Sexual Reproduction In Flowering Plants

Sexual reproduction in flowering plants, also known as angiosperms, involves the fusion of male and female gametes to produce a zygote, which develops into a seed. 

Key Stages of Sexual Reproduction

1. Pre-Fertilization:
   • Structure of Flower: The flower consists of four whorls: calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (pistil).
   • Gamete Formation:
     • Pollen Grains: Male gametes are produced in the anther of the stamen.
     • Ovules: Female gametes (egg cells) are produced in the ovules within the ovary of the pistil.
2. Pollination: The transfer of pollen grains from the anther of one flower to the stigma of another or the same flower.
   • Types: Self-pollination and cross-pollination.
3. Fertilization
   • Double Fertilization: A unique process where one pollen grain produces two male gametes. One fuses with the egg cell to form the zygote, and the other fuses with the secondary nucleus to form the endosperm (a nutritive tissue).
4. Post-Fertilization:
   • Seed Formation
   • Fruit Formation: The ovary of the flower develops into a fruit, which protects the seeds.

Significance of Sexual Reproduction

• Genetic Variation: Sexual reproduction leads to genetic variation in offspring, which increases their chances of survival in changing environments.
• Adaptation: Genetic variation allows plants to adapt to new conditions and evolve over time.
• New Varieties: Breeding programs utilize sexual reproduction to develop new and improved plant varieties with desirable traits.

Exercise

1. Name the parts of an angiosperm flower in which development of male and female gametophyte take place

Ans : 

Male Gametophyte (Pollen Grain):

• Anther: The pollen grains are produced within the anther, which is part of the stamen.

Female Gametophyte (Embryo Sac):

• Ovule: The embryo sac develops within the ovule, which is contained within the ovary of the pistil.

2. Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events.

Ans : 

Feature	Microsporogenesis	Megasporogenesis
Location	Anther of the stamen	Ovule within the ovary
Starting Cell	Pollen mother cell	Megaspore mother cell
Cell Division	Meiosis (two divisions)	Meiosis (two divisions)
Number of Functional Products	All four	One
Structure Formed	Pollen grains	Embryo sac
Function	Involved in pollination and fertilization	Contains the egg cell for fertilization

3. Arrange the following terms in the correct developmental sequence: Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes

Ans : 

Sporogenous tissue

Pollen mother cell

Microspore tetrad

Pollen grain

Male gametes

4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule

Ans : 

5. What is meant by monosporic development of female gametophyte?

Ans : 

Monosporic development refers to the development of the female gametophyte (embryo sac) from a single functional megaspore.

In angiosperms, meiosis of the megaspore mother cell produces four megaspores. Usually, only one of these megaspores remains functional, while the others degenerate. This functional megaspore undergoes mitotic divisions to form the eight-celled embryo sac.

6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte

Ans : 

Explanation

The embryo sac consists of:

1. 3 cells at the micropylar end:
   • Egg cell: A large cell located centrally, surrounded by synergids.
   • Synergids: Two small cells flanking the egg cell. They play a role in pollen tube guidance and fertilization.
2. 2 polar nuclei: Located in the central cell, these nuclei fuse to form the endosperm nucleus after fertilization.
3. 3 cells at the chalazal end:
   • Antipodals: Three cells located at the opposite end of the embryo sac from the micropyle. Their function is not fully understood, but they may play a role in nutrient transport or embryo development.

7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer.

Ans : 

Chasmogamous Flowers

• Definition: Flowers that open fully, exposing their reproductive organs to external agents.
• Characteristics:
  • Large, showy petals to attract pollinators.
  • Abundant pollen production.
  • Well-developed stigma for pollen reception.
• Examples: Roses, lilies, sunflowers

Cleistogamous Flowers

• Definition: Flowers that never open and self-pollinate within the flower bud.
• Characteristics:
  • Small, inconspicuous, and often hidden within the leaves.
  • Reduced or absent petals.
  • Stamens and pistils enclosed within the bud.
• Examples: Some species of violets, grasses, and commelina

Cross-Pollination in Cleistogamous Flowers

Cross-pollination is not possible in cleistogamous flowers.

Reasons:

1. Closed Structure: Since cleistogamous flowers never open, there is no exposure to external agents that could transfer pollen from another flower.
2. Self-Pollination: The enclosed nature of cleistogamous flowers ensures that pollen from the same flower reaches the stigma, leading to self-pollination.
3. Reduced Reproductive Organs: Cleistogamous flowers often have reduced or modified reproductive organs, further limiting the possibility of cross-pollination.

