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Sexual Reproduction in Flowering Plants – CBSE NCERT Study Resources

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12th

12th - Biology

Sexual Reproduction in Flowering Plants

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Overview of the Chapter

This chapter explores the process of sexual reproduction in flowering plants, detailing the structures involved, the mechanisms of pollination, fertilization, and the development of seeds and fruits. It aligns with the CBSE Grade 12 Biology curriculum, providing a comprehensive understanding of plant reproductive biology.

Flower Structure

Flowers are the reproductive structures of angiosperms. A typical flower consists of four whorls:

  • Calyx (sepals)
  • Corolla (petals)
  • Androecium (male reproductive part - stamens)
  • Gynoecium (female reproductive part - pistil)

Stamen: The male reproductive organ, consisting of an anther and a filament.

Pistil: The female reproductive organ, composed of the stigma, style, and ovary.

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma. It can occur through:

  • Self-pollination (autogamy and geitonogamy)
  • Cross-pollination (xenogamy)

Agents of pollination include wind, water, insects, birds, and other animals.

Fertilization

After pollination, the pollen tube grows through the style to reach the ovary. Double fertilization occurs, involving:

  • Fusion of one male gamete with the egg (forming a zygote)
  • Fusion of the second male gamete with the polar nuclei (forming the endosperm)

Double Fertilization: A unique process in flowering plants where two fusion events occur, leading to the formation of both embryo and endosperm.

Post-Fertilization Changes

After fertilization, the following developments occur:

  • The ovule develops into a seed.
  • The ovary matures into a fruit.
  • The zygote forms the embryo, and the endosperm provides nutrition.

Seed and Fruit Formation

Seeds contain an embryo, stored food (endosperm or cotyledons), and a protective seed coat. Fruits develop from the ovary wall and aid in seed dispersal.

Parthenocarpy: The development of fruit without fertilization, resulting in seedless fruits.

Apomixis and Polyembryony

Apomixis is a form of asexual reproduction that mimics sexual reproduction, producing seeds without fertilization. Polyembryony refers to the occurrence of more than one embryo in a seed.

All Question Types with Solutions – CBSE Exam Pattern

Explore a complete set of CBSE-style questions with detailed solutions, categorized by marks and question types. Ideal for exam preparation, revision and practice.

Very Short Answer (1 Mark) – with Solutions (CBSE Pattern)

These are 1-mark questions requiring direct, concise answers. Ideal for quick recall and concept clarity.

Question 1:
Define microsporogenesis.
Answer:
Definition: Formation of microspores from microspore mother cells via meiosis.
Question 2:
Name the female gametophyte in flowering plants.
Answer:
Embryo sac (7-celled, 8-nucleate structure).
Question 3:
What is the function of tapetum in anthers?
Answer:
Provides nutrition and sporopollenin to developing microspores.
Question 4:
Identify the pollinating agent for maize.
Answer:
Wind (anemophily).
Question 5:
Which chemical induces pollen tube growth?
Answer:
Boron and calcium.
Question 6:
Define double fertilization.
Answer:
Definition: Fusion of one sperm with egg (zygote) and second sperm with polar nuclei (endosperm).
Question 7:
What is apomixis?
Answer:
Definition: Asexual reproduction mimicking sexual reproduction (e.g., citrus).
Question 8:
Name the cells in a mature pollen grain.
Answer:
  • Vegetative cell
  • Generative cell
Question 9:
Which part of the pistil receives pollen?
Answer:
Stigma (sticky surface for pollen adhesion).
Question 10:
What is polyembryony? Give an example.
Answer:
Definition: Formation of multiple embryos (e.g., mango).
Question 11:
Why is bagging done in artificial hybridization?
Answer:
Prevents contamination from unwanted pollen.
Question 12:
Name the layer protecting pollen from UV radiation.
Answer:
Sporopollenin (most resistant organic material).
Question 13:
What triggers pollen grain germination?
Answer:
Sugary secretion (stigma exudates).
Question 14:
Define dioecy in plants.
Answer:
Definition: Male and female flowers on separate plants (e.g., papaya).
Question 15:
Name the part of the flower that develops into a fruit after fertilization.
Answer:

The ovary of the flower develops into the fruit after fertilization.

Question 16:
What is the function of the tapetum in the anther?
Answer:

The tapetum provides nourishment to the developing pollen grains and contributes to the formation of the pollen wall.

Question 17:
Define double fertilization in flowering plants.
Answer:

Double fertilization is a process where:
1. One sperm fuses with the egg to form the zygote.
2. The other sperm fuses with the two polar nuclei to form the triploid endosperm.

Question 18:
What is the significance of synergids in the embryo sac?
Answer:

Synergids guide the pollen tube toward the egg for fertilization and secrete chemicals to attract it.

Question 19:
Name the type of pollination that occurs in closed flowers.
Answer:

Cleistogamy is the type of pollination that occurs in closed flowers, ensuring self-pollination.

Question 20:
What is the role of endosperm in seed development?
Answer:

The endosperm provides nutrition to the developing embryo, supporting its growth and germination.

Question 21:
Differentiate between autogamy and geitonogamy.
Answer:

  • Autogamy: Self-pollination within the same flower.
  • Geitonogamy: Transfer of pollen from one flower to another on the same plant.

Question 22:
Why is the embryo sac called a monosporic structure?
Answer:

The embryo sac is called monosporic because it develops from a single functional megaspore out of the four produced by meiosis.

Question 23:
Name the chemical that prevents self-pollination in some flowers.
Answer:

Self-incompatibility proteins prevent self-pollination by inhibiting pollen germination or pollen tube growth.

Question 24:
What is the ploidy level of the primary endosperm nucleus?
Answer:

The primary endosperm nucleus is triploid (3n) as it forms by the fusion of one sperm with two polar nuclei.

