Sexual Reproduction in Flowering Plants

Key Concepts

Flower - A Fascination Organ

Flowers are not just aesthetic objects; they are the sites of sexual reproduction in angiosperms. A typical flower has four whorls: calyx, corolla, androecium (male), and gynoecium (female).

Pre-fertilisation: Structures and Events

Several hormonal and structural changes lead to the differentiation and development of the floral primordium.

1. Stamen, Microsporangium and Pollen Grain

Stamen: Consists of a long stalk (filament) and a terminal bilobed structure (anther).
Anther: Typically bilobed and dithecous. It is a four-sided (tetragonal) structure with four microsporangia.
Microsporogenesis: The process of formation of microspores from a pollen mother cell (PMC) through meiosis.

Microsporogenesis Process

Pollen Grain: Represents the male gametophyte.
- Exine: Hard outer layer made of sporopollenin, the most resistant organic material. It has germ pores.

Sporopollenin Sporopollenin is one of the most resistant organic materials known. It can withstand high temperatures and strong acids or alkalis. No enzyme that degrades sporopollenin is so far known, allowing pollen grains to be well-preserved as fossils.

Intine: Inner thin wall made of cellulose and pectin.
Cells: Mature pollen has a vegetative cell (large, food reserve) and a generative cell (small, spindle-shaped, gives rise to two male gametes).

2. The Pistil, Megasporangium (ovule) and Embryo sac

Gynoecium: Can be monocarpellary or multicarpellary (syncarpous or apocarpous).
Ovule: Attached to the placenta by a funicle. Parts include hilum, integuments, micropyle, chalaza, and nucellus.
Megasporogenesis: Formation of megaspores from the megaspore mother cell (MMC) via meiosis.
Embryo Sac (Female Gametophyte): Formed from a single functional megaspore (monosporic development). A mature embryo sac is 8-nucleate and 7-celled.

The Mature Embryo Sac Although the embryo sac is 8-nucleate, it is 7-celled because the large central cell contains two polar nuclei that eventually fuse to form a single diploid secondary nucleus.

Egg Apparatus: 2 synergids + 1 egg cell.
Antipodals: 3 cells at the chalazal end.
Central Cell: Contains 2 polar nuclei.

Pollination

The transfer of pollen grains from the anther to the stigma.

Autogamy: Within the same flower.
Geitonogamy: Different flower of the same plant.
Xenogamy: Different flower of a different plant (Genetically different).
Agents: Abiotic (wind, water) and Biotic (bees, butterflies, birds, bats, etc.).

Double Fertilisation

A unique event in angiosperms.

Double Fertilisation Unique to flowering plants, it involves two types of fusion: Syngamy (forming the zygote) and Triple Fusion (forming the triploid endosperm). Since two types of fusion occur in the embryo sac, the phenomenon is termed double fertilisation.

Syngamy: One male gamete + Egg cell → Zygote (2n).
Triple Fusion: Second male gamete + 2 Polar Nuclei → Primary Endosperm Nucleus (PEN) (3n).

Post-fertilisation: Structures and Events

Endosperm: Develops from the primary endosperm cell; provides nutrition to the embryo.
Embryo: Develops at the micropylar end. Stages: Proembryo → Globular → Heart-shaped → Mature.
Seed: Fertilised ovule.
- Albuminous: Retain endosperm (e.g., wheat, maize).
- Non-albuminous: No residual endosperm (e.g., pea, groundnut).
- Perisperm: Persistent nucellus (e.g., black pepper).
Fruit: Ripened ovary.
- True Fruits: Develop from ovary.
- False Fruits: Thalamus also contributes (e.g., apple, strawberry).
- Parthenocarpic: Formed without fertilisation (e.g., banana).

Apomixis and Polyembryony

Apomixis: Production of seeds without fertilisation (asexual reproduction mimicking sexual).
Polyembryony: Occurrence of more than one embryo in a seed (common in Citrus and Mango).

