Fertilization in Flowering Plants

Introduction

• Previous discussions on reproduction in flowering plants covered:
  • Formation of male and female gametes.
• This session focuses on the fusion of these gametes: fertilization.

Bisexual Flower

• Contains both male and female parts.
• Example used: Self-pollinating flower (male gamete can fertilize female gamete of the same flower).

Process of Fertilization

1. Pollen Release

   • Released from anthers as pollen grains.
   • Pollination: Pollen grains fall onto the female part (stigma) of the flower.
   • Pollination agents: Wind, insects (e.g., bees).

2. Pollen Grain Details

   • Acts like a bag holding male gametes.
   • Contains two haploid male gametes.

3. Female Gametes

   • Located in the ovary, within ovules.
   • Each ovule contains one female gamete (embryo sac).

4. Pollen Tube Formation

   • Male gametes travel down the style via the pollen tube.
   • Pollen tube aids in reaching the ovule.

Structure of the Embryo Sac

• Antipodal Cells: Role in fertilization unclear, believed to disintegrate.
• Synergids: Guide pollen tube with filiform structures.
• Central Cell Nuclei (Polar Nuclei): Haploid, important for forming endosperm.
• Egg Cell (Female Gamete): Haploid, fuses with male gamete.

Post-Pollen Tube Entry

• Pollen tube disintegrates after gametes release.
• Synergids disintegrate after guiding pollen tube.

Key Fertilization Events

1. Syngamy (Fertilization)

   • Male gamete fuses with egg cell to form zygote (diploid).
   • Zygote develops into an embryo.

2. Formation of Primary Endosperm Nucleus

   • Second male gamete fuses with two polar nuclei.
   • Results in triploid primary endosperm nucleus (nutritive tissue).
   • Endosperm develops before zygote to provide nourishment.

Ploidy Calculations

• Embryo Sac: All nuclei are haploid.
• Primary Endosperm Nucleus: Formed by fusion of three haploid nuclei, thus triploid (3n).

Double Fertilization

• Unique to angiosperms.
• Involves both syngamy and triple fusion.

Example Question on Chromosome Numbers

• Given: Each haploid nucleus in an embryo sac has 10 chromosomes.
• Question: Chromosome number in primary endosperm nucleus after double fertilization?
  • Calculation: 2 female nuclei (10 chromosomes each) + 1 male nucleus (10 chromosomes) = 30 chromosomes total in primary endosperm nucleus.