Question 25:
List two agents of cross-pollination.
Answer:

  • Wind (Anemophily)
  • Insects (Entomophily)

Question 26:
What happens to the antipodal cells after fertilization?
Answer:

Antipodal cells degenerate after fertilization as they have no further role in seed development.

Very Short Answer (2 Marks) – with Solutions (CBSE Pattern)

These 2-mark questions test key concepts in a brief format. Answers are expected to be accurate and slightly descriptive.

Question 1:
What is the function of the tapetum in anther development?
Answer:

The tapetum is the innermost nutritive layer of the anther wall.
It provides nourishment to developing pollen grains.
It also secretes enzymes and sporopollenin for pollen wall formation.

Question 2:
Why is the embryo sac called monosporic in most flowering plants?
Answer:

The embryo sac is called monosporic because it develops from a single functional megaspore (out of four produced by meiosis).
The other three megaspores degenerate.

Question 3:
Name the chemical components of the pollen wall.
Answer:

The pollen wall consists of two layers:
Exine (outer layer): Made of sporopollenin, the most resistant organic material.
Intine (inner layer): Made of cellulose and pectin.

Question 4:
How does cleistogamy ensure self-pollination?
Answer:

In cleistogamy, flowers remain closed and never open.
Since the anthers and stigma lie close together, self-pollination occurs automatically.
Examples: Viola, Oxalis.

Question 5:
Explain the role of pollen kitt in pollination.
Answer:

Pollen kitt is a sticky, oily layer on pollen grains of entomophilous flowers.
It helps pollen grains:
1. Adhere to insect bodies.
2. Protect from UV radiation.
3. Attract pollinators with its scent.

Question 6:
What is polyembryony? Give one example.
Answer:

Polyembryony is the formation of multiple embryos from a single fertilized egg or other cells of the embryo sac.
Example: Citrus fruits where nucellar cells form additional embryos.

Question 7:
Why are watermelon seeds scattered in the field called apitomous seeds?
Answer:

Apitomous seeds (like watermelon) are non-albuminous seeds where the endosperm is completely consumed by the developing embryo.
Food is stored in cotyledons instead.

Question 8:
Describe the function of filiform apparatus in synergids.
Answer:

The filiform apparatus is a finger-like projection in synergids that:
1. Increases surface area for nutrient absorption.
2. Releases chemical signals to guide the pollen tube.
3. Helps in pollen tube entry into the embryo sac.

Short Answer (3 Marks) – with Solutions (CBSE Pattern)

These 3-mark questions require brief explanations and help assess understanding and application of concepts.

Question 1:
Explain the significance of double fertilization in flowering plants.
Answer:

Double fertilization is a unique process in flowering plants where two male gametes fuse with different female gametes.
One male gamete fuses with the egg cell to form a diploid zygote, which develops into the embryo.
The other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus, which develops into the endosperm, providing nutrition to the growing embryo.
This ensures efficient use of resources and enhances seed viability.

Question 2:
Describe the structure and function of the embryo sac in angiosperms.
Answer:

The embryo sac is the female gametophyte in angiosperms, typically 7-celled and 8-nucleate.
It consists of:

  • One egg cell (fuses with a male gamete to form the zygote)
  • Two synergids (guide the pollen tube)
  • Three antipodal cells (degenerate after fertilization)
  • One central cell with two polar nuclei (forms endosperm after fertilization)

Its primary function is to facilitate fertilization and support early embryo development.

Question 3:
Differentiate between autogamy and geitonogamy with examples.
Answer:

Autogamy is self-pollination within the same flower (e.g., pea plants), ensuring genetic uniformity but reducing variation.
Geitonogamy is transfer of pollen from one flower to another on the same plant (e.g., maize), genetically similar to autogamy but requires pollinating agents.
Both maintain parental traits but limit genetic diversity.

Question 4:
How does pollination differ from fertilization in flowering plants?
Answer:

Pollination is the transfer of pollen grains from the anther to the stigma, which can be abiotic (wind/water) or biotic (insects/birds).
Fertilization occurs after pollination, where the male gamete fuses with the female gamete inside the ovule.
Pollination is a physical process, while fertilization is a biochemical event leading to zygote formation.

Question 5:
What is apomixis? How is it beneficial in agriculture?
Answer:

Apomixis is asexual reproduction in plants where seeds form without fertilization.
It produces genetically identical offspring, preserving desirable traits (e.g., citrus, mango).
Benefits include:

  • Faster crop propagation
  • Reduced hybrid seed production cost
  • Maintenance of hybrid vigor
It is widely used in horticulture and seed industries.

Question 6:
Explain the role of pollen-pistil interaction in preventing incompatible pollination.
Answer:

The pollen-pistil interaction ensures only compatible pollen grains germinate on the stigma.
Steps:

  • Pollen adhesion and hydration on stigma
  • Chemical recognition by proteins/glycoproteins
  • Incompatible pollen is inhibited via self-incompatibility mechanisms
  • Compatible pollen forms a pollen tube for fertilization

This prevents inbreeding and promotes genetic diversity.

Question 7:
Describe the role of tapetum in pollen development.
Answer:

The tapetum is the innermost nutritive layer of the anther wall.

Its functions include:

  • Providing nutrients and enzymes for developing pollen grains.
  • Secreting sporopollenin, which forms the outer wall (exine) of pollen.
  • Producing pollenkitt for pollen adhesion and protection.
Without tapetum, pollen grains would lack structural integrity and viability.

Question 8:
How does embryo sac formation occur in angiosperms?
Answer:

The embryo sac develops from the megaspore mother cell (MMC) via megasporogenesis:

1. MMC undergoes meiosis to form 4 haploid megaspores.
2. Three degenerate; one functional megaspore develops.
3. The megaspore undergoes mitosis (3 times) to form 8-nucleate embryo sac.
4. Mature embryo sac has:

  • 1 egg cell + 2 synergids (at micropylar end).
  • 1 central cell with 2 polar nuclei.
  • 3 antipodal cells (at chalazal end).