8. Mention two strategies evolved to prevent self-pollination in flowers

Ans : 

Two Strategies to Prevent Self-Pollination in Flowers

1. Spatial Separation of Male and Female Reproductive Organs:
   • Dichogamy: This refers to the maturation of male and female reproductive organs at different times within the same flower.
     • Protandry: Stamens mature earlier than pistils.
     • Protogyny: Pistils mature earlier than stamens.
   • Heterostyly: The stamens and pistils are at different heights within the flower, making it difficult for pollen from the same flower to reach the stigma.
2. Self-Incompatibility:
   • Genetic Mechanism: Plants have evolved mechanisms that prevent pollen from the same plant or a closely related plant from fertilizing its ovules.
   • Types:
     • Gametophytic Self-Incompatibility (GSI): The pollen grain’s genotype determines its compatibility.
     • Sporophytic Self-Incompatibility (SSI): The genotype of the pollen parent determines compatibility.

9. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species?

Ans : 

Self-incompatibility is a genetic mechanism that prevents self-pollination in flowering plants. It is a mechanism that ensures cross-pollination and genetic diversity.

Why Self-Pollination Doesn’t Lead to Seed Formation

• Pollen Tube Inhibition: The pollen tube, which carries the male gametes, is prevented from growing down the style to reach the ovule. This inhibition can occur at various points along the pollen tube’s journey.
• Stigma Rejection: The stigma may actively reject the pollen grain, preventing it from adhering or germinating.
• Ovule Rejection: Even if the pollen tube reaches the ovule, the ovule itself may reject the pollen grain, preventing fertilization.

10.What is bagging technique? How is it useful in a plant breeding programme?

Ans : 

Bagging Technique

Bagging is a technique used in plant breeding to isolate flowers from unwanted pollination. This is done by covering the flowers with a bag before they open, preventing pollen from other plants or even from the same plant from reaching the stigma.

Uses of Bagging in Plant Breeding:

1. Preventing Self-Pollination: By isolating flowers, bagging ensures that only pollen from a desired source can reach the stigma, preventing self-pollination.
2. Maintaining Purity: Bagging is crucial for maintaining the purity of a particular variety or cultivar. It prevents unwanted cross-pollination that could introduce foreign genes and alter the desired traits.
3. Producing Hybrids: Bagging is used in hybrid breeding programs to control the parentage of the offspring. By selectively bagging flowers, breeders can ensure that only pollen from the desired male parent reaches the female parent.
4. Studying Inheritance Patterns: Bagging can be used to study inheritance patterns by isolating flowers with known genotypes and observing the phenotypes of their offspring.

11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion

Ans : 

Triple fusion is a unique process that occurs in angiosperms (flowering plants) during fertilization.

Where and How it Takes Place:

• Location: Triple fusion takes place in the central cell of the embryo sac within the ovule.
• Process:
  • The pollen tube, carrying two male gametes, penetrates the ovule through the micropyle.
  • One male gamete fuses with the egg cell to form the zygote
  • The other male gamete fuses with the two polar nuclei to form the endosperm nucleus.  

Nuclei Involved:

1. Male gamete
2. Two polar nuclei

12. Why do you think the zygote is dormant for sometime in a fertilised ovule?

Ans : 

The dormancy of the zygote in a fertilized ovule is an adaptation that serves several important purposes:

1. Favorable Conditions for Germination: The zygote remains dormant until environmental conditions are optimal for germination. 
2. Nutrient Accumulation: During dormancy, the endosperm, a tissue formed during triple fusion, continues to accumulate nutrients. These nutrients provide the young seedling with the necessary energy and resources for growth.
3. Seed Dispersal: Dormancy allows for seed dispersal, which is essential for colonizing new habitats and reducing competition for resources. Seeds can be dispersed by various means, such as wind, water, animals, or human activities.
4. Protection: The seed coat, a protective layer that forms around the embryo and endosperm, provides physical protection during dormancy. It shields the developing embryo from harsh environmental conditions, such as desiccation, temperature extremes, and predators.
5. Synchronization with Seasonal Cycles: In many plants, seed dormancy is synchronized with seasonal cycles. This ensures that the seedling germinates at the most favorable time of year, when there is sufficient sunlight, water, and nutrients available for growth.

13. Differentiate between: 

(a) hypocotyl and epicotyl; (b) coleoptile and coleorrhiza; 

(c) integument and testa; (d) perisperm and pericarp

Ans : 

1. 

Feature	Hypocotyl	Epicotyl
Location	Below cotyledons	Above cotyledons
Function	Anchors seedling	Forms shoot and leaves
Development	From embryonic stem	From embryonic stem
Structure	Cylindrical	Cylindrical
Found in	Dicots and monocots	Dicots and monocots

2. 

Feature	Coleoptile	Coleorrhiza
Location	Monocots	Monocots
Function	Protects shoot	Protects root
Development	From leaf primordia	From root primordia
Structure	Hollow tube	Tube-like
Found in	Monocots	Monocots

3.

Feature	Integument	Testa
Location	Ovule	Seed coat
Function	Protects ovule	Protects seed
Development	From ovule	From integuments
Structure	Layers	Outer layer of seed coat
Found in	All flowering plants	All flowering plants

4. 

