Chapter 25 – Cones and Flowers and Reproductive Organs

Cones, Flowers, and Reproductive Organs

Introduction

  • Seed plants utilize cones (gymnosperms) and flowers (angiosperms) for sexual reproduction.
  • Plants exhibit alternation between sporophyte and gametophyte generations.
  • Cones, flowers, and seeds are crucial for enabling seed plants to reproduce without water.

Gymnosperm Life Cycle

  • Trees produce both male and female cones.
  • Pollen is released from male cones and carried by wind to female cones.
  • The pollen grain germinates, forming a pollen tube that grows into the ovule.
  • Sperm travels through the pollen tube to fertilize the egg.
  • The resulting zygote develops into an embryo encased within a seed.

Flower Structure

  • Typical flower produces both male and female gametes.
  • Sepals: Green, leaf-like structures that protect the developing flower.
  • Calyx: Collective term for all the sepals of a flower.
  • Petals: Usually the second circle of flower parts, often brightly colored to attract pollinators.
  • Corolla: Collective term for all the petals of a flower.
  • Stamens: Male reproductive structures, each consisting of an anther and a filament.
  • Anther: Contains microsporangia that produce pollen.
  • Filament: Long, thin stalk supporting the anther.
  • Carpels: Innermost circle of flower parts, containing the pistil.
  • Pistil: Female reproductive structure, consisting of the ovary, style, and stigma.
  • Ovary: Base of the pistil, containing the ovules.
  • Style: Stalk connecting the stigma and the ovary.
  • Stigma: Top of the style, often sticky to capture pollen.

Female Gametophyte

  • Ovules: Located inside the ovary, containing the embryo sac.
  • Embryo sac: Membrane containing eight nuclei.
  • Polar nuclei: The two central nuclei within the embryo sac.
  • Egg nucleus: One of the eight nuclei, which enlarges and awaits fertilization.

Male Gametophyte

  • Pollen chambers: Located inside the anthers, produce pollen grains.
  • Pollen grains: Produce two haploid nuclei: the tube nucleus and the generative nucleus.
  • Tube nucleus: One of the haploid nuclei, which eventually disintegrates.
  • Generative nucleus: The other haploid nucleus, which divides to form two sperm cells.

Pollination

  • Pollination: The transfer of pollen from the anther to the stigma.
  • Self-pollination: Pollen transfer occurs within the same flower.
  • Cross-pollination: Pollen transfer occurs between different flowers.

Fertilization in Angiosperms

  • Pollen grain lands on the stigma and grows a pollen tube.
  • The generative nucleus divides, forming two sperm nuclei within the pollen grain.
  • Sperm cells travel down the style to the ovary and ovule.
  • Double fertilization:
    • One sperm fuses with the egg nucleus to form the zygote.
    • The other sperm fuses with the two polar nuclei to form a triploid nucleus, known as the endosperm (provides food for the embryo).

Seed Formation

  • Seeds protect the embryo from environmental stressors such as dryness and salinity.
  • Seeds provide nourishment and protection for the developing embryos.

Seed Structure

  • Cotyledon: Seed leaves containing stored food for germination.
  • Radicle: The first attachment point to the cotyledon that develops into the plant's primary root.
  • Hypocotyl: Located between the radicle and the epicotyl.
  • Epicotyl: Located above the hypocotyl and will develop into the plant’s stem.

Seed Germination

  • Various parts of the seedling are named according to their attachment point to the cotyledon.