Question 9:
Why is apomixis considered an advantage in agriculture?
Answer:

Apomixis is aseed production without fertilization, offering agricultural benefits:

  • Preserves hybrid vigour: Offspring are genetically identical to the parent.
  • Reduces cost: Eliminates need for repeated hybrid seed production.
  • Ensures crop uniformity: Maintains desirable traits like disease resistance.
  • Examples: Citrus, Mango.
However, it limits genetic diversity, making crops vulnerable to environmental changes.

Question 10:
Explain the outbreeding devices in flowering plants to prevent self-pollination.
Answer:

Plants use outbreeding devices to promote cross-pollination:

  • Dichogamy: Male and female reproductive parts mature at different times (e.g., Sunflower).
  • Herkogamy: Physical barriers prevent self-pollination (e.g., Hibiscus).
  • Self-incompatibility: Biochemical rejection of self-pollen (e.g., Brassica).
  • Unisexuality: Separate male and female flowers (e.g., Papaya).
These mechanisms enhance genetic diversity and adaptability.

Long Answer (5 Marks) – with Solutions (CBSE Pattern)

These 5-mark questions are descriptive and require detailed, structured answers with proper explanation and examples.

Question 1:
Explain the double fertilization process in flowering plants with reference to the synergids and central cell. How does this ensure genetic diversity?
Answer:
Theoretical Framework

Double fertilization is a unique process in angiosperms where one sperm cell fuses with the egg to form a zygote (2n), and the other fuses with the central cell (2n) to form triploid endosperm (3n). Synergids guide the pollen tube to the embryo sac.

Evidence Analysis
  • Our textbook shows that the central cell provides nutrients for embryo development.
  • The synergids degenerate after pollen tube entry, ensuring precise sperm delivery.
Critical Evaluation

This process increases genetic diversity as two male gametes contribute to offspring and nutritive tissue. Cross-pollination further enhances variability.

Future Implications

Understanding this mechanism aids in hybrid crop development, improving agricultural yields.

Question 2:
Compare autogamy and geitonogamy with examples. Why is xenogamy evolutionarily significant?
Answer:
Theoretical Framework

Autogamy is self-pollination within the same flower (e.g., peas), while geitonogamy occurs between different flowers of the same plant (e.g., maize). Both limit genetic variation.

Evidence Analysis
  • Our experiments show autogamy ensures reproductive assurance but reduces adaptability.
  • Geitonogamy still requires pollinator assistance despite being genetically similar to autogamy.
Critical Evaluation

Xenogamy (cross-pollination) introduces new alleles, enhancing species survival. Orchids evolved intricate mechanisms to promote xenogamy.

Future Implications

Conserving pollinator species becomes crucial for maintaining genetic diversity in crops.

Question 3:
Analyze the role of tapetum in pollen development. How does its malfunction affect microsporogenesis?
Answer:
Theoretical Framework

The tapetum is the innermost nutritive layer of the anther that secretes enzymes and sporopollenin for pollen wall formation during microsporogenesis.

Evidence Analysis
  • Microscopic studies show tapetum provides lipids for pollen coat development.
  • Mutation studies reveal tapetal dysfunction causes pollen abortion due to improper exine formation.
Critical Evaluation

Without functional tapetum, pollen grains become sterile, directly impacting plant fertility. This is exploited in hybrid seed production.

Future Implications

Research on tapetal genes could lead to male-sterile lines for efficient hybrid breeding programs.

Question 4:
Describe the apomixis phenomenon with two examples. How does it challenge Mendelian inheritance patterns?
Answer:
Theoretical Framework

Apomixis is asexual seed formation without fertilization, producing clones of the mother plant (e.g., citrus, dandelions).

Evidence Analysis
  • Citrus varieties show identical offspring through nucellar embryony.
  • Dandelions use diplospory where megaspore mother cells bypass meiosis.
Critical Evaluation

This bypasses genetic recombination, creating populations with reduced adaptability but preserving desirable traits.

Future Implications

Engineering apomixis in crops could revolutionize agriculture by fixing hybrid vigor, though ecological risks exist.

Question 5:
Analyze the role of tapetum in pollen development and its significance in sporopollenin formation. Provide two examples of pollen-related adaptations.
Answer:
Theoretical Framework

The tapetum is the innermost nutritive layer of the anther that supports microspore development. It secretes sporopollenin, the most resistant organic material known.

Evidence Analysis
  • We studied how defective tapetum leads to male sterility, as observed in transgenic cotton.
  • Pollen adaptations: Orchids produce pollinia for efficient transfer, while hydrophilous plants like Vallisneria release buoyant pollen.
Critical Evaluation

Sporopollenin's durability allows pollen fossils to persist for millions of years, aiding paleobotanical studies.

Future Implications

Understanding tapetal function can help develop male-sterile lines for hybrid seed production in wheat.

Question 6:
Compare autogamy and geitonogamy with respect to genetic consequences. How do plants like Viola and Oxalis avoid these?
Answer:
Theoretical Framework

Autogamy (self-pollination on same flower) and geitonogamy (between flowers of same plant) both reduce genetic variability but ensure reproductive assurance.

Evidence Analysis
  • Viola produces cleistogamous flowers (obligate autogamy) and chasmogamous flowers (potential cross-pollination).
  • Oxalis exhibits heterostyly with pin-thrum flowers to prevent selfing.
Critical Evaluation

While these mechanisms maintain population stability, prolonged selfing increases recessive allele expression, as seen in Mendel's pea experiments.

Future Implications

Studying these systems helps design crop varieties with balanced outcrossing rates, crucial for climate resilience.

Question 7:
Evaluate the outbreeding devices in flowering plants with reference to dichogamy and herkogamy. How do these compare with artificial hybridization techniques?
Answer:
Theoretical Framework

Outbreeding devices prevent self-pollination through temporal (dichogamy) or spatial (herkogamy) separation of reproductive structures.

