Chapter Summary and Practice

A complete account from flower to seed

Sexual reproduction in flowering plants coordinates the production of haploid gametophytes with protected transfer of male gametes to female gametophytes. The androecium contains stamens bearing anthers; the gynoecium contains one or more carpels enclosing ovules. Flower initiation, organ differentiation, meiosis, pollination, recognition and fertilisation are all distinct processes. A student can explain the chapter as a developmental sequence only by tracking where each event occurs, which cells participate and whether each nucleus is diploid, haploid or triploid.

A typical immature anther has two lobes with four microsporangia in total. Each microsporangium is bounded by epidermis, endothecium, middle layers and tapetum. Diploid pollen mother cells in the sporogenous tissue undergo meiosis, yielding haploid tetrads. Microspores separate and mature into pollen grains. The pollen exine contains highly resistant sporopollenin and bears germination apertures; the intine lies inside. A two-celled grain has vegetative and generative cells. A three-celled grain has a vegetative cell and two male gametes because the generative cell has already divided.

Inside the ovary, an ovule attaches to the placenta by its funicle. The hilum is the junction with the ovule body; integuments surround the nucellus while leaving the micropyle open. A diploid megaspore mother cell undergoes meiosis to create four haploid megaspores, usually only one of which is functional in common monosporic development. Three free-nuclear mitoses yield eight nuclei, followed by cell organisation. The characteristic seven-cell female gametophyte contains one egg, two synergids, three antipodals and a central cell with two polar nuclei. The synergids' filiform apparatus helps direct a pollen tube to the egg apparatus.

Pollination pathways and selective reproductive interactions

Pollination merely deposits pollen on a stigma. Autogamy transfers pollen within one flower; geitonogamy transfers pollen between flowers of the same plant; xenogamy moves pollen between different plants. Cleistogamous flowers remain closed and can self-pollinate without external agents, while chasmogamous flowers open and expose the reproductive structures. Wind, water and animal pollination each favour different floral traits: exposed stamens and feathery stigmas in many wind-pollinated grasses, water-surface or submerged pollen transport in selected aquatic plants, and visual or chemical rewards in many animal-pollinated plants.

Outbreeding devices can discourage self-fertilisation. Different times of pollen shedding and stigma receptivity, spatial separation of stamens and stigma, genetically encoded self-incompatibility and separate sexes are distinct mechanisms. Monoecious plants have separate male and female flowers on the same individual, whereas dioecious plants separate male and female flowers between plants. When compatible pollen reaches a receptive stigma, a pollen tube grows through the style and commonly enters an ovule via the micropyle and a synergid. In controlled crosses, emasculation removes anthers before release from a bisexual female-parent flower; bagging prevents unintended pollen until chosen pollen is applied.

Two fertilisation products develop into different tissues

The pollen tube delivers two male gametes into the female gametophyte. Syngamy joins one gamete (n) with the egg (n), producing the zygote (2n). Triple fusion unites the other male gamete (n) with the central cell's two polar nuclei (n + n), producing the primary endosperm nucleus, usually 3n. These two events are the distinctive double fertilisation of flowering plants. The resulting endosperm provides nutrition, frequently undergoing an early free-nuclear stage before cellularisation. The embryo develops from the zygote, passing through early proembryo, globular and other stages as its axis and cotyledons are established.

Dicot embryos typically contain two cotyledons, an epicotyl with plumule and a hypocotyl ending in a radicle. Grass embryos typically have a single cotyledon, the scutellum, and protective sheaths called coleoptile around the embryonic shoot and coleorrhiza around the embryonic root. In a mature seed, food reserves may remain in the endosperm, reside in cotyledons or include residual nucellar perisperm. The ovule becomes the seed, its integuments mature into the seed coats, and the ovary develops into a fruit whose wall is called the pericarp.

The exceptions reveal what each reproductive process means

A fruit sometimes develops from the ovary together with accessory floral tissues; apple flesh is a familiar example of such an accessory fruit. Parthenocarpy describes fruit formation without fertilisation, often producing seedlessness. Apomixis describes formation of embryos and seeds without the usual egg–sperm fusion, for example from unreduced egg cells or maternal nucellar tissues. Polyembryony describes more than one embryo in a seed and need not imply that all embryos formed in the same manner. These phenomena should not be equated: fruit formation, embryo origin and embryo number are separate biological properties.

Follow developmental origin when interpreting diagrams

A labelled ovule diagram must put the micropyle opposite the chalaza and show the funicle connecting to maternal placental tissue. A mature embryo-sac diagram should place three antipodals at the chalazal pole, the egg and two synergids at the micropylar pole, and two polar nuclei within one central cell. A pollen-tube diagram should trace movement through the stigma and style into the micropyle rather than showing free sperm swimming externally. Good biological diagrams are evidence for a mechanism; labels and polarity are as important as colour or decorative detail.

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