Lecture Notes on the Role of Flowers in Plant Reproduction

Introduction to Flowers

• Flowers serve as a source of visual pleasure due to their variety in shapes and colors.
• They are not just decorative; they play a critical role in plant reproduction.

The Structure of Flowers

• Larger, simpler flowers of monocotyledons illustrate flower structure.
• Flower parts include:
  • Sepals: Three that overlap the three petals.
  • Petals: Enclose the stamens.
  • Stamens: Six stamens produce pollen, each with a filament and an anther.
  • Anther: Contains four microsporangia nourished by tapetum.

Pollen Formation

• Microsporangium consists of diploid sporicite cells that divide by meiosis to form haploid microspore cells (tetrad formation).
• The formation of pollen includes the development of a protective outer layer and the division into generative and tube cells.
• Mature pollen grains are released when anthers dry and open up.

Female Reproductive Structures

• Carpals: Three carpals contain enlarged ovaries and are fused to form stigma and style.
• Ovule Formation:
  • Megasporangium forms within the ovary.
  • Integuments cover megasporangium, leaving a micropyle.
  • Diploid sporocyte in sporangium divides to form haploid megaspore cells.
  • Only one survives and develops into an embryo sac.

Pollination and Fertilization

• Pollination: Transfer of pollen to stigma via wind, insects, etc.
• Pollen grain absorbs water, swells, and produces a pollen tube with two sperm cells.
• Pollen tube grows down the style, through the micropyle, to fertilize the ovule.
• Double Fertilization:
  • One sperm fertilizes egg cell forming zygote.
  • Other sperm fuses with two polar nuclei forming primary endosperm cell.

Seed and Fruit Development

• Post-fertilization changes include withering of flower parts and ovary expansion.
• Seed Formation:
  • Zygote develops into an embryo.
  • Primary endosperm cell divides to form nutrient tissue.
• Monocotyledons vs. Dicotyledons:
  • Nutrient accumulation differs, leading to seed dormancy.
• Fruit Formation:
  • Ovary wall expands due to hormonal signals.
  • Produces fruit that aids in seed dispersal.

Conclusion

• Flowers are crucial for efficient plant reproduction.
• Diversity in flowering plants contributes significantly to ecosystems and human agriculture.
• The variety of habitats supports an extensive range of species, from large to microscopic.