Evidence Analysis
  • In sunflower, protandry (male-first dichogamy) ensures cross-pollination.
  • Herkogamy examples: Primula's pin-thrum flowers and Hibiscus's exerted stigmas.
Critical Evaluation

Artificial hybridization emulates these mechanisms through emasculation and bagging, as practiced in maize breeding programs since 1920s.

Future Implications

Modern CRISPR technology could enhance natural outbreeding systems for sustainable agriculture, reducing manual hybridization labor.

Question 8:
Compare autogamy and geitonogamy in flowering plants, highlighting their evolutionary trade-offs. Cite one example each.
Answer:
Theoretical Framework

Autogamy (self-pollination within the same flower) ensures seed set without pollinators, while geitonogamy (between flowers of the same plant) reduces genetic variability.

Evidence Analysis
  • Pea plants (Pisum sativum) exhibit autogamy due to closed flowers, ensuring reproductive assurance.
  • Corn (Zea mays) shows geitonogamy, increasing pollen waste but maintaining some diversity.
Critical Evaluation

Autogamy limits adaptation to changing environments, whereas geitonogamy balances resource allocation and genetic health.

Future Implications

Studying these systems helps design crops with optimal yield and resilience, like self-pollinating rice varieties.

Question 9:
Describe the outbreeding devices in flowering plants with emphasis on dichogamy and heterostyly. How do they prevent inbreeding depression?
Answer:
Theoretical Framework

Outbreeding devices promote cross-pollination. Dichogamy (temporal separation of male/female phases) and heterostyly (differing style lengths) ensure pollen transfer between plants.

Evidence Analysis
  • In sunflower (Helianthus), protandry (male-first dichogamy) prevents selfing.
  • Primula vulgaris exhibits heterostyly, where pin/thrum flowers enforce cross-pollination.
Critical Evaluation

These mechanisms reduce homozygosity, minimizing harmful recessive traits. Current data shows heterostylous species have 30% higher fitness.

Future Implications

Applying these principles can restore genetic diversity in endangered species, like reintroducing heterostylous orchids.

Question 10:
Compare autogamy and geitonogamy with examples. Why is xenogamy evolutionarily advantageous?
Answer:
Theoretical Framework

Autogamy is self-pollination within the same flower (e.g., wheat), while geitonogamy occurs between flowers of the same plant (e.g., maize). Xenogamy involves cross-pollination between genetically different plants.

Evidence Analysis
  • Our textbook highlights that xenogamy introduces new alleles, as observed in sunflower and papaya.
  • It reduces inbreeding depression, enhancing adaptability.
Critical Evaluation

Though autogamy ensures reproductive assurance, xenogamy fosters genetic variability, crucial for survival in changing environments.

Future Implications

Breeders use this knowledge to develop disease-resistant crops.

Question 11:
Describe the role of tapetum in pollen development. How does its malfunction affect microsporogenesis?
Answer:
Theoretical Framework

The tapetum is the innermost nutritive layer of the anther, providing enzymes and sporopollenin for pollen wall formation during microsporogenesis.

Evidence Analysis
  • Our textbook cites its role in supplying callase to release microspores from tetrads, as in rice and tomato.
  • If dysfunctional, pollen grains become sterile due to incomplete exine formation.
Critical Evaluation

This underscores the tapetum's role in male fertility. Sterility can be exploited for hybrid seed production.

Future Implications

Research on tapetal genes may help engineer apomixis in crops.

Question 12:
Analyze the significance of embryo sac formation in megasporogenesis. How does its structure support fertilization?
Answer:
Theoretical Framework

The embryo sac (female gametophyte) forms via meiosis of the megaspore mother cell, typically developing into a 7-celled, 8-nucleate structure.

Evidence Analysis
  • As per NCERT, the egg apparatus (1 egg + 2 synergids), central cell (2 polar nuclei), and antipodals facilitate double fertilization, e.g., in lily and mustard.
  • The synergids secrete chemotropins to guide pollen tubes.
Critical Evaluation

This precise organization ensures efficient fertilization and endosperm formation, critical for seed development.

Future Implications

Manipulating embryo sac cells could enable synthetic seed technology.

Question 13:
Explain the process of double fertilization in flowering plants with a well-labeled diagram. Highlight the significance of this process.
Answer:

In flowering plants, double fertilization is a unique process where two male gametes fuse with different female gametophyte cells. Here's a step-by-step explanation:


Process:
1. Pollination: Pollen grains land on the stigma and form a pollen tube.
2. Pollen tube growth: The tube grows through the style, carrying two male gametes.
3. Entry into ovule: The pollen tube enters the ovule through the micropyle and releases the two male gametes.
4. First fertilization: One male gamete fuses with the egg cell to form a diploid zygote (2n).
5. Second fertilization: The other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (3n).

Diagram: (Draw a labeled diagram showing pollen tube entry, male gametes, egg cell, polar nuclei, and resulting structures.)

Significance:
  • Ensures seed formation by creating a zygote (future embryo).
  • Provides nutritive tissue (endosperm) for embryo development.
  • Enhances genetic diversity by combining male and female traits.

This process is crucial for sexual reproduction in angiosperms, ensuring successful seed and fruit development.

Question 14:
Describe the structure of a mature embryo sac (female gametophyte) in angiosperms. How does it prepare for fertilization?
Answer:

The embryo sac is the female gametophyte in angiosperms, typically 7-celled and 8-nucleate at maturity. Its structure includes:


Components:
  • Egg apparatus: 1 egg cell (female gamete) + 2 synergids (guide pollen tube).
  • Central cell: Contains 2 polar nuclei (fuse to form endosperm).
  • Antipodal cells: 3 cells (degenerate after fertilization).

Preparation for Fertilization:
1. The synergids secrete chemotropins to attract the pollen tube.
2. The egg cell remains haploid (n) for fusion with a male gamete.
3. Polar nuclei align centrally for triple fusion.

