Section 1: Flower Structure and Gametophyte Development
A typical angiosperm flower has four whorls: calyx, corolla, androecium, and gynoecium. The androecium's stamens (filament + anther) produce pollen through microsporogenesis: pollen mother cells (2n) in microsporangia undergo meiosis to form haploid microspore tetrads; each microspore becomes a two-celled pollen grain (vegetative cell with tube nucleus + generative cell). The pollen wall comprises exine (sporopollenin, highly resistant outer layer) and intine (cellulose/pectin, inner, forms pollen tube). Germ pores are exine-free zones for pollen tube emergence.
The gynoecium's pistils (stigma + style + ovary) contain ovules where megasporogenesis occurs: megaspore mother cells (2n) undergo meiosis to form four megaspores — three degenerate, one functional megaspore undergoes three free nuclear mitotic divisions to form the 7-celled, 8-nucleate embryo sac. Structure: egg + 2 synergids (micropylar end), central cell (2 polar nuclei), and 3 antipodals (chalazal end).
Section 2: Pollination
Pollination is pollen transfer from anther to stigma. Types: autogamy (same flower, no variation), geitonogamy (same plant, different flower — genetically self, ecologically cross, no new variation), xenogamy (different plants — true cross-pollination, produces variation). Outbreeding devices — self-incompatibility, dicliny, dichogamy — promote xenogamy. Cleistogamy (Commelina) ensures obligate self-pollination. Pollination agents: wind (light pollen, feathery stigma), insects (bright petals, nectar, sticky pollen), water (mucilaginous pollen — Vallisneria), birds (red flowers, copious nectar — Bombax), bats (nocturnal, strong scent — Kigelia).
Section 3: Double Fertilization
After germination on the stigma, the pollen tube grows through the style (guided by the vegetative nucleus) and enters the embryo sac through the micropyle via a synergid. Two sperm cells are released. Syngamy: sperm + egg = zygote (2n). Triple fusion: sperm + 2 polar nuclei = PEN (3n). Together, these constitute double fertilization — unique to angiosperms. The zygote develops into the embryo; the PEN develops into the triploid endosperm.
Section 4: Post-Fertilization and Special Modes
Post-fertilization: ovule → seed; ovary → fruit; integuments → seed coat (testa + tegmen); PEN → endosperm (3n); zygote → embryo (2n). Embryo stages: globular → heart → torpedo → mature (radicle, plumule, cotyledons, hypocotyl). Special modes: apomixis (seeds without fertilization, agricultural value for hybrid vigor), parthenocarpy (seedless fruits — banana), polyembryony (multiple embryos — Citrus).