Sexual Reproduction in Flowering Plants

SEXUAL REPRODUCTION IN FLOWERING PLANTS

Overview of Sexual Reproduction

• Sexual reproduction involves the fusion of male and female gametes to form a zygote, leading to the production of seeds.

• Flowering plants (angiosperms) are characterized by their reproductive structures found within flowers.

Structural Components of Flowers

Male Reproductive Organs (Stamen)

• Stamen consists of two parts:

  • Filament: Long and slender stalk attached to the thalamus.

  • Anther: Terminal bilobed structure, each lobe containing two thecae (dithecous).

  • Anther has a longitudinal groove separating the thecae.

  • Each theca consists of 4 microsporangia that develop in pollen sacs, containing pollen grains.

Female Reproductive Organs (Pistil or Carpel)

• The pistil consists of three parts:

  • Stigma: Landing platform for pollen grains.

  • Style: Slender part beneath the stigma.

  • Ovary: Basal bulging part of the pistil containing ovules.

  • The ovary houses the placenta, where ovules arise.

Microsporangium Structure

• Microsporangium has:

  • Outer Layers: Epidermis, Endothecium, and Middle Layer for protection and facilitating dehiscence.

  • Tapetum: Inner layer that nourishes developing pollen grains.

  • Contains sporogenous tissue responsible for pollen formation.

Microsporogenesis

• Microsporogenesis is the process of formation of microspores from microspore mother cells through meiosis.

• Each microspore develops into a pollen grain after the tetrad dissociates. Pollen grains are released when the anther dehisces.

Pollen Grain Characteristics

• Pollen grain is the male gametophyte, typically spherical (25-50 μm in diameter) with two layers:

  • Exine: Outermost layer made of sporopollenin, resistant to high temperatures and chemical degradation.

  • Intine: Inner wall composed of cellulose and pectin, surrounds the cytoplasm and nucleus of the pollen grain.

• Germpore: Aperture on exine through which the pollen tube grows.

• Pollen grain typically consists of two cells:

  • Vegetative Cell: Larger, with an irregularly shaped nucleus and abundant food reserves.

  • Generative Cell: Smaller, spindle-shaped, divides mitotically to form two male gametes.

• Pollen Viability: Refers to the period that pollen can germinate after landing on a stigma.

Female Reproductive Structures

Ovule (Megasporangium)

• The ovule consists of:

  • Attached to placenta via the funicle, with the point of attachment known as the hilum.

  • Protective layers (integuments) encasing the nucellus which contains the megaspore mother cell.

  • Each ovule typically contains one functional megaspore that develops into the female gametophyte (embryo sac).

Megasporogenesis

• The process of forming megaspores from megaspore mother cells.

• Typically, one functional megaspore remains after meiotic division, developing into the embryo sac within the ovule.

Embryo Sac Development

• The functional megaspore undergoes three mitotic divisions (free nuclear division) to form an eight-nucleate embryo sac with:

  • 6 antipodal cells and 2 polar nuclei in the center.

  • Egg and synergid cells situated at the micropylar end of the embryo sac.

Pollination

• Defined as the process of transferring pollen grains from the anther to stigma. Types include:

  • Autogamy: Self-pollination within the same flower.

  • Complete autogamy is rare; it requires synchronized pollen release and receptivity.

  • Examples include flowers of Viola, Oxalis, and Commelina.

  • Geitonogamy: Transfer of pollen from one flower to another on the same plant, functionally cross-pollination but genetically similar to autogamy.

  • Xenogamy: Transfer of pollen to a stigma of a different plant leading to genetic diversity.

Pollination Agents

• Pollination occurs through:

  • Anemophily: Wind pollination. Characterized by light, non-sticky pollen grains with large, feathery stigmas (e.g., corn).

  • Hydrophily: Water pollination, found in several genera like Vallisneria and Hydrilla.

  • Zoophily: Pollination by animals, primarily involving insects, birds, and mammals. Flowers adapted for this include large, colorful, fragrant blooms rich in nectar to attract pollinators.

Pollen-Pistil Interaction

• Not all pollen grains germinate on stigma; compatibility is critical for successful fertilization.

• Pollen tubes grow through the style to reach ovules, carrying male gametes.

Fertilization Process

Double Fertilization

• Upon entering a synergid, the pollen tube releases two male gametes:

  • One fuses with the egg cell (syngamy) to form zygote (2n).

  • The other fuses with two polar nuclei (triple fusion) to form the primary endosperm nucleus (3n). This process is unique to angiosperms.

Seed and Fruit Development

Seed Formation

• Involves the transformation of ovules into seeds and ovaries into fruits.

• A typical seed has:

  • Seed coat: Tough covering that remains after fertilization.

  • Cotyledons: The plant's first leaves, storing food for embryo development.

• Seeds can be classified into:

  • Exalbuminous: Seeds with no residual endosperm (e.g., pea, bean).

  • Albuminous: Seeds that retain part of the endosperm during embryonic development (e.g., wheat).

Fruit Development

• Fertilized ovaries transform into fruits, containing seeds. Fruits can be:

  • True Fruits: Resulting from ovary tissue (e.g., mango).

  • False Fruits: Composed of other floral parts along with the ovary (e.g., apple, strawberry).

  • Parthenocarpic Fruits: Develop without fertilization (e.g., banana).

Apomixis and Polyembryony

• Apomixis: A form of asexual reproduction where seeds are produced without fertilization, typical in some plants.

• Polyembryony: Occurrence of more than one embryo within a single seed, seen in various species including some citrus plants.

Conclusion

• Understanding the sexual reproduction process in flowering plants is essential for agriculture, horticulture, and biodiversity conservation, influencing practices like hybridization and crop improvement programs.

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