Activities (for understanding, not part of notes)

Observe pollen germination in a 10% sucrose solution.
Dissect different flowers to identify syncarpous and apocarpous ovaries.
Examine various seeds (monocots vs. dicots) after soaking them.
Squeeze orange seeds to observe polyembryony.

Key Concepts

Flowers are not just aesthetic objects; they are the sites of sexual reproduction in angiosperms. A typical flower has four whorls: calyx, corolla, androecium (male), and gynoecium (female).

Several hormonal and structural changes lead to the differentiation and development of the floral primordium.

Stamen: Consists of a long stalk (filament) and a terminal bilobed structure (anther).

Anther: Typically bilobed and dithecous. It is a four-sided (tetragonal) structure with four microsporangia.

Microsporogenesis: The process of formation of microspores from a pollen mother cell (PMC) through meiosis.

Pollen Grain: Represents the male gametophyte.

Exine: Hard outer layer made of sporopollenin, the most resistant organic material. It has germ pores.

Intine: Inner thin wall made of cellulose and pectin.

Cells: Mature pollen has a vegetative cell (large, food reserve) and a generative cell (small, spindle-shaped, gives rise to two male gametes).

Gynoecium: Can be monocarpellary or multicarpellary (syncarpous or apocarpous).

Ovule: Attached to the placenta by a funicle. Parts include hilum, integuments, micropyle, chalaza, and nucellus.

Megasporogenesis: Formation of megaspores from the megaspore mother cell (MMC) via meiosis.

Embryo Sac (Female Gametophyte): Formed from a single functional megaspore (monosporic development). A mature embryo sac is 8-nucleate and 7-celled.

Egg Apparatus: 2 synergids + 1 egg cell.

Antipodals: 3 cells at the chalazal end.

Central Cell: Contains 2 polar nuclei.

The transfer of pollen grains from the anther to the stigma.

Autogamy: Within the same flower.

Geitonogamy: Different flower of the same plant.

Xenogamy: Different flower of a different plant (Genetically different).

Agents: Abiotic (wind, water) and Biotic (bees, butterflies, birds, bats, etc.).

A unique event in angiosperms.

Syngamy: One male gamete + Egg cell → Zygote (2n).

Triple Fusion: Second male gamete + 2 Polar Nuclei → Primary Endosperm Nucleus (PEN) (3n).

Endosperm: Develops from the primary endosperm cell; provides nutrition to the embryo.

Embryo: Develops at the micropylar end. Stages: Proembryo → Globular → Heart-shaped → Mature.

Seed: Fertilised ovule.

Albuminous: Retain endosperm (e.g., wheat, maize).
Non-albuminous: No residual endosperm (e.g., pea, groundnut).
Perisperm: Persistent nucellus (e.g., black pepper).

Fruit: Ripened ovary.

True Fruits: Develop from ovary.
False Fruits: Thalamus also contributes (e.g., apple, strawberry).
Parthenocarpic: Formed without fertilisation (e.g., banana).

Apomixis: Production of seeds without fertilisation (asexual reproduction mimicking sexual).

Polyembryony: Occurrence of more than one embryo in a seed (common in Citrus and Mango).

Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

Key Concepts

Flower - A Fascination Organ

Pre-fertilisation: Structures and Events

1. Stamen, Microsporangium and Pollen Grain

Microsporogenesis Process

2. The Pistil, Megasporangium (ovule) and Embryo sac

Pollination

Double Fertilisation

Post-fertilisation: Structures and Events

Apomixis and Polyembryony

Activities (for understanding, not part of notes)

On this page

Sexual Reproduction in Flowering Plants

Sexual Reproduction in Flowering Plants

Key Concepts

Flower - A Fascination Organ

Pre-fertilisation: Structures and Events

1. Stamen, Microsporangium and Pollen Grain

Microsporogenesis Process

2. The Pistil, Megasporangium (ovule) and Embryo sac

Pollination

Double Fertilisation

Post-fertilisation: Structures and Events

Apomixis and Polyembryony

Activities (for understanding, not part of notes)

On this page