Sexual Reproduction in Flowering Plants: Class 12 Biology Notes

Sexual Reproduction in Flowering Plants: Class 12 Biology Notes. Flowers are the glorious sites of sexual reproduction in angiosperms. This chapter covers the structural and functional aspects of plant reproduction, from the formation of gametes to the development of seeds and fruits.

1. Structure of a Flower

A typical flower consists of four distinct whorls arranged on a swollen stalk base called the thalamus (or receptacle).

Non-Essential (Accessory) Whorls

• Calyx: The outermost whorl made of sepals. They protect the flower in the bud stage.

• Corolla: The second whorl made of petals. Often brightly colored to attract pollinators.

Essential (Reproductive) Whorls

• Androecium: The male reproductive organ, composed of a cluster of stamens.

• Gynoecium (Pistil/Carpel): The female reproductive organ, located at the center.

2. Development of the Male Gametophyte (Microsporogenesis)

The stamen consists of a long, slender stalk called the filament and a terminal, usually bilobed structure called the anther.

Structure of an Anther

• A typical angiosperm anther is bilobed and dithecous (each lobe has two thecae).

• It is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners.

Anther Wall Layers

From outermost to innermost:

1. Epidermis: Protective single layer.

2. Endothecium: Fibrous layer that helps in anther dehiscence (splitting) due to its hygroscopic nature.

3. Middle Layers: 1–3 layers that degenerate at maturity.

4. Tapetum: Innermost layer with dense cytoplasm and multiple nuclei. It nourishes the developing pollen grains.

Microsporogenesis

The process of formation of microspores from a Pollen Mother Cell (PMC) through meiosis.

Pollen Grain (The Male Gametophyte)

• Exine: Hard outer layer made of sporopollenin (one of the most resistant organic materials known; withstands high temperatures and strong acids/alkalis). It has prominent apertures called germ pores where sporopollenin is absent.

• Intine: Thin, continuous inner wall made of cellulose and pectin.

Cell Composition at Maturity

• Vegetative Cell: Bigger, contains abundant food reserves and a large, irregularly shaped nucleus.

• Generative Cell: Small, floats in the cytoplasm of the vegetative cell. It divides mitotically to give rise to two male gametes.

💡 Note: In over 60% of angiosperms, pollen grains are shed at the 2-celled stage (vegetative + generative). In the remaining 40%, the generative cell divides to form two male gametes before shedding (3-celled stage).

3. Development of the Female Gametophyte (Megasporogenesis)

The gynoecium represents the female component. A pistil consists of three parts: Stigma (landing platform for pollen), Style (elongated tube), and Ovary (swollen basal part). Inside the ovary is the ovarian cavity (locule), which houses the ovules (megasporangia) attached via a placenta.

Structure of an Anatropous (Inverted) Ovule

• Funiculus: Stalk attaching the ovule to the placenta.

• Hilum: Junction where the body of the ovule fuses with the funiculus.

• Integuments: Protective outer coats.

• Micropyle: Small opening at the tip where integuments are absent.

• Chalaza: Basal part of the ovule, opposite the micropylar end.

• Nucellus: Central mass of parenchymatous tissue containing reserve food.

Megasporogenesis

The process of formation of megaspores from the Megaspore Mother Cell (MMC).

1. A single MMC (2n) differentiates in the micropylar region of the nucellus.

2. The MMC undergoes meiosis to produce a linear tetrad of four haploid megaspores.

3. In most flowering plants, three megaspores degenerate, and only one remains functional.

Megagametogenesis (Embryo Sac Formation)

The functional megaspore expands and undergoes three successive free-nuclear mitotic divisions:

Cell walls are then laid down, leading to the organization of the typical female gametophyte:

The 7-Celled, 8-Nucleate Embryo Sac

• Egg Apparatus (at micropylar end): Consists of two synergids and one egg cell. Synergids feature special cellular thickenings called the filiform apparatus, which guides the pollen tube into the synergid.

• Antipodal Cells (at chalazal end): Three cells that generally degenerate before or shortly after fertilization.

• Central Cell: A large central cell containing two polar nuclei.

4. Pollination: Types, Agencies, and Examples

Pollination is the transfer of pollen grains from the anther to the stigma of a pistil.

Types of Pollination

Agencies of Pollination

Abiotic Agents

• Wind (Anemophily): Pollen grains are light, non-sticky, and often winged. Stamens are well-exposed, and stigmas are large and feathery to trap wind-borne pollen. Flowers often lack color and nectar. Examples: Corn/Maize, Grasses, Date Palm.

• Water (Hydrophily): Quite rare (limited to about 30 genera, mostly monocots).

  • Epihydrophily (Surface): Vallisneria (female flowers reach the surface on long stalks; male flowers break off and float to them).

  • Hypohydrophily (Submerged): Zostera (sea grasses; long, ribbon-like pollen grains float underwater).

Biotic Agents (Zoophily)

Plants use insects (entomophily), birds (ornithophily), or bats (chiropterophily). Flowers are typically large, colorful, fragrant, and rich in nectar to serve as animal rewards.

• Examples: Rose, Jasmine, Amorphophallus (provides a safe egg-laying site), Yucca (shares an obligate mutual relationship with the Yucca moth).