Note: The embryo sac's organization ensures efficient fertilization and nutrient allocation post-fertilization, crucial for seed development.

Question 15:
Compare autogamy, geitonogamy, and xenogamy in flowering plants. Discuss their advantages and disadvantages.
Answer:

These are three types of pollination strategies in flowering plants:


1. Autogamy:
  • Definition: Self-pollination within the same flower.
  • Advantages: Ensures seed set even in isolation; preserves parental traits.
  • Disadvantages: Reduces genetic diversity; may lead to inbreeding depression.

2. Geitonogamy:
  • Definition: Pollination between different flowers of the same plant.
  • Advantages: Guarantees pollination if cross-pollination fails.
  • Disadvantages: Genetically similar to autogamy; no new gene combinations.

3. Xenogamy:
  • Definition: Cross-pollination between flowers of different plants.
  • Advantages: Enhances genetic diversity; healthier offspring.
  • Disadvantages: Depends on pollinators; may fail in their absence.

Plants often evolve mechanisms (e.g., dichogamy, self-incompatibility) to promote xenogamy.

Question 16:
Describe the structure of a mature embryo sac in angiosperms. How does it facilitate fertilization?
Answer:

A mature embryo sac (female gametophyte) in angiosperms is 7-celled and 8-nucleated. Its structure and role in fertilization are as follows:


Structure:
  • Egg apparatus: 1 egg cell + 2 synergids (at the micropylar end).
  • Central cell: Contains 2 polar nuclei (later fuse to form secondary nucleus).
  • Antipodal cells: 3 cells at the chalazal end (degenerate after fertilization).

Role in Fertilization:
  • The synergids guide the pollen tube via chemical signals.
  • The egg cell fuses with a male gamete to form the zygote.
  • The central cell undergoes triple fusion to form nutritive endosperm.

This precise arrangement ensures successful double fertilization, crucial for seed formation.

Question 17:
Describe the structure of a mature embryo sac (female gametophyte) in flowering plants. Explain the role of each component in reproduction.
Answer:

A mature embryo sac is a 7-celled, 8-nucleate structure formed after megagametogenesis. Its components and roles are:


Structure and Functions:
1. Egg Apparatus (3 cells):
- Egg cell: Fuses with a male gamete to form the zygote.
- Synergids (2 cells): Guide the pollen tube via chemotropism and secrete attractants.

2. Central Cell (1 cell, 2 polar nuclei):
- Fuses with the second male gamete to form triploid endosperm (nutritional tissue).

3. Antipodal Cells (3 cells):
- Degenerate after fertilization; their exact role is unclear but may assist in nutrient transport.

Key Notes:
  • The embryo sac is haploid (n) except for the central cell (2n before fertilization).
  • It is enclosed within the ovule and connected to the plant via the funicle.
Question 18:
Explain the process of double fertilization in flowering plants with a well-labelled diagram. Highlight the significance of this process.
Answer:

In flowering plants, double fertilization is a unique process where two male gametes fuse with different female gametophyte cells. Here's a step-by-step explanation:


Process:
1. Pollination occurs when pollen grains land on the stigma.
2. The pollen tube grows through the style, carrying two male gametes.
3. Upon reaching the ovule, one male gamete (n) fuses with the egg cell (n) to form a diploid zygote (2n).
4. The second male gamete (n) fuses with the two polar nuclei (n+n) to form a triploid primary endosperm nucleus (3n).

Significance:
  • The zygote develops into an embryo, ensuring genetic continuity.
  • The endosperm provides nutrition to the developing embryo.
  • It promotes genetic diversity due to the fusion of male and female gametes.

Diagram Requirement: A well-labelled diagram showing pollen tube entry, syngamy, and triple fusion must be drawn for full marks.
Question 19:
Describe the structure of a mature embryo sac in angiosperms. How does it prepare for fertilization?
Answer:

The embryo sac (female gametophyte) is a 7-celled, 8-nucleate structure formed after megasporogenesis. Its components are:


  • Egg apparatus: 1 egg cell + 2 synergids (at micropylar end).
  • Central cell: Contains 2 polar nuclei (later fuse to form secondary nucleus).
  • Antipodal cells: 3 cells at the chalazal end (degenerate after fertilization).

Preparation for fertilization:
1. Synergids secrete chemicals to guide the pollen tube.
2. The egg cell becomes receptive for fusion with a male gamete.
3. Polar nuclei align to form a diploid secondary nucleus for triple fusion.
4. The micropyle opens to allow pollen tube entry.


Note: The embryo sac ensures successful double fertilization by organizing its nuclei strategically.

Question 20:
Describe the process of double fertilization in flowering plants with a labeled diagram. Explain the significance of this process.
Answer:

In flowering plants, double fertilization is a unique process where two male gametes fuse with different female gametophyte cells. Here's a step-by-step explanation:


Process:
1. Pollination: Pollen grains land on the stigma and form a pollen tube.
2. Pollen tube growth: The tube grows through the style, carrying two male gametes.
3. Entry into ovule: The pollen tube enters the ovule through the micropyle and releases the two gametes.
4. First fertilization: One male gamete fuses with the egg cell to form a diploid zygote (2n).
5. Second fertilization: The other male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (3n).

Significance:
  • Ensures seed formation (zygote develops into embryo).
  • Provides nutrition to the developing embryo (endosperm acts as a food reserve).
  • Maintains genetic diversity through sexual reproduction.

