Samacheer Kalvi 12th Biology Chapter 1 Exercise Solutions

Chapter 1 — Asexual and Sexual Reproduction in Plants Exercise Solutions

Reproduction questions test definition accuracy and ability to name examples. For diagrams, practise labelling the embryo sac with all 8 cells.

How to Use This Page

1. Start with the Key Concepts table for a quick refresher on every important term in this chapter. 2. Work through MCQs and Fill in the Blanks to test recall of facts and definitions. 3. Use True or False to catch common misconceptions — pay attention to corrections for false statements. 4. Practise Match the Following to link terms with their correct descriptions. 5. Read Brief Answers for 2-mark questions — keep answers to 3-5 sentences. 6. Study Detailed Answers for 5-mark questions — use the structured sub-points format. 7. Review Common Mistakes to avoid losing marks on similar errors. 8. Tick off the Exam Checklist before your exam to ensure nothing is missed.

Key Concepts (Quick Recap)

I. Choose the Best Answer (MCQs)

1. Which of the following is a natural method of vegetative propagation? (a) Grafting (b) Layering (c) Runner in Oxalis (d) Tissue culture

2. Double fertilisation is a characteristic feature of: (a) Gymnosperms (b) Angiosperms (c) Pteridophytes (d) Bryophytes

3. In the embryo sac, the egg apparatus consists of: (a) One egg cell and three antipodal cells (b) One egg cell and two synergids (c) Two polar nuclei and one egg cell (d) Three antipodal cells

4. Apomixis is the production of: (a) Fruits without seeds (b) Seeds without fertilisation (c) Seeds with double fertilisation (d) Vegetative propagation through roots

5. Which agent of pollination is involved in hydrophily? (a) Wind (b) Water (c) Insects (d) Birds

6. Parthenocarpy results in: (a) Seedless fruits (b) Fruits with many seeds (c) Seeds without embryo (d) Fruits with hard seed coat

7. Emasculation is performed to prevent: (a) Cross-pollination (b) Self-pollination (c) Double fertilisation (d) Fragmentation

8. The ploidy of the primary endosperm nucleus is: (a) n (b) 2n (c) 3n (d) 4n

9. Which of the following is an outbreeding device? (a) Self-pollination (b) Cleistogamy (c) Herkogamy (d) Autogamy

10. How many cells are present in a mature embryo sac (Polygonum type)? (a) 5 (b) 6 (c) 7 (d) 8

11. In which organism does fragmentation occur? (a) Yeast (b) Spirogyra (c) Mucor (d) Mango

12. Dichogamy refers to: (a) Self-pollination within the same flower (b) Maturation of stamens and pistils at different times (c) Production of seedless fruits (d) Formation of seeds without fertilisation

II. Fill in the Blanks

1. The transfer of pollen from anther to stigma is called pollination. 2. The embryo sac is also known as the female gametophyte. 3. Double fertilisation includes syngamy and triple fusion. 4. The primary endosperm nucleus has a ploidy of 3n. 5. Seedless fruits are produced by the process of parthenocarpy. 6. In vegetative propagation through runners, the example plant is Oxalis. 7. Removal of anthers before they dehisce is called emasculation. 8. The fertilisation of male gamete with two polar nuclei is called triple fusion. 9. When pollen is carried by wind, the type of pollination is called anemophily. 10. Heterostyly is an example of an outbreeding device.

III. True or False (with corrections for false ones)

IV. Match the Following

V. Answer Briefly (2-mark)

1. Differentiate between self-pollination and cross-pollination. Self-pollination (autogamy) involves the transfer of pollen from anther to stigma of the same flower. Cross-pollination (allogamy) involves transfer of pollen from anther of one flower to stigma of another flower on a different plant. Self-pollination maintains purity of traits but reduces genetic variation, while cross-pollination increases genetic variation and produces healthier offspring.

2. What is apomixis? Give two examples. Apomixis is the production of seeds without fertilisation. The embryo develops directly from the nucellus or from an unreduced egg cell without fusion of gametes. It is genetically equivalent to asexual reproduction. Examples include mango (Mangifera indica) and Citrus species. Apomixis helps maintain parental characters in the offspring.

3. Describe the structure of the embryo sac. The mature embryo sac (Polygonum type) is a 7-celled, 8-nucleate structure. At the micropylar end, the egg apparatus consists of one egg cell and two synergids. At the chalazal end, three antipodal cells are present. The large central cell contains two polar nuclei. The embryo sac is the female gametophyte of angiosperms and is the site of double fertilisation.

