Chapter 6: Sexual Reproduction in Flowering Plants

6.1 Structure of a Flower

Flowers are the reproductive structures of angiosperms, containing male and female floral organs essential for sexual reproduction.

1. Floral Structures and Functions

• Pedicel: Flower stalk that provides physical support and connects the flower to the stem.
• Receptacle: Swollen terminal end of the pedicel that bears and holds floral whorls.
• Sepals (Calyx): Outermost green leaf-like structures that protect the flower bud prior to opening.
• Petals (Corolla): Brightly colored, scented structures (often with nectaries) that attract insect and bird pollinators.
• Stamen (Androecium - Male Reproductive Organ):
  • Anther: Lobed sac containing microsporangia (pollen sacs) where pollen grains develop via meiosis.
  • Filament: Slender stalk that elevates and exposes the anther to wind or pollinators.
• Pistil / Carpel (Gynoecium - Female Reproductive Organ):
  • Stigma: Sticky receptive surface at the top of the style that captures pollen grains.
  • Style: Elongated neck structure connecting stigma to ovary, supporting pollen tube growth.
  • Ovary: Swollen basal structure housing one or more ovules.
  • Ovule: Structure inside ovary containing the megasporangium where the embryo sac develops.

6.2 Formation of Pollen Grains and Embryo Sac

1. Formation of Pollen Grains (Microsporogenesis)

1. Inside the anther, each pollen sac contains numerous diploid ($2n$) microspore mother cells (microsporocytes).
2. Each microspore mother cell undergoes meiosis to produce four haploid ($n$) microspores (a microspore tetrad).
3. Each haploid microspore undergoes mitosis to form a pollen grain containing two nuclei:
   • Tube nucleus: Directs the growth of the pollen tube down the style.
   • Generative nucleus: Later divides by mitosis to form two male gametes (sperm cells).
4. The pollen wall thickens, forming a tough, sculptured outer wall (exine) and inner wall (intine).

2. Formation of the Embryo Sac (Megasporogenesis)

1. Inside the ovule, a single diploid ($2n$) megaspore mother cell (megasporocyte) undergoes meiosis to yield four haploid ($n$) megaspores.
2. Three of the four megaspores degenerate, leaving one functional megaspore.
3. The functional megaspore undergoes three consecutive mitotic divisions without cytokinesis, producing 8 haploid nuclei within a single cell.
4. Cytokinesis reorganizes these 8 nuclei into 7 distinct cells forming the mature embryo sac:
   • 3 Antipodal cells: Located at the chalazal end (opposite the micropyle).
   • 2 Polar nuclei: Located in a large central cell.
   • 1 Egg cell (female gamete): Positioned at the micropylar end.
   • 2 Synergid cells: Flanking the egg cell at the micropylar end to guide the pollen tube.

6.3 Pollination and Development of Tube to Double Fertilization

1. Pollination

Transfer of pollen grains from an anther to a mature sticky stigma of a flower (Self-pollination or Cross-pollination).

2. Growth of Pollen Tube and Double Fertilization

1. Sucrose solution secreted by the stigma stimulates pollen germination.
2. The tube nucleus secretes digestive enzymes (cellulase and pectinase) to digest style tissues, creating a path for the growing pollen tube toward the micropyle.
3. The generative nucleus follows behind the tube nucleus down the style and undergoes mitosis to form two haploid male gametes ($n$).
4. Upon reaching the ovule, the pollen tube enters through the micropyle, penetrates a degenerating synergid cell, and releases both male gametes into the embryo sac. The tube nucleus disintegrates.
5. Double Fertilization Process:
   • First Fertilization: One haploid male gamete ($n$) fuses with the haploid egg cell ($n$) to form a diploid zygote ($2n$).
   • Second Fertilization: The second haploid male gamete ($n$) fuses with the two polar nuclei ($2n$) in the central cell to form a triploid endosperm nucleus ($3n$).

6.4 Development of Seeds and Fruits

1. Post-Fertilization Structural Changes

• Diploid Zygote ($2n$): Undergoes mitosis to develop into the plant embryo (consisting of plumule, radicle, and cotyledon/s).
• Triploid Endosperm Nucleus ($3n$): Undergoes rapid mitosis to form endosperm tissue, a nutrient storage tissue (rich in starch, lipids, and proteins) that nourishes the developing embryo.
• Ovule: Develops into the seed; integuments harden to form the protective seed coat (testa).
• Ovary: Enlarges and develops into the fruit; ovary wall matures into the pericarp (exocarp, mesocarp, and endocarp).
• Floral Parts: Stigma, style, stamens, petals, and sepals usually wither, dry up, and drop off.

2. Importance of Double Fertilization

• Ensures endosperm tissue develops only when an egg cell has been successfully fertilized, preventing waste of plant metabolic resources.
• Triploid endosperm provides a specialized nutritive reserve tailored for embryo survival during seed dormancy and early germination.