flowers lecture 13

Sexual Reproduction in Plants

• Overview: Sexual reproduction is vital for the propagation of plant species, involving the alternation between haploid and diploid generations.

Key Components

• Common Ancestors: Land plants, such as liverworts, mosses, and ferns, share common ancestors that contributed to the diversity of modern plants.

• Stomata: Small openings that facilitate gas exchange.

• Sporophyte and Gametophyte:

  • Sporophyte:

    • Diploid (2n).

    • Multicellular and produces spores through meiosis.

  • Gametophyte:

    • Haploid (n).

    • Multicellular and produces gametes through mitosis.

Evolutionary Lineage of Plants

• Lycophytes: Includes club mosses and relatives.

• Monilophytes: Includes ferns and horsetails.

• Seed Plants: Include gymnosperms and angiosperms.

• Reproductive Processes:

  • Meiosis leads to the formation of haploid spores.

  • Fertilization results in a diploid zygote, which develops into a sporophyte.

Gametophyte and Sporophyte Nutrition

• Nutritional Independence: In ferns, sporophytes and gametophytes are nutritionally independent from each other.

• The seed plant sporophyte nourishes developing gametophytes, essential for sexual reproduction.

Angiosperm Sexual Reproduction

• Process:

  • Male gametophyte develops into pollen.

  • Female gametophyte develops into an embryo sac.

• Flower Structure:

  • Sepals: Protect bud.

  • Petals: Attract pollinators.

  • Stamens: Produce pollen.

  • Pistil: Contains ovules to produce embryo sacs.

• Floral Organization: Arranged in whorls for efficient reproduction.

Fertilization and Embryo Development

• Double Fertilization:

  • One sperm fuses with the egg to form a zygote.

  • The second sperm fuses with the polar nuclei, contributing to the formation of the triploid endosperm (nutritive tissue).

• Developmental Stages:

  • Gametophyte Development: Microspores develop into pollen; megaspores form the embryo sac.

  • Fertilization leads to the formation of seeds.

Pollination Mechanisms

• Types:

  • Wind Pollination: No need for attraction.

  • Animal Pollination: Involves attracting pollinators like bees and birds to transfer pollen.

• Pollen Tube Growth: The pollen tube carries sperm to the ovule, guided by gene expression.

Genetic Variability in Pollination

• Outcrossing: Encouraged via external pollination to avoid inbreeding, which can accumulate defects.

• Self-incompatibility: Prevents fertilization if the pollen is genetically too similar, promoting genetic diversity.

Embryogenesis and Fruit Development

• Embryo Formation:

  • The zygote develops into an embryo while reliant on maternal tissue.

  • Morphogenesis leads to the organization of cotyledons and apical meristems.

• Fruit Formation:

  • The ovary wall matures into a fruit that aids in seed dispersal.

  • Various strategies like fleshy or dry fruits to address different dispersal mechanisms.

Dormancy and Germination

• Dormancy: Embryo enters a dormant phase to wait for favorable conditions, with hormonal regulation (e.g., abscisic acid).

• Germination: Triggered by gibberellin, facilitating the breakdown of endosperm and supporting seedling growth.