4. What are outbreeding devices? Name three types. Outbreeding devices are structural or temporal mechanisms in flowers that prevent self-pollination and promote cross-pollination. Three types are: (i) Dichogamy — stamens and pistils mature at different times (protandry or protogyny). (ii) Herkogamy — physical barrier between anther and stigma preventing self-pollination. (iii) Heterostyly — flowers have styles of different lengths (pin and thrum types) within the same species.

5. Define parthenocarpy. How is it useful? Parthenocarpy is the development of fruit without fertilisation, resulting in seedless fruits. It is useful in horticulture because seedless fruits are commercially preferred (e.g., bananas, seedless grapes, watermelons). Parthenocarpy can be induced artificially by applying growth hormones like gibberellic acid to flowers.

6. What is self-incompatibility? Self-incompatibility is a genetic mechanism in flowering plants that prevents self-pollination from producing viable seeds. It is controlled by S-genes. When pollen carrying the same S-allele as the pistil lands on the stigma, pollen tube growth is inhibited. This ensures cross-pollination and maintains genetic diversity. Examples include tobacco and potato.

VI. Answer in Detail (5-mark)

1. Explain double fertilisation in angiosperms with a diagram description. Double fertilisation is unique to angiosperms and involves two fusion events:

• Pollination and pollen tube entry: The pollen grain lands on the stigma, germinates, and the pollen tube grows through the style into the ovule, entering through the micropyle.
• Discharge of male gametes: Two male gametes are released into the embryo sac through the pollen tube.
• Syngamy (first fusion): The first male gamete (n) fuses with the egg cell (n) to form the zygote (2n). This is called syngamy and restores the diploid condition.
• Triple fusion (second fusion): The second male gamete (n) fuses with the two polar nuclei (n + n) in the central cell to form the primary endosperm nucleus (3n). This is called triple fusion.

• Diagram description: Draw a longitudinal section of an ovule showing the embryo sac. Label the egg cell, synergids, polar nuclei, antipodal cells, pollen tube, and the two male gametes. Show arrows for syngamy and triple fusion.

2. Describe the types of asexual reproduction in plants with examples. Asexual reproduction produces genetically identical offspring (clones) without gamete fusion:

• Vegetative propagation: New plants arise from vegetative parts. Examples — runners (Oxalis), bulbs (Allium/onion), rhizomes (ginger, Zingiber), tubers (potato, Solanum tuberosum), offset (water hyacinth, Eichhornia).
• Budding: An outgrowth (bud) forms on the parent body and detaches to form a new individual. Example — yeast (Saccharomyces).
• Fragmentation: The parent body breaks into fragments, each of which grows into a new organism. Example — Spirogyra (filamentous green alga).
• Sporulation: Spores are produced which germinate to form new individuals. Examples — Mucor (fungi), ferns (pteridophytes produce spores on sporophylls).
• Artificial methods: Layering (jasmine), grafting (mango), and tissue culture (micropropagation) are human-assisted methods of vegetative propagation used in horticulture and agriculture.

3. Explain the different types of pollination and their agents. Pollination is the transfer of pollen from anther to stigma:

• Self-pollination (autogamy): Pollen transfers within the same flower. Requires synchronised maturity of anther and stigma. Seen in cleistogamous flowers (e.g., Viola).
• Cross-pollination (allogamy): Pollen transfers from one flower to another on a different plant. Promotes genetic variation.
• Agents of cross-pollination:
• Anemophily (wind) — light, non-sticky pollen; feathery stigma; well-exposed stamens. Examples: grasses, maize, wheat.
• Hydrophily (water) — pollen floats on water surface. Examples: Vallisneria (epihydrophily), Zostera (hypohydrophily).
• Entomophily (insects) — bright coloured, fragrant flowers; nectar present; sticky pollen. Examples: sunflower, orchids.
• Ornithophily (birds) — large, brightly coloured flowers; copious nectar. Examples: Bignonia, Agave.
• Chiropterophily (bats) — large, dull-coloured flowers that open at night. Examples: Kigelia (sausage tree).
• Contrivances for cross-pollination: Dichogamy, herkogamy, heterostyly, self-incompatibility.

