Reproduction in Plants: Flower, Pollination, Fertilization, Seed and Fruit
Most questions here are counting questions - chromosomes through the sporogenesis pathway - plus naming the cells of the embryo sac and matching fruit parts to their origins.
Core rules
- Microsporogenesis: pollen mother cell (2n) → meiosis → tetrad of microspores (n) → pollen grain, released 2-celled in most plants and 3-celled in some.
- Megasporogenesis: one megaspore mother cell → meiosis → four megaspores, three degenerate; the functional one undergoes three mitotic divisions to give an 8-nucleate, 7-celled Polygonum-type embryo sac - egg, two synergids with filiform apparatus, three antipodals, and a central cell with two polar nuclei.
- Pollination: self versus cross; agents include anemophily, hydrophily (Vallisneria, Zostera), entomophily, ornithophily and chiropterophily, each with recognisable adaptations such as feathery stigmas, pollenkitt and cleistogamy.
- Double fertilization: syngamy gives the zygote (2n); triple fusion gives the primary endosperm nucleus (3n) and then the endosperm. Endosperm types: nuclear, cellular, helobial.
- Fruit develops from the ovary wall (pericarp); false fruits develop with the thalamus, as in apple and cashew. Parthenocarpy is fruit without fertilization; apomixis is seed without fertilization.
Worked example
If n = 12 in the pollen grain, then the zygote is 2n = 24, the primary endosperm nucleus is 3n = 36, an antipodal cell is n = 12, the integument is 2n = 24 and the pericarp is 2n = 24 because it arises from the ovary wall.
Traps that cost marks
Calling the central cell diploid before fusion, although its two polar nuclei are each haploid until triple fusion; saying all endosperm is triploid (Gnetum is diploid); attributing the fleshy part of an apple to the ovary.