Biology · Ch 1 — Sexual Reproduction in Flowering Plants
Summary
Summary
- Structure of a flower: The male reproductive unit is the stamen (anther + filament); the female unit is the pistil/carpel (stigma, style, ovary). The anther is a dithecous structure with four microsporangia (two per lobe). Inside each microsporangium, sporogenous tissue undergoes meiosis to form microspore tetrads.
- Microsporogenesis & male gametophyte: Each microspore (pollen grain) has two cells — a small generative cell and a large vegetative cell. The generative cell divides mitotically to form two male gametes (sperm cells). Pollen grains are shed at the 2-celled or 3-celled stage. The outer wall (exine) is made of sporopollenin — the most resistant organic material known.
- Megasporogenesis & female gametophyte: In the ovule, one megaspore mother cell undergoes meiosis to produce four haploid megaspores; three degenerate, one functional megaspore develops into the embryo sac (female gametophyte). The typical Polygonum-type embryo sac has 7 cells and 8 nuclei: egg apparatus (2 synergids + 1 egg), 3 antipodal cells, and a central cell with two polar nuclei.
- Pollination: Transfer of pollen from anther to stigma. Self-pollination (autogamy, geitonogamy) vs. cross-pollination (xenogamy). Agents: wind (anemophily), water (hydrophily), insects/animals (entomophily/zoophily). Adaptations include nectar, scent, colour, and pollenkitt.
- Double fertilisation: A unique feature of angiosperms. One male gamete fuses with the egg cell → zygote (2n). The other male gamete fuses with the two polar nuclei → triploid primary endosperm nucleus (3n). This is double fertilisation; the product is a triploid endosperm.
- Post-fertilisation events: The zygote develops into an embryo (with suspensor, radicle, plumule, cotyledons). The primary endosperm nucleus divides to form endosperm (nuclear type — free nuclei; or cellular type — wall formation). The ovule becomes the seed; the ovary becomes the fruit. …