Feature	Perisperm	Pericarp
Location	Seed	Fruit
Function	Nutrient reserve	Fruit wall
Development	From nucellus	From ovary wall
Structure	Parenchymatous tissue	Fleshy or dry
Found in	Some gymnosperms and angiosperms	All flowering plants

14. Why is apple called a false fruit? Which part(s) of the flower forms the fruit?

Ans : 

Apple is considered a false fruit because the fleshy part we eat is not derived from the ovary of the flower. Instead, the fleshy part is formed from the receptacle, which is a swollen part of the flower stalk.

The true fruit in an apple is the small, hard core that contains the seeds. This core is derived from the ovary of the flower.

15. What is meant by emasculation? When and why does a plant breeder employ this technique? 

Ans : 

Emasculation is a technique used in plant breeding to remove the anthers (male reproductive organs) from a flower. This prevents self-pollination and allows for controlled cross-pollination with pollen from a desired male parent.

When and why a plant breeder employs this technique:

• Hybrid Breeding: Emasculation is essential for producing hybrids. By removing the anthers of a female parent, the breeder can control which pollen grains fertilize the ovules, leading to the desired hybrid offspring.
• Maintaining Pure Lines: Emasculation can be used to maintain the purity of a particular variety or cultivar. By preventing self-pollination, breeders can ensure that the offspring retain the specific traits of the parent plant.
• Studying Inheritance Patterns: Emasculation can be used in genetic studies to control the parentage of offspring and examine how traits are inherited.

16.If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?

Ans : 

Parthenocarpy is the development of fruit without fertilization. This can be induced by applying growth substances.

Here are some fruits that might be suitable candidates for parthenocarpy induction:

1. Tomatoes: Tomatoes are often used in parthenocarpy studies due to their commercial importance and the ease of inducing fruit set. Growth substances like gibberellic acid can be applied to promote fruit development without pollination.
2. Cucumbers: Parthenocarpy can be induced in cucumbers using auxins like naphthalene acetic acid (NAA). This can be beneficial for producing seedless cucumbers, which are preferred by many consumers.
3. Eggplants: Eggplants can also be induced to develop parthenocarpic fruits using growth substances. This can be useful for increasing yield and improving fruit quality.
4. Bananas: Although bananas naturally produce parthenocarpic fruits, the application of growth substances can be used to enhance fruit size and yield.
5. Citrus fruits: While less common, parthenocarpy has been induced in some citrus fruits, such as oranges and grapefruit. This can be useful for producing seedless varieties.

Why these fruits?

• Economic Importance: These fruits are all commercially important, and inducing parthenocarpy can have significant economic benefits.
• Consumer Preference: Seedless varieties of these fruits are often preferred by consumers, making parthenocarpy induction a desirable practice.
• Ease of Induction: These fruits are relatively easy to induce parthenocarpy using available growth substances.

17. Explain the role of tapetum in the formation of pollen-grain wall.

Ans : 

It is a layer of nutritive cells that surrounds the pollen mother cell during microsporogenesis (the process of pollen grain formation).

Here are the key functions of the tapetum in pollen grain wall formation:

1. Nutrient Supply: The tapetum provides essential nutrients and substances to the developing pollen grains. These nutrients are necessary for the growth and differentiation of the pollen grain cells.
2. Synthesis of Pollen Wall Components: The tapetum synthesizes various components that make up the pollen grain wall. These include:
   • Sporopollenin: A highly resistant polymer that forms the outer layer of the pollen grain wall, providing protection and durability.
   • Callose: A carbohydrate that forms a temporary layer around the developing pollen grains, providing support and protection during early development.
   • Proteins: Proteins involved in pollen grain development, such as enzymes and signaling molecules.
3. Release of Pollen Wall Components: The tapetum releases the synthesized components into the locule (the cavity within the anther) where the pollen grains are developing. These components are then incorporated into the developing pollen grain wall.
4. Degeneration: After providing nutrients and synthesizing pollen wall components, the tapetum undergoes degeneration. The remnants of the tapetum may contribute to the formation of the pollen grain wall.

18. What is apomixis and what is its importance?

Ans : 

Apomixis is a type of asexual reproduction in plants that results in the formation of seeds without fertilization. In other words, it is the production of offspring that are genetically identical to the parent plant without the involvement of meiosis or fertilization.

Importance of Apomixis:

1. Preservation of Genetic Identity: Apomixis allows plants to maintain their genetic identity across generations, ensuring the continuity of desirable traits.
2. Rapid Propagation: Apomixis can be a faster and more efficient way to propagate plants compared to sexual reproduction, as it does not require pollination or fertilization.
3. Maintenance of Clones: Apomixis is used to produce clones of plants with desirable traits, such as high yield, disease resistance, or specific characteristics.
4. Evolutionary Significance: Apomixis can play a role in the evolution of plant species by allowing them to adapt to specific environments without the need for genetic recombination.