Flowering Plants: Sexual Reproduction

• Sexual Reproduction in Flowering Plants

Pre-fertilisation: Structures and Events

• Prior to fertilisation, floral primordium develops, followed by the differentiation and development of floral structures (stamens and pistils).

The Flower

• A flower consists of four whorls: calyx, corolla, androecium, and gynoecium.
• Androecium and gynoecium are the reproductive organs.
• A typical stamen consists of a filament and an anther.
• The anther is usually bilobed, with each lobe containing two theca (dithecous).
• Anther Structure: It has four microsporangia located at the corners, two in each lobe.
• Microsporangia develop further and become pollen sacs, containing pollen grains.
• Microsporogenesis: It is the process of formation of microspores from a pollen mother cell (PMC) through meiosis.
• As the anther matures, the microspores separate and develop into pollen grains.
• Pollen grains represent the male gametophytes.
• Pollen grains have a two-layered wall: the exine (outer, made of sporopollenin) and the intine (inner, made of cellulose and pectin).
• Sporopollenin: It is a resistant organic material that can withstand high temperatures, strong acids, and alkalis.
• The germ pore is an aperture on the exine where sporopollenin is absent, allowing the pollen tube to emerge.
• Pollen grains are shed at a 2-celled stage (vegetative and generative cell) in most angiosperms.
• In some species, pollen grains are shed at a 3-celled stage (vegetative cell and two male gametes).
• Pollen grains of some plants cause allergies and respiratory problems.
• Pollen viability varies from plant to plant; some retain viability for months, while others for only 30 minutes.
• Pollen grains can be stored in liquid nitrogen (-196°C) in pollen banks.

The Pistil, Megasporangium (Ovule) and Embryo Sac

• The pistil represents the female reproductive part of the flower and may consist of a single carpel (monocarpellary) or multiple carpels (multicarpellary).
• Syncarpous: Condition where carpels are fused.
• Apocarpous: Condition where carpels are free.
• Each pistil has three parts: stigma, style, and ovary.
• The ovules are present inside the ovary.
• Megasporangium (Ovule): A typical ovule consists of a funicle (stalk), hilum (junction between ovule and funicle), integuments (protective layers), nucellus (tissue enclosing the embryo sac), and chalaza (basal part).
• The embryo sac is located within the nucellus.
• Megasporogenesis: The process of formation of megaspores from the megaspore mother cell (MMC) through meiosis.
• Usually, one megaspore is functional, while the other three degenerate.
• The functional megaspore develops into the female gametophyte (embryo sac).
• Megagametogenesis: The development of the embryo sac from a single functional megaspore.
• The nucleus of the functional megaspore divides mitotically to form two nuclei, which move to opposite poles, forming a 2-nucleate embryo sac.
• Subsequent mitotic divisions result in 4-nucleate and 8-nucleate stages.
• The 8-nucleate embryo sac organizes with six nuclei surrounded by cell walls and the remaining two (polar nuclei) fusing to form the secondary nucleus in the central cell.
• The egg apparatus consists of one egg cell and two synergids, located at the micropylar end.
• Antipodals are three cells present at the chalazal end.
• A typical angiosperm embryo sac at maturity is 8-nucleate and 7-celled.

Pollination

• Pollination is the transfer of pollen grains from the anther to the stigma.
• Autogamy: Self-pollination within the same flower.
• Geitonogamy: Transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.
• Xenogamy: Transfer of pollen grains from the anther of one flower to the stigma of a different plant.
• Agents of Pollination: Abiotic (wind, water) and biotic (animals).
• Anemophily: Pollination by wind.
• Hydrophily: Pollination by water.
• Zoophily: Pollination by animals (insects, birds, bats, etc.).
• Adaptations in flowers for different pollination methods:
  • Wind-pollinated flowers: small, non-showy, produce large quantities of pollen, light, and non-sticky pollen grains.
  • Water-pollinated flowers: small, inconspicuous, pollen grains protected by a mucilaginous covering.
  • Insect-pollinated flowers: large, colorful, fragrant, nectar-rich.
• Outbreeding Devices: Mechanisms to prevent self-pollination and promote cross-pollination:
  • Dichogamy (pollen release and stigma receptivity not synchronized).
  • Self-incompatibility (genetic mechanism preventing self-pollination).
  • Herkogamy (physical barrier preventing self-pollination).
  • Production of unisexual flowers.

Pollen-Pistil Interaction

• Pollen-pistil interaction involves the recognition of pollen by the stigma, followed by promotion or inhibition of pollen tube growth.
• Compatible pollen grains germinate on the stigma and grow through the style to reach the ovule.
• The pollen tube enters the embryo sac through the micropyle and releases two male gametes.

Artificial Hybridisation

• Artificial hybridisation is a method to improve crop varieties by crossing different plants.
• Process involves emasculation (removal of anthers) and bagging (covering the stigma to prevent contamination).
• Desired pollen is dusted on the stigma, and the flower is rebagged to allow fruit development.

Double Fertilisation

• Double fertilisation is a unique event in angiosperms involving two fusions.
• One male gamete fuses with the egg cell to form a zygote (syngamy).
• The other male gamete fuses with the polar nuclei to form the primary endosperm nucleus (PEN) in the central cell (triple fusion).
• The PEN develops into the endosperm, which provides nourishment to the developing embryo.

Post-fertilisation: Structures and Events

• Following fertilisation, the zygote develops into the embryo, the ovule develops into the seed, and the ovary develops into the fruit.

Endosperm

• Endosperm development usually precedes embryo development.
• The primary endosperm cell divides repeatedly to form a triploid endosperm tissue.
• Endosperm may be completely consumed by the developing embryo (e.g., pea, groundnut) or persist in the mature seed (e.g., castor, coconut).
• Types of endosperm development: Nuclear, Cellular, and Helobial.

Embryo

• The zygote divides to form the proembryo, which then develops into the embryo.
• A typical dicot embryo consists of an embryonal axis and two cotyledons.
• The epicotyl (plumule) is the portion of the embryonal axis above the cotyledon attachment.
• The hypocotyl (radicle) is the portion below the cotyledon attachment.
• A typical monocot embryo has one cotyledon (scutellum), a coleorhiza (protective sheath covering the radicle), and a coleoptile (protective sheath covering the plumule).

Seed

• The seed is the fertilised ovule.
• Seed Coat: It is formed from the integuments of the ovule.
• Micropyle: it facilitates entry of oxygen and water into the seed during germination.
• Hilum: Scar on the seed coat where it was attached to the fruit.
• Albuminous seeds: Seeds that retain a part of the endosperm (e.g., wheat, maize, barley).
• Non-albuminous seeds: Seeds that do not retain any endosperm (e.g., pea, groundnut).
• Perisperm: Remnant of nucellus in some seeds (e.g., black pepper, beet).
• Seed dormancy: A state of inactivity, helping to survive unfavorable conditions.

Fruit

• The fruit is the mature ovary developed after fertilisation.
• The wall of the ovary forms the fruit wall or pericarp.
• True fruits develop only from the ovary.
• False fruits develop from other floral parts along with the ovary (e.g., apple, strawberry).
• Parthenocarpic fruits develop without fertilisation (e.g., banana).

Apomixis and Polyembryony

• Apomixis: A form of asexual reproduction that mimics sexual reproduction, where seeds are produced without fertilisation.
• Polyembryony: The occurrence of more than one embryo in a seed (e.g., citrus, mango).