowering Plants

Real CBSE board exam questions from past papers — free to attempt.

CBSE 2026 · Set 57/1/1 · 1 mark

Identify the plant in which emasculation is not required for artificial hybridization.

  • Rice
  • Wheat
  • Pea
  • Papaya

CBSE 2026 · Set 57/1/1 · 2 marks

(a) (i) How does a farmer use dormancy of seed to his advantage? (1 mark) (ii) Differentiate between pea seed and castor seed. (1 mark)

CBSE 2026 · Set 57/1/1 · 3 marks

(a) Do all pollen grains remain viable for the same length of time? Support your answer with two suitable examples. (2.5 marks) (b) How are pollen grains stored in pollen banks? (0.5 marks)

CBSE 2026 · Set 57/1/1 · 3 marks

Draw a neat diagram of megasporangium of an angiosperm and label any six parts. (0.5 × 6 = 3 marks)

CBSE 2026 · Set 57/1/2 · 3 marks

(a) Do all pollen grains remain viable for the same length of time? Support your answer with two suitable examples. (2½ marks) (b) How are pollen grains stored in pollen banks? (½ mark)

CBSE 2026 · Set 57/1/2 · 5 marks

(a) In angiosperms, the female gametophyte develops from a single cell of megasporangium. (i) Name that cell of megasporangium and describe the sequential changes it undergoes to form a mature female gametophyte. (3 marks) (ii) Draw a well labelled diagram of mature female gametophyte. Label any four parts. (2 marks) OR (b) [Hindi text not included - refer to original for complete OR section]

CBSE 2026 · Set 57/1/3 · 1 mark

Perisperm present in seed of black pepper is remnant of

  • Ovary
  • Nucellus
  • Anther
  • Megaspore

CBSE 2026 · Set 57/1/3 · 2 marks

How does a farmer use dormancy of seed to his advantage? Differentiate between pea seed and castor seed.

CBSE 2026 · Set 57/1/3 · 2 marks

Write any two advantages of producing plants by micro-propagation.

CBSE 2026 · Set 57/1/3 · 3 marks

Do all pollen grains remain viable for the same length of time? Support your answer with two suitable examples. How are pollen grains stored in pollen banks?

CBSE 2026 · Set 57/1/3 · 3 marks

Draw a neat diagram of megasporangium of an angiosperm and label any six parts.

CBSE 2026 · Set 57/2/1 · 1 mark

Given below are stages of embryo sac formation from megaspore mother cell. Identify the correct option for the cell division marked a, b, c and d.

  • a – Meiosis-I, b – No division, c – Mitosis, d – Meiosis-II
  • a – Meiosis-I, b – Meiosis-II, c – No division, d - Mitosis
  • a – Mitosis, b – Meiosis-I, c – Meiosis-II, d – No division
  • a – No division, b – Mitosis, c – Meiosis-I, d – Meiosis-II

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Class 12 Biology Previous Year Questions, Chapter-wise

All 1074 CBSE board questions from 2022–2026, sorted by chapter and question type. Free PDF download.

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