Diagram: (Draw and label: pollen tube, male gametes, egg cell, polar nuclei, zygote, and endosperm nucleus).
Question 21:
Explain the process of double fertilization in flowering plants with a well-labeled diagram. Highlight the significance of this process in the life cycle of angiosperms.
Answer:

Double fertilization is a unique process in flowering plants (angiosperms) involving the fusion of two male gametes with two different female gametes. Here's a step-by-step explanation:


1. Pollination: Pollen grains land on the stigma and form a pollen tube that grows towards the ovary.
2. Entry into Ovule: The pollen tube enters the ovule through the micropyle and releases two male gametes.
3. First Fertilization: One male gamete fuses with the egg cell to form a diploid zygote (2n), which develops into the embryo.
4. Second Fertilization: The second male gamete fuses with the two polar nuclei to form a triploid primary endosperm nucleus (3n), which develops into the endosperm, providing nutrition to the embryo.
5. Diagram: (Draw a labeled diagram showing pollen tube entry, syngamy, and triple fusion).


Significance:

  • Ensures genetic variation due to fusion of gametes.
  • Endosperm formation supports embryo development.
  • Unique to angiosperms, enhancing their evolutionary success.

Question 22:
Describe the structure and function of the embryo sac in a mature angiosperm ovule. How does it prepare for fertilization?
Answer:

The embryo sac (female gametophyte) is a critical structure within the ovule of flowering plants. Its structure and functions are as follows:


Structure:

  • Typically 7-celled and 8-nucleated (monosporic development).
  • Contains one egg cell (female gamete), two synergids (guide pollen tube), three antipodal cells (degenerate after fertilization), and two polar nuclei (fuse to form endosperm).


Function:

  • The egg cell fuses with a male gamete to form the zygote.
  • The polar nuclei participate in triple fusion to form endosperm.
  • Synergids secrete chemicals to attract the pollen tube.


Preparation for Fertilization:
1. The embryo sac matures after megasporogenesis and megagametogenesis.
2. It positions the egg apparatus near the micropyle for easy pollen tube entry.
3. Synergids release chemoattractants to guide the pollen tube.

Case-based Questions (4 Marks) – with Solutions (CBSE Pattern)

These 4-mark case-based questions assess analytical skills through real-life scenarios. Answers must be based on the case study provided.

Question 1:
In a study of double fertilization, researchers observed that one sperm fuses with the egg and the other with the polar nuclei. Explain the significance of this process and its outcome in flowering plants.
Answer:
Case Deconstruction

Double fertilization involves two sperm cells: one fertilizes the egg to form a diploid zygote, and the other fuses with polar nuclei to form triploid endosperm.

Theoretical Application
  • The zygote develops into an embryo, ensuring genetic continuity.
  • The endosperm provides nutrition for the developing embryo, critical for seed viability.
Critical Evaluation

This process is unique to angiosperms, enhancing reproductive efficiency. For example, in maize, endosperm stores starch, while in coconut, it forms liquid endosperm.

Question 2:
A farmer observed apomixis in certain citrus varieties where seeds developed without fertilization. Analyze how this phenomenon differs from sexual reproduction and its agricultural benefits.
Answer:
Case Deconstruction

Apomixis is asexual seed formation where embryos develop from diploid cells without meiosis or fertilization.

Theoretical Application
  • It preserves parental traits, unlike sexual reproduction which introduces variation.
  • Farmers can propagate hybrid varieties like mango or citrus without losing desirable traits.
Critical Evaluation

This reduces breeding time and cost, but limits genetic diversity, making crops vulnerable to diseases. For example, apomictic guava varieties maintain consistent fruit quality.

Question 3:
In an experiment, self-incompatibility was observed in Brassica plants. Discuss its mechanism and how it promotes outcrossing in flowering plants.
Answer:
Case Deconstruction

Self-incompatibility prevents pollen from the same plant from fertilizing ovules, ensuring cross-pollination.

Theoretical Application
  • It involves S-allele recognition, where pollen tubes are inhibited if they share alleles with the pistil.
  • Examples include mustard and petunia, which rely on pollinators for genetic diversity.
Critical Evaluation

This mechanism reduces inbreeding depression but may limit reproduction in isolated populations. Our textbook shows how this system maintains heterozygosity.

Question 4:
Scientists engineered a male sterile rice line to simplify hybrid seed production. Explain the biological basis of male sterility and its utility in agriculture.
Answer:
Case Deconstruction

Male sterility results from mitochondrial mutations disrupting pollen development, as seen in cytoplasmic male sterility (CMS).

Theoretical Application
  • It forces cross-pollination, enabling large-scale hybrid seed production.
  • For example, CMS lines in rice and sunflower reduce manual emasculation costs.
Critical Evaluation

While CMS boosts yield, reliance on maintainer lines complicates breeding. Our studies show how restorer genes are essential for fertility restoration in hybrids.

Question 5:
In a study of double fertilization, researchers observed that the synergids degenerate after pollen tube entry. Analyze the role of synergids and the significance of their degeneration in ensuring successful fertilization.
Answer:
Case Deconstruction

Synergids are specialized cells in the embryo sac that guide the pollen tube toward the egg. Their degeneration releases chemotropic signals, ensuring precise pollen tube discharge.

Theoretical Application
  • Degeneration prevents polyspermy by allowing only one sperm to fuse with the egg.
  • It triggers the release of calcium ions, activating the zygote.
Critical Evaluation

Our textbook shows synergid degeneration as a checkpoint for fertilization. For example, in Arabidopsis, mutants with persistent synergids show failed fertilization, proving their necessity.

Question 6:
A farmer observed parthenocarpy in his banana orchard but not in mango trees. Compare the biological basis of parthenocarpy and its agricultural implications for these two species.
Answer:
Case Deconstruction

Parthenocarpy is fruit development without fertilization, often induced by hormones like auxins or gibberellins.

Theoretical Application
  • Bananas are naturally parthenocarpic, ensuring seedless fruits, while mangoes require pollination.
  • Farmers use synthetic auxins to induce parthenocarpy in crops like cucumbers.
Critical Evaluation

We studied that parthenocarpy boosts yield but may reduce genetic diversity. For example, seedless watermelons are commercially valuable but depend entirely on human intervention.

