Sexual Reproduction in Flowering Plants

The One Thing

Sexual reproduction in flowering plants depends on the coordinated formation of male and female gametophytes, pollination, compatible pollen–pistil interaction and fertilisation. In angiosperms, double fertilisation produces both a diploid zygote and triploid endosperm; subsequently, the ovule develops into a seed and the ovary into a fruit.

Reactions, Processes and Experiments

Key Terms

• Flower: The reproductive structure of an angiosperm, consisting mainly of sepals, petals, stamens and carpels.
• Stamen: The male reproductive organ made up of a filament and anther.
• Anther: The pollen-producing part of the stamen, usually containing four pollen sacs or microsporangia.
• Microsporogenesis: The formation of haploid microspores from diploid microspore mother cells by meiosis.
• Pollen grain: The male gametophyte, usually consisting of a vegetative cell and a generative cell at the time of release.
• Carpel or pistil: The female reproductive organ consisting of stigma, style and ovary.
• Ovary: The basal swollen part of the carpel that contains one or more ovules.
• Ovule: The structure inside the ovary that contains the female gametophyte and develops into a seed after fertilisation.
• Megasporogenesis: The formation of haploid megaspores from a diploid megaspore mother cell by meiosis.
• Embryo sac: The female gametophyte, typically consisting of seven cells and eight nuclei.
• Typical embryo sac: A monosporic, seven-celled and eight-nucleate structure with an egg apparatus, two polar nuclei and three antipodal cells.
• Egg apparatus: The group at the micropylar end consisting of one egg cell and two synergids.
• Synergids: Two cells beside the egg that help guide the pollen tube; each contains a filiform apparatus.
• Polar nuclei: Two nuclei in the central cell that may fuse to form the diploid secondary nucleus.
• Antipodals: Three cells located at the chalazal end of the embryo sac.
• Pollination: The transfer of pollen grains from the anther to the stigma.
• Autogamy: Pollination between the anther and stigma of the same flower.
• Geitonogamy: Transfer of pollen between different flowers of the same plant; it is functionally cross-pollination but genetically similar to self-pollination.
• Xenogamy: Transfer of pollen from a flower of one plant to a flower of another genetically different plant of the same species.
• Outbreeding devices: Features such as unisexuality, dichogamy, herkogamy, self-incompatibility and spatial separation that reduce self-pollination.
• Pollen-pistil interaction: The recognition process by which the stigma 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.
• Fertilisation: Fusion of male and female gametes to form a zygote.
• Double fertilisation: A characteristic angiosperm process involving syngamy and triple fusion in the same embryo sac.
• 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 formed after triple fusion that supports the developing embryo.
• Embryo: The young sporophyte formed from the zygote, usually developing into a radicle, plumule and cotyledons.
• Seed: A mature ovule containing an embryo, stored food or endosperm, and a protective seed coat.
• Seed coat: The protective covering of a seed, mainly formed from the integuments of the ovule.
• Fruit: The mature ovary, sometimes along with other floral parts, formed after fertilisation.
• Apomixis: Formation of seeds without fertilisation.
• Polyembryony: The occurrence of more than one embryo in a single seed.

Easily Confused

• Pollination and fertilisation: Pollination transfers pollen from anther to stigma; fertilisation is the fusion of male and female gametes.
• Autogamy and geitonogamy: Autogamy occurs within the same flower; 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.
• Syngamy and triple fusion: Syngamy forms the diploid zygote by fusion with the egg; triple fusion forms the triploid primary endosperm nucleus by fusion with two polar nuclei.
• Zygote and primary endosperm nucleus: The zygote is diploid and develops into the embryo; the primary endosperm nucleus is triploid and develops into endosperm.
• Ovule and seed: The ovule develops into the seed after fertilisation.
• Ovary and fruit: The ovary develops into the fruit after fertilisation.
• Apomixis and parthenocarpy: Apomixis produces seeds without fertilisation; parthenocarpy produces fruits without fertilisation.
• Albuminous and non-albuminous seeds: Albuminous seeds retain endosperm at maturity; non-albuminous seeds generally use most of their endosperm during embryo development.
• Dicot and monocot embryos: Dicot embryos have two cotyledons; monocot embryos generally have one cotyledon called the scutellum.

What Gets Asked

• Identify the structures and functions of a flower: Questions may require the four whorls—calyx, corolla, androecium and gynoecium—or the parts of the stamen, anther, carpel and ovule. Marks are lost by confusing the anther with the entire stamen or the ovary with the ovule.
• Describe microsporogenesis and the structure of pollen: Answers should state that diploid microspore mother cells undergo meiosis to form haploid microspore tetrads, and should distinguish the sporopollenin-rich exine from the cellulose- and pectin-rich intine.
• Describe megasporogenesis and the typical embryo sac: The required details include one functional megaspore, degeneration of the other three, and the seven-celled, eight-nucleate arrangement: three antipodals, two synergids, one egg and one central cell with two polar nuclei.
• Distinguish types of pollination: Autogamy, geitonogamy and xenogamy must be identified by the source and destination of the pollen. A common error is to call geitonogamy genetically equivalent to xenogamy; geitonogamy is genetically similar to self-pollination.
• Explain double fertilisation using equations: Syngamy must be written as n + n = 2n, while triple fusion must be written as n + n + n = 3n. Reversing the products or omitting the two polar nuclei loses marks.
• Trace post-fertilisation changes: The expected sequence is zygote to embryo, primary endosperm nucleus to endosperm, ovule to seed, integuments to seed coat and ovary to fruit. Confusing the ovule–seed relationship with the ovary–fruit relationship is a frequent error.