Sexual Reproduction in Flowering Plants

The One Thing

Sexual reproduction in flowering plants proceeds through coordinated flower structure, gamete formation, pollination, pollen-tube growth and double fertilization. Following fertilization, the ovule develops into a seed and the ovary develops into a fruit.

Reactions, Processes and Experiments

Key Terms

• Flower: The reproductive structure of a flowering plant, made up of sepals, petals, stamens and carpels.
• Stamen: The male reproductive organ consisting of a filament and anther.
• Anther: The terminal part of the stamen that produces and releases pollen grains.
• Microsporogenesis: The formation of haploid microspores from diploid microspore mother cells through meiosis.
• Pollen grain: The male gametophyte, usually having an exine, intine, vegetative cell and generative cell.
• Carpel or pistil: The female reproductive organ consisting of stigma, style and ovary.
• Stigma: The sticky or receptive surface on which pollen grains land and germinate.
• Ovary: The basal part of the carpel containing one or more ovules.
• Ovule: A structure inside the ovary that contains the female gametophyte and develops into a seed after fertilization.
• Megasporogenesis: The formation of haploid megaspores from a diploid megaspore mother cell through meiosis.
• Embryo sac: The female gametophyte, commonly seven-celled and eight-nucleate, containing the egg apparatus, central cell and three antipodals.
• Pollination: The transfer of pollen grains from the anther to the stigma of a flower.
• Autogamy: Pollination within the same flower.
• Geitonogamy: Transfer of pollen between different flowers of the same plant.
• Xenogamy: Transfer of pollen from a flower of one plant to the stigma of a flower on a genetically different plant of the same species.
• Pollen-pistil interaction: The recognition process through which the pistil accepts compatible pollen and rejects incompatible pollen.
• Pollen tube: A tube that grows from a germinating pollen grain through the stigma and style toward the ovule.
• Fertilization: Fusion of male and female gametes to form a zygote.
• Double fertilization: The occurrence of syngamy and triple fusion in the same embryo sac, a characteristic feature of flowering plants.
• Syngamy: Fusion of one male gamete with the egg to form a diploid zygote.
• Triple fusion: Fusion of the second male gamete with the two polar nuclei to form the triploid primary endosperm nucleus.
• Endosperm: The nutritive tissue that develops from the primary endosperm nucleus and supports embryo development.
• Embryogenesis: The development of the zygote into an embryo.
• Seed: The mature ovule containing an embryo, stored food and protective coverings.
• Fruit: The mature ovary, usually formed after fertilization and containing seeds.
• Apomixis: Formation of seeds without fertilization.
• Parthenocarpy: Development of fruit without fertilization, often resulting in seedless fruits.
• Polyembryony: The formation of more than one embryo in a single seed.

Easily Confused

• Autogamy and geitonogamy: Autogamy occurs within the same flower, whereas geitonogamy occurs between different flowers of the same plant.
• Geitonogamy and xenogamy: Geitonogamy involves flowers on the same plant, whereas xenogamy involves genetically different plants of the same species.
• Pollination and fertilization: Pollination is pollen transfer from anther to stigma; fertilization is fusion of male and female gametes.
• Syngamy and triple fusion: Syngamy forms the diploid zygote through fusion with the egg; triple fusion forms the triploid primary endosperm nucleus through fusion with the two polar nuclei.
• Double fertilization and syngamy: Double fertilization includes both syngamy and triple fusion, whereas syngamy is only one of these two events.
• Apomixis and parthenocarpy: Apomixis produces seeds without fertilization, whereas parthenocarpy produces fruits without fertilization.
• Ovule and seed: The ovule develops into the seed after fertilization.
• Ovary and fruit: The ovary develops into the fruit after fertilization.
• Endosperm and embryo: Endosperm is nutritive tissue supporting embryo development; the embryo develops from the zygote.

What Gets Asked

• Flower structure: Questions may require identification of the four whorls—calyx, corolla, androecium and gynoecium—and the functions of sepals and petals. Marks are lost by confusing the male androecium with the female gynoecium.
• Anther and microsporangium structure: Questions may ask for the bilobed, dithecous anther, its four microsporangia and the four wall layers: epidermis, endothecium, middle layers and tapetum. Marks are lost by omitting tapetum as the nutritive innermost layer.
• Male and female gametophyte formation: Questions may test microsporogenesis, the two-celled pollen grain, megasporogenesis and the seven-celled, eight-nucleate embryo sac. Marks are lost by stating that the embryo sac is seven-nucleate rather than seven-celled and eight-nucleate.
• Pollination types: Questions may distinguish autogamy, geitonogamy and xenogamy. Marks are lost by treating transfer between flowers of the same plant as xenogamy.
• Pollen-pistil interaction and pollen-tube growth: Questions may ask what happens after compatible pollen reaches the stigma, including moisture absorption, germination, tube growth through the stigma and style, entry through the micropyle and release of two male gametes into a synergid. Marks are lost by treating pollination alone as sufficient for seed formation.
• Double fertilization and post-fertilization changes: Questions may require the equations n + n = 2n and n + n + n = 3n, followed by the fates of the zygote, primary endosperm nucleus, ovule and ovary. Marks are lost by reversing the relationships: the ovule becomes the seed and the ovary becomes the fruit.
• Percentage calculations: Questions may require use of the pollen viability and seed germination formulas. Marks are lost by placing the total tested or total sown in the numerator rather than the denominator.