Question 7:
Scientists engineered a self-incompatible tobacco plant to express a self-compatible gene from tomato. Predict the ecological and evolutionary consequences of this genetic modification.
Answer:
Case Deconstruction

Self-incompatibility prevents inbreeding, while self-compatibility allows single plants to reproduce independently.

Theoretical Application
  • Ecologically, it may reduce pollinator dependence but increase genetic uniformity.
  • Evolutionarily, it could lead to loss of adaptive diversity over generations.
Critical Evaluation

Our textbook shows that self-compatibility can aid crop breeding, as seen in Brassica. However, it risks inbreeding depression, as observed in some wheat varieties.

Question 8:
In an experiment, apomixis was induced in maize by silencing the meiotic genes
Answer:
Case Deconstruction

Apomixis is asexual seed formation, bypassing meiosis and fertilization, common in grasses like Poa.

Theoretical Application
  • Silencing meiotic genes clones maternal traits, preserving hybrid vigor.
  • Farmers could replant seeds without losing desirable traits, reducing costs.
Critical Evaluation

We studied that apomixis could democratize hybrid seeds, as seen in Taraxacum. However, ecological risks include reduced adaptability to environmental changes.

Question 9:
In a study of self-incompatibility in flowering plants, researchers observed that pollen from the same plant failed to fertilize ovules. Analyze the genetic and biochemical mechanisms behind this phenomenon.
Answer:
Case Deconstruction

Self-incompatibility prevents inbreeding by rejecting pollen from the same plant. Our textbook shows it involves S-alleles encoding specific proteins in pollen and pistil.

Theoretical Application
  • Genetic control: S-alleles determine compatibility; identical alleles block pollen tube growth.
  • Biochemical rejection: RNases or glycoproteins degrade incompatible pollen RNA.
Critical Evaluation

Examples: Brassica uses sporophytic rejection, while Nicotiana employs gametophytic systems. This ensures genetic diversity.

Question 10:
A farmer observed apomixis in citrus crops, where seeds developed without fertilization. Explain how this impacts agricultural productivity and genetic diversity.
Answer:
Case Deconstruction

Apomixis produces clones of the parent plant. We studied it in Citrus and Mangifera, where embryos form from diploid egg cells.

Theoretical Application
  • Productivity: Guarantees uniform high-yield crops.
  • Diversity loss: Limits adaptation to pests/climate change.
Critical Evaluation

While beneficial for hybrid preservation, long-term monoculture risks are evident in wheat apomixis experiments.

Question 11:
Compare porogamy, mesogamy, and chalazogamy based on pollen tube entry paths in ovules. Support with examples from NCERT.
Answer:
Case Deconstruction

Pollen tube entry varies across species. Our textbook highlights porogamy (via micropyle) in Lilium.

Theoretical Application
TypePathExample
PorogamyMicropyleLilium
MesogamyIntegumentsCucurbita
ChalazogamyChalazaCasuarina
Critical Evaluation

These adaptations optimize fertilization efficiency in different floral structures.

Question 12:
Critically evaluate how polyembryony in Citrus and mango benefits horticulturists but challenges plant breeders.
Answer:
Case Deconstruction

Polyembryony produces multiple embryos per seed. We studied it in Citrus (nucellar embryos) and mango (adventive embryos).

Theoretical Application
  • Benefits: Clonal propagation maintains hybrid vigor.
  • Challenges: Obstructs cross-breeding efforts.
Critical Evaluation

While ensuring crop consistency, it reduces genetic recombination opportunities, as seen in Kinnow orchards.

Question 13:

A farmer observed that his Brassica campestris (mustard) crop showed poor fruit and seed formation despite healthy flowering. Upon examination, he found that the pollen grains were not reaching the stigma effectively. Suggest two possible reasons for this issue and explain how it can be resolved using artificial methods.

Answer:

The poor fruit and seed formation in Brassica campestris could be due to:

  • Lack of pollinators: Insects like bees may be absent, hindering pollination.
  • Unfavorable environmental conditions: High humidity or rain may wash away pollen grains.

To resolve this, the farmer can use artificial pollination techniques:
1. Hand pollination: Collect pollen from anthers using a brush and transfer it to the stigma.
2. Emasculation and bagging: Remove anthers from flowers before they dehisce and cover them with bags to prevent contamination, later dusting with desired pollen.

Question 14:

In an experiment, a student observed that the embryo sac of a flowering plant contained eight nuclei but only seven cells. Explain the structure of the mature embryo sac (female gametophyte) and the significance of this arrangement in double fertilization.

Answer:

The mature embryo sac (monosporic, Polygonum type) consists of seven cells and eight nuclei arranged as:
1. Egg apparatus: One egg cell and two synergids at the micropylar end.
2. Central cell: Contains two polar nuclei (may fuse to form a diploid secondary nucleus).
3. Antipodal cells: Three cells at the chalazal end (degenerate after fertilization).

This arrangement is crucial for double fertilization:
- The egg cell fuses with one male gamete to form the zygote (2n).
- The polar nuclei fuse with the second male gamete to form the triploid endosperm (3n), which nourishes the embryo.

Question 15:

In a botanical study, a student observed that some flowers of a plant species had long styles and short stamens, while others had short styles and long stamens. The student hypothesized that this was an adaptation to promote cross-pollination.

a) Identify the type of floral adaptation described here.
b) Explain how this adaptation ensures cross-pollination.
c) Name one plant that exhibits this adaptation.

Answer:

a) The described floral adaptation is called heterostyly, specifically distyly (a form of heterostyly where flowers have two distinct style and stamen lengths).

b) This adaptation ensures cross-pollination because:

  • Insects visiting a flower with long styles and short stamens will pick up pollen on their bodies at a specific height.
  • When they visit a flower with short styles and long stamens, the pollen is deposited precisely on the stigma, promoting cross-pollination between different floral morphs.

c) An example of a plant exhibiting this adaptation is Primula vulgaris (Primrose).

