Angiosperm Reproduction Notes

Flower Structure and Function

  • Flowers are the reproductive parts of angiosperms, attracting pollinators through bright colors, smells, and nectar.
  • Key structures include: Stigma, Style, Ovary, Anther, Filament, Petal, Sepal, Receptacle, and Ovule.

Sexual Reproduction in Angiosperms

  • Sporophytes produce microspores (pollen grains) and megaspores via meiosis in the flower.
  • Gametophytes are highly reduced; sexual reproduction occurs within the flower.
  • Stamens (microsporophylls) are the male parts, comprising the anther and filament. Anthers contain pollen sacs (microsporangia).
  • Carpels (megasporophylls) are the female parts, consisting of the stigma, style, and ovary, which contains ovules (megasporangia).

Ovule Structure

  • The ovule contains the nucellus, megasporocyte, integuments, micropyle, and funiculus.

Female Gametophyte Formation

  • In the nucellus, the megasporocyte undergoes meiosis to produce 4 megaspores (n).
  • 3 megaspores degenerate, leaving one functional megaspore.
  • The nucleus of the remaining megaspore undergoes 3 mitotic divisions, forming 8 nuclei: 3 antipodal, 2 polar nuclei, 2 synergids, and 1 egg cell.
  • The female gametophyte (embryo sac) contains only 8 nuclei and is retained within the ovule.

Male Gametophyte Formation

  • Microspores are produced in the microsporangia (pollen sacs) of the anthers via meiosis.
  • Microspores develop into pollen grains (male gametophytes).
  • Pollen is transferred to the stigma via wind, insects, water, or other animals.

Pollen Development

  • The microspore nucleus divides into a tube nucleus and a generative nucleus.
  • The generative nucleus divides into 2 sperm cells.
  • The mature male gametophyte contains 3 nuclei: 2 sperm nuclei and 1 tube nucleus.

Pollination and Fertilization

  • Pollen on the stigma forms a pollen tube, which grows down the style to the ovule through the micropyle.
  • 2 sperm cells are released into the embryo sac.
  • One sperm nucleus (n) fuses with the egg to form a zygote (2n).
  • The other sperm nucleus (n) fuses with the 2 polar nuclei (2n) to form a triploid primary endosperm nucleus (3n), a process called double fertilization.

Post-Fertilization

  • The triploid primary endosperm nucleus develops into the endosperm (3n), which serves as a food reserve.
  • Antipodals and synergids degenerate, and the micropyle closes.
  • The integuments harden, forming the seed coat; the ovule becomes the seed.
  • The ovary enlarges, forming the fruit, which contains the seeds.

Seed and Fruit Development

  • The ovule becomes the seed, containing the embryo, food store, and seed coat.
  • The ovary develops into the fruit, containing one or many seeds.
  • Seeds and fruits are dispersed by wind, insects, animals, or self-dispersal.