5. Outbreeding Devices

Continuous self-pollination leads to inbreeding depression, which reduces vigor and fertility. Plants have evolved several mechanisms to prevent self-pollination and encourage cross-pollination:

• Dichogamy: Pollen release and stigma receptivity are not synchronized. Either the anther matures first (Protandry) or the stigma matures first (Protogyny).

• Herkogamy: Physical barriers separate the anther and stigma, preventing self-contact.

• Self-Incompatibility: A genetic mechanism that prevents self-pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

• Dioecy: Production of unisexual flowers on separate plants (e.g., Papaya). This prevents both autogamy and geitonogamy.

6. Pollen-Pistil Interaction

This covers all events from pollen deposition on the stigma until the pollen tube enters the ovule. It acts as a dynamic chemical dialogue.

1. The pistil recognizes whether the pollen is compatible (right type) or incompatible (wrong type) through chemical components.

2. If compatible, the pollen grain germinates on the stigma, producing a pollen tube through a germ pore.

3. The contents of the pollen grain move into the pollen tube. The tube grows through the tissues of the style and reaches the ovary.

4. The pollen tube enters the ovule through the micropyle and enters one of the synergids through the filiform apparatus.

7. Double Fertilization

Discovered by Nawaschin, this unique event is characteristic only of angiosperms.

1. After entering a synergid, the pollen tube releases two male gametes into the cytoplasm of the cell.

2. Syngamy: One male gamete (n) moves toward the egg cell (n) and fuses with it to form a diploid Zygote (2n), which eventually develops into the embryo.

3. Triple Fusion: The second male gamete (n) moves toward the central cell and fuses with the two polar nuclei (2n) to form a triploid Primary Endosperm Nucleus (PEN) (3n).

Because two types of fusions take place in an embryo sac, the phenomenon is termed Double Fertilization. The central cell becomes the Primary Endosperm Cell (PEC).

8. Post-Fertilization Events

Following double fertilization, these coordinated events transform the flower structures into a seed and fruit.

Development of Endosperm

Endosperm development precedes embryo development to guarantee an available food supply for the growing embryo.

• Free-nuclear type: The PEN undergoes successive nuclear divisions without wall formation, creating a large number of free nuclei (e.g., coconut water).

• Cellular type: Nuclear division is immediately followed by cell wall formation (e.g., the white kernel surrounding coconut water).

Development of the Embryo (Embryogenesis)

The embryo develops at the micropylar end of the embryo sac.

• Stages of Dicot Embryo Development:

  

• Structure of Dicot Embryo: Consists of an embryonal axis and two cotyledons. The portion of the axis above the cotyledons is the epicotyl (terminates in the plumule); the portion below is the hypocotyl (terminates in the radicle, covered by a root cap).

• Structure of Monocot Embryo: Contains only one cotyledon, termed the scutellum (positioned laterally). The embryonal axis features a plumule enclosed in a protective sheath called the coleoptile, and a radicle enclosed in a sheath called the coleorhiza.

Development of Seed and Formation of Fruit

• The ovule matures into a seed.

• The ovary matures into a fruit.

Seed Structure and Types

The outer integument becomes the testa (outer seed coat), and the inner integument becomes the tegmen (inner seed coat).

• Non-albuminous (Exalbuminous) Seeds: Endosperm is completely consumed during embryo development (e.g., Pea, Groundnut).

• Albuminous Seeds: Retain a portion of endosperm as it is not completely used up (e.g., Wheat, Maize, Barley, Castor).

• Perisperm: Remnants of the nucellus that persist in some seeds (e.g., Black Pepper, Beet).

As the seed matures, its water content is reduced, and its metabolic activity slows down. The embryo enters a state of inactivity called dormancy.

Fruit Types

The wall of the ovary develops into the wall of the fruit, called the pericarp.

• True Fruits: Develop solely from the ovary (e.g., Mango, Tomato).

• False Fruits: The thalamus or other floral parts contribute to fruit formation along with the ovary (e.g., Apple, Strawberry, Cashew).

9. Special Modes of Reproduction

• Apomixis: A form of asexual reproduction that mimics sexual reproduction, where seeds are formed without fertilization (e.g., Asteraceae, grasses). This can happen when a diploid egg cell forms without meiosis and develops directly into an embryo.

• Parthenocarpy: Development of fruits without fertilization. These fruits are naturally seedless (e.g., Banana). Parthenocarpy can also be induced artificially using plant growth hormones like auxins.

• Polyembryony: The occurrence of more than one embryo in a seed. Frequently observed in citrus varieties like Orange and Mango, where nucellar cells surrounding the embryo sac project into it and develop into multiple embryos.

10. Significance of Fruit Formation and Seed Dispersal

• Protection: The fruit pericarp acts as a protective shield for tender, developing seeds against harsh weather conditions and herbivores.

• Seed Dispersal Mechanisms: Fruits develop adaptations (hooks, wings, vibrant colors, fibrous husks) to leverage wind, water, or animals for dispersal. This prevents intense competition with the parent plant for space, sunlight, and nutrients.

• Colonization: Efficient dispersal allows species to spread into new geographical niches, enhancing biodiversity and survival.

• Agricultural Base: Perennation structures like dormant seeds permit food storage over extended periods and serve as the foundation for seasonal agriculture.

Mind Map- Sexual Reproduction in Flowering Plants: Class 12 Biology Notes