Sexual Reproduction in Flowering Plants

Angiosperms reproduce sexually by flowers. The process involves Pre-fertilization, Double Fertilization, and Post-fertilization events.

Stamen: Consists of filament (stalk) and anther (bilobed, dithecous).

A. Structure of Microsporangium

• Surrounded by 4 layers:
  1. Epidermis, Endothecium, Middle layers: Protection & dehiscence.
  2. Tapetum: Innermost. Nourishes developing pollen grains. Cells have dense cytoplasm & multiple nuclei.

B. Microsporogenesis

Formation of Microspores from Pollen Mother Cell (PMC) through Meiosis. Forms Microspore Tetrad.

C. Pollen Grain (Male Gametophyte)

• Exine (Outer): Made of Sporopollenin (Most resistant organic material known). Has Germ Pores (Sporopollenin absent).
• Intine (Inner): Cellulose & Pectin.
• Cells:
  1. Vegetative Cell: Larger, food reserve.
  2. Generative Cell: Smaller, floats in cytoplasm. Divides mitotically to give 2 male gametes.
• Shedding stage: 60% angiosperms shed at 2-celled stage; 40% at 3-celled stage.

Pistil: Stigma, Style, Ovary. (Apocarpous/Syncarpous).

A. Structure of Ovule (Megasporangium)

• Funicle: Stalk.
• Hilum: Junction between ovule and funicle.
• Integuments: Protective envelopes.
• Micropyle: Small opening. Chalaza: Basal part.
• Nucellus: Mass of cells with food reserve.

B. Megasporogenesis

MMC (2n) undergoes Meiosis → 4 Megaspores. In most, 3 degenerate, 1 remains functional (Monosporic development).

C. Embryo Sac (Female Gametophyte)

7-celled, 8-nucleate structure:

• Egg Apparatus (Micropylar end): 2 Synergids (with Filiform apparatus) + 1 Egg cell.
• Antipodals (Chalazal end): 3 cells.
• Central Cell: Large, with 2 Polar Nuclei.

Transfer of pollen to stigma.

Agents of Pollination

• Abiotic: Wind (Anemophily - Corn cob, Grasses), Water (Hydrophily - Vallisneria, Hydrilla, Zostera).
• Biotic: Insects (Entomophily), Birds, Bats. (Floral rewards: Nectar, Pollen, Safe place for eggs e.g., Amorphophallus, Yucca).

Outbreeding Devices

To encourage cross-pollination: Non-synchrony, Heterostyly, Self-incompatibility (Genetic), Unisexual flowers.

Unique to Angiosperms. Involves two fusion events:

1. Syngamy: One male gamete (n) + Egg cell (n) → Zygote (2n).
2. Triple Fusion: Second male gamete (n) + Two polar nuclei (n+n) → Primary Endosperm Nucleus (PEN, 3n).

Endosperm

Develops before embryo to provide nutrition. Free nuclear (Coconut water) or Cellular (Coconut kernel).

Embryo

• Dicot: Embryonal axis + 2 Cotyledons. Stages: Proembryo → Globular → Heart-shaped → Mature.
• Monocot: One cotyledon (Scutellum). Coleoptile (shoot tip sheath) & Coleorhiza (root cap sheath).

Seed & Fruit

• Seed: Fertilized Ovule. Albuminous (Wheat, Maize, Castor) vs Non-albuminous (Pea, Groundnut). Perisperm (Persistent nucellus - Black pepper, Beet).
• Fruit: Fertilized Ovary.
  True Fruit: From ovary only.
  False Fruit: Thalamus also contributes (Apple, Strawberry).
  Parthenocarpic Fruit: Without fertilization (Banana).

• Apomixis: Production of seeds without fertilization. Mimics sexual reproduction. (Asteraceae, Grasses). Hybrids farmers can use apomictic seeds to avoid segregation of loose hybrid characters.
• Polyembryony: Occurrence of more than one embryo in a seed. (Citrus, Mango).

Structures and events leading to formation of gametes (Stamen, Pistil).

Four layers: Epidermis, Endothecium, Middle Layers, Tapetum.

Innermost layer. Nourishes developing pollen grains. Multinucleated cells.

Occupies center of microsporangium. Cells differentiate into PMC (Pollen Mother Cells).

Process of formation of microspores from PMC through meiosis.

Structure of Exine (outer layer of pollen). Hardest organic material known.

Apertures on exine where sporopollenin is absent. Pollen tube emergence site.

Larger, abundant foo reserve, irregular nucleus.

Small, floats in cytoplasm of vegetative cell. Spindle shaped. Divides to form male gametes.

Carrot grass (Parthenium) causes pollen allergy. came as contaminant with imported wheat.

Rice/Wheat: 30 minutes. Rosaceae/Leguminoseae/Solanaceae: Several months.

Storing pollen in liquid nitrogen (-196 deg C) for pollen banks.

Syncarpous (Fused - Papaver). Apocarpous (Free - Michelia).

Attached to placenta by Funicle. Junction is Hilum.

Encircle the nucellus except at the tip (Micropyle).

Embryo sac. Formed from a single megaspore (Monosporic development).

7-celled and 8-nucleate structure at maturity.

Two Synergids + One Egg Cell. Loacted at micropylar end.

Special cellular thickening in synergids. Guides pollen tube entry.

Three cells located at the Chalazal end.

Largest cell. Contains 2 polar nuclei. Fuses with male gamete to form PEN.

Pollination between different flowers of same plant. Genetically autogamy.

Transfer of pollen grains to stigma of different plant. Genetically different.

Pollen light/non-sticky. Stigma feathery. Single ovule per ovary. e.g. Corn, Grasses.

Rare (30 genera, monocots). Vallisneria (Surface), Zostera (Submerged). Pollen covered by mucilage.

Large, colourful, fragrant, nectar rich flowers. Pollen/Stigma sticky.

Obligate symbiotic relationship. Moth deposits egg in ovary, also pollinates.

Removal of anthers from flower bud before dehiscence (Artificial hybridization).

Syngamy + Triple Fusion. Unique to Angiosperms.

Primary Endosperm Nucleus. Formed by fusion of male gamete + 2 polar nuclei.

Free-nuclear endosperm. White kernel is cellular endosperm.

Single large cotyledon of grass family (Monocot).

Hollow foliar structure enclosing the shoot apex (Plumule).

Persistent Nucellus. Found in Black Pepper, Beet.

True: from ovary. False: Thalamus also contributes (Apple, Strawberry, Cashew).