Question 16:

A farmer noticed that his papaya orchard had poor fruit set despite healthy flowering. Upon consulting an agricultural expert, he learned that papaya plants are dioecious.

a) What does the term dioecious mean in flowering plants?
b) Why did the farmer's orchard show poor fruit set?
b) Suggest a practical solution to improve fruit production in his orchard.

Answer:

a) Dioecious refers to plant species where male and female reproductive structures are present on separate individual plants (e.g., male and female papaya plants).

b) The poor fruit set likely occurred because:

  • The orchard may have had mostly male plants, which produce pollen but no fruits.
  • Insufficient female plants or lack of pollination agents (like wind or insects) to transfer pollen from male to female flowers.

c) The farmer can:

  • Introduce more female papaya plants to the orchard.
  • Ensure proper spacing between male and female plants for effective pollination.
  • Use manual pollination techniques if natural pollinators are scarce.

Question 17:

In a botanical study, a student observed that some flowers of a plant species had long styles and short stamens, while others had short styles and long stamens. The student hypothesized that this was an adaptation to promote cross-pollination.

a) Identify the type of floral adaptation described here.
b) Explain how this adaptation prevents self-pollination.
c) Name one other mechanism in flowers that prevents self-pollination.

Answer:

a) The described adaptation is called heterostyly, specifically distyly, where flowers have either long styles and short stamens (pin flowers) or short styles and long stamens (thrum flowers).

b) This prevents self-pollination because:

  • In pin flowers, pollen is deposited on the body of pollinators at a lower position, while the stigma is positioned higher.
  • In thrum flowers, pollen is deposited higher on the pollinator, while the stigma is positioned lower.
  • Thus, pollen from one morph cannot reach the stigma of the same morph, ensuring cross-pollination.

c) Another mechanism is dichogamy, where anthers and stigmas mature at different times (e.g., protandry or protogyny).

Question 18:

A farmer noticed that his maize crops produced more kernels when he planted them in alternating rows with another crop, compared to planting maize alone. He learned that maize is wind-pollinated.

a) Why does alternating rows improve pollination in maize?
b) Describe two floral adaptations in maize that aid wind pollination.
c) How does this observation highlight the ecological importance of cross-pollination?

Answer:

a) Alternating rows reduces competition for wind-borne pollen and increases airflow, ensuring better pollen dispersal and higher chances of cross-pollination.

b) Adaptations in maize:

  • Exserted stamens: Anthers hang outside the flowers to release pollen easily into the wind.
  • Feathery stigmas: Large, sticky stigmas protrude to trap airborne pollen efficiently.

c) Cross-pollination ensures:

  • Genetic diversity, leading to healthier offspring with better adaptability.
  • Higher yield due to reduced inbreeding depression, as seen in the farmer's observation.

Question 19:
A farmer observed that his paddy crop showed poor seed setting despite healthy flowering. Upon examination, it was found that pollen grains failed to germinate on the stigma. Explain the possible reasons and suggest two measures to improve seed setting.
Answer:

The poor seed setting in paddy could be due to incompatibility between pollen and stigma or environmental factors affecting pollen viability. Possible reasons include:

  • Self-incompatibility: The pollen and stigma may be genetically incompatible, preventing pollen tube formation.
  • High temperature or humidity: Extreme conditions can desiccate pollen or inhibit germination.

Measures to improve seed setting:

  • Cross-pollination: Introduce compatible pollen from another plant to ensure fertilization.
  • Optimal irrigation: Maintain proper moisture levels to support pollen germination and tube growth.
Question 20:
In an experiment, a student removed the anthers from a flower before they dehisced and covered the flower with a bag. Later, no fruit formation was observed. Analyze the steps and explain why this happened, along with the significance of the student's actions.
Answer:

The absence of fruit formation occurred because:

  • Emasculation: Removing anthers prevented pollen release, eliminating chances of self-pollination.
  • Bagging: The cover blocked external pollen, ensuring no cross-pollination occurred.

Significance:

  • The experiment demonstrates controlled pollination techniques used in hybridization.
  • It highlights the role of pollination in fruit development, as no pollen means no fertilization or fruit set.

For fruit formation, the student should have manually transferred compatible pollen to the stigma after emasculation.

Question 21:
A farmer observed that his maize crops were not producing seeds despite healthy flowering. Upon examination, he found that the pollen grains were non-functional. Explain the possible reasons for this observation and suggest a solution to ensure seed production.
Answer:

The non-functional pollen grains in maize could be due to several reasons:

  • Environmental stress: High temperatures or drought during pollen development can lead to sterility.
  • Genetic factors: Mutations or chromosomal abnormalities may disrupt pollen formation.
  • Nutritional deficiencies: Lack of essential nutrients like boron can impair pollen viability.

To ensure seed production, the farmer can:

  • Use cross-pollination by introducing healthy pollen from another plant.
  • Maintain optimal growing conditions, including proper irrigation and balanced fertilization.
  • Apply boron supplements if deficiency is suspected, as it plays a key role in pollen tube growth.

Additionally, selecting disease-resistant and climate-resilient maize varieties can prevent such issues in the future.

Question 22:
In a botanical study, students observed that some flowers of a species had long styles, while others had short styles. Explain the significance of this variation in the context of sexual reproduction and how it promotes outbreeding.
Answer:

The variation in style length within a species is an example of heterostyly, a mechanism that promotes outbreeding and genetic diversity. Here's how it works:

  • Flowers with long styles (pin flowers) have stamens positioned lower, while those with short styles (thrum flowers) have stamens positioned higher.
  • This spatial separation ensures that pollen from a pin flower is more likely to land on the stigma of a thrum flower, and vice versa.
  • Such cross-pollination reduces self-fertilization, preventing inbreeding depression and enhancing genetic variability.

This adaptation is particularly beneficial for plant populations as it increases their resilience to environmental changes and diseases. Examples of plants exhibiting heterostyly include Primula (primrose) and Linum (flax).

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