Sexual reproduction in angiosperms (flowering plants) is a multi-step process involving the formation of male and female gametophytes, pollination, and a unique double fertilization event that produces both the embryo and the nutritive endosperm simultaneously.
Flower Structure
A typical angiosperm flower is organized into four whorls. From outside to inside: the calyx (sepals, protective), the corolla (petals, attractive to pollinators), the androecium (male reproductive whorl comprising stamens), and the gynoecium (female reproductive whorl comprising pistils). Each stamen has a filament (stalk) and an anther (pollen-producing head). Each pistil consists of a stigma (sticky or feathery surface that receives pollen), a style (tube through which the pollen tube grows), and an ovary (containing one or more ovules).
Male Gametophyte Development (Microsporogenesis)
Inside the anther, microsporangia (pollen sacs) house diploid pollen mother cells (PMC, 2n). These PMCs undergo meiosis to produce microspore tetrads, each microspore being haploid (n). Each microspore develops by mitosis into a two-celled mature pollen grain: a larger vegetative cell (with a tube nucleus that directs pollen tube growth) and a smaller generative cell (that later divides to produce two sperm cells). The pollen wall has two layers: the outer exine, composed of sporopollenin (the most chemically and thermally resistant biological material known, withstanding strong acids, strong alkalis, high temperatures, and enzymatic degradation), and the inner intine, made of cellulose and pectin. Germ pores are thin exine regions (absent sporopollenin) through which the pollen tube emerges during germination.
Female Gametophyte Development (Megasporogenesis)
Inside each ovule, a diploid megaspore mother cell (MMC, 2n) undergoes meiosis to produce four megaspores. Three degenerate; one functional megaspore survives. This functional megaspore undergoes three rounds of free nuclear (mitotic) divisions to produce 8 nuclei. These cellularize to form the seven-celled, eight-nucleate embryo sac. The organization: egg cell + two synergids = egg apparatus at the micropylar end; one large central cell with two polar nuclei in the middle; three antipodal cells at the chalazal end. Synergids have a filiform apparatus that secretes chemotropic signals to guide the pollen tube. Antipodal cells serve a nutritive function and degenerate after fertilization. The central cell's two polar nuclei participate in triple fusion during fertilization.
Pollination
Pollination is the transfer of pollen from anther to stigma. Autogamy is self-pollination within the same flower (ensures seed set; no genetic variation). Geitonogamy transfers pollen between different flowers of the same plant — it is ecologically equivalent to cross-pollination (requires a pollination agent) but genetically equivalent to self-pollination (same genotype, no new genetic material). Xenogamy is true cross-pollination between flowers of different plants, introducing genetic variation. Plants promote xenogamy through outbreeding devices: self-incompatibility (pistil rejects own-genotype pollen), dicliny (unisexual flowers — monoecious or dioecious), and dichogamy (anthers and pistils mature at different times — protandry or protogyny). Cleistogamous flowers (e.g., Commelina benghalensis) never open and ensure obligate autogamy. Pollination agents include wind (anemophily: light pollen, feathery stigma, no colour/nectar), insects (entomophily: sticky pollen, bright petals, nectar, fragrance), water (hydrophily: mucilaginous pollen — Vallisneria, Zostera), birds (ornithophily: bright red/orange flowers, copious nectar, no fragrance — Bombax, Butea), and bats (chiropterophily: large dull flowers, strong fruity scent, nocturnal — Kigelia, Bauhinia).
Double Fertilization
After landing on a compatible stigma, the pollen grain germinates: the intine grows through a germ pore to form the pollen tube, guided by the vegetative nucleus. The generative cell divides to produce two sperm cells inside the growing tube. The tube enters the ovule through the micropyle, enters a synergid (which degenerates), and bursts to release both sperm cells. One sperm fuses with the egg cell (syngamy) to form the diploid zygote (2n). Simultaneously, the other sperm fuses with the two polar nuclei (triple fusion) to form the triploid primary endosperm nucleus (PEN, 3n). This simultaneous occurrence of syngamy and triple fusion is called double fertilization — unique to angiosperms.
Post-Fertilization Development
The zygote undergoes sequential mitotic divisions: proembryo → globular → heart → torpedo → mature embryo (containing radicle, plumule, hypocotyl, and cotyledons). The PEN (3n) undergoes repeated mitosis to form the triploid endosperm (nutritive tissue). Post-fertilization transformations: ovule → seed; integuments → seed coat (outer integument → testa; inner integument → tegmen); ovary → fruit; ovary wall → pericarp. The endosperm is triploid (3n); the embryo, seed coat, and pericarp are all diploid (2n — maternal tissue for seed coat and pericarp).
Special Reproductive Modes
Apomixis is asexual seed production without fertilization — embryos develop from diploid nucellar cells or unreduced egg cells, producing clonal offspring. It preserves hybrid vigor in agriculture. Parthenocarpy is fruit development without fertilization, producing seedless fruits (banana). Polyembryony is the presence of multiple embryos in one seed (Citrus — nucellar polyembryony).