4. Describe the development of the embryo sac (Polygonum type). The Polygonum type is the most common embryo sac development pattern in angiosperms:

• Megaspore mother cell (MMC): A diploid (2n) cell in the nucellus differentiates into the MMC.
• Meiosis: The MMC undergoes meiosis to form four haploid megaspores arranged in a linear tetrad.
• Functional megaspore: Only the chalazal megaspore remains functional; the other three degenerate.
• Megagametogenesis: The functional megaspore undergoes three successive mitotic divisions (nuclear divisions without cell wall formation) to produce 8 nuclei.
• Organisation: These 8 nuclei arrange into 7 cells — 3 cells at micropylar end (1 egg cell + 2 synergids = egg apparatus), 3 antipodal cells at chalazal end, and 1 central cell with 2 polar nuclei.

Types of Asexual Reproduction — Quick List

1. Vegetative propagation — runners (Oxalis), bulbs (Allium), rhizomes (ginger) 2. Budding — Yeast (not a plant, but mentioned in context) 3. Fragmentation — Spirogyra 4. Sporulation — Mucor, ferns (spores) 5. Layering — Jasmine (artificial) 6. Grafting — Mango (artificial)

Embryo Sac Structure (7-cell, 8-nucleate)

The mature embryo sac (Polygonum type) contains:

• 1 Egg cell + 2 Synergids = Egg apparatus (3 cells at micropylar end)
• 2 Polar nuclei (central cell, later form endosperm)
• 3 Antipodal cells (at chalazal end)

Total: 7 cells, 8 nuclei

Double Fertilisation — Steps

1. Pollen tube enters through micropyle. 2. Two male gametes discharged into embryo sac. 3. First male gamete + egg cell → Zygote (2n) — Syngamy 4. Second male gamete + 2 polar nuclei → Primary Endosperm Nucleus (3n) — Triple fusion 5. Both events together = Double fertilisation

Short Answer Points

• Pollination is the transfer of pollen from anther to stigma.
• Self-pollination (autogamy) — pollen lands on stigma of same flower.
• Cross-pollination (allogamy) — pollen from another flower.
• Agents of pollination: wind (anemophily), water (hydrophily), insects (entomophily), birds (ornithophily).
• Outbreeding devices: dichogamy, herkogamy, heterostyly, self-incompatibility.

Worked Examples

Example 1 — Double fertilisation (model answer)

In angiosperms one male gamete fuses with the egg (syngamy, n + n → 2n zygote), and the second male gamete fuses with the two polar nuclei (triple fusion, n + n + n → 3n primary endosperm nucleus). Because two fusion events occur, it is called double fertilisation.

Example 2 — Ploidy quick reference

Example 3 — Embryo sac structure

Common Mistakes to Avoid

1. ❌ Confusing syngamy with triple fusion ✅ Syngamy = male gamete + egg cell → zygote (2n); Triple fusion = male gamete + 2 polar nuclei → PEN (3n)

2. ❌ Stating the embryo sac has 8 cells ✅ The embryo sac has 7 cells and 8 nuclei

3. ❌ Writing that parthenocarpy and apomixis are the same ✅ Parthenocarpy = seedless fruit without fertilisation; Apomixis = seed without fertilisation

4. ❌ Forgetting that double fertilisation is unique to angiosperms ✅ Double fertilisation (syngamy + triple fusion) is the defining feature of angiosperms only

5. ❌ Confusing self-incompatibility with self-pollination ✅ Self-incompatibility prevents self-pollination from producing seeds; it promotes cross-pollination

6. ❌ Writing that vegetative propagation produces genetically varied offspring ✅ Vegetative propagation produces genetically identical offspring (clones)

7. ❌ Listing emasculation as removal of stigma ✅ Emasculation is the removal of anthers from a flower before they dehisce

8. ❌ Confusing anemophily (wind) with hydrophily (water) ✅ Anemophily = wind pollination; Hydrophily = water pollination

Exam Checklist

• [ ] Can you define asexual reproduction and list its types with examples?
• [ ] Can you describe vegetative propagation with at least 4 examples?
• [ ] Do you know the structure of the embryo sac (7 cells, 8 nuclei) and can you label a diagram?
• [ ] Can you explain double fertilisation step by step with ploidy at each stage?
• [ ] Can you differentiate between syngamy and triple fusion?
• [ ] Can you define apomixis and parthenocarpy and distinguish between them?
• [ ] Can you list the agents of pollination with examples for each?
• [ ] Can you explain outbreeding devices (dichogamy, herkogamy, heterostyly, self-incompatibility)?
• [ ] Can you define emasculation and explain its purpose?
• [ ] Do you know the ploidy of egg cell, zygote, and primary endosperm nucleus?
• [ ] Can you describe the development of the Polygonum type embryo sac?
• [ ] Can you differentiate self-pollination from cross-pollination with advantages and disadvantages?