Reproduction and Developmental Biology in Plants
Flower to seed
The exam tests microsporogenesis, megasporogenesis, embryo sac structure, pollination, double fertilisation, endosperm, embryo and seed development, and apomixis.
Core rules
- The microspore mother cell undergoes meiosis to form a tetrad of microspores; each becomes a pollen grain.
- The megaspore mother cell undergoes meiosis to form four megaspores; usually three degenerate and one forms the embryo sac.
- The mature embryo sac is 8-nucleate and 7-celled: one egg, two synergids, three antipodals and a central cell with two polar nuclei.
- Double fertilisation is unique to angiosperms: syngamy forms the zygote and triple fusion forms the triploid primary endosperm nucleus.
- Endosperm may be nuclear or cellular; it nourishes the embryo.
- Apomixis is seed formation without fertilisation; polyembryony is the presence of more than one embryo in a seed.
Worked example
Count the cells in a mature embryo sac. Reasoning: eight nuclei are produced, but the central cell contains two polar nuclei, so the number of cells is seven: egg (1), synergids (2), antipodals (3) and central cell (1). This is why the embryo sac is described as 8-nucleate and 7-celled.
Traps
Double fertilisation does not mean two zygotes. Endosperm is usually triploid, not diploid. Pollen may be shed at the 2-celled or 3-celled stage. Apomixis produces seeds without fertilisation; it is not the same as vegetative propagation.