Structure and Pollen Formation #
- Pollen: Tiny grains produced in the flower’s anther.
- Anther: Part of the flower’s stamen; contains pollen.
- Microsporangia: Small compartments in anther, each with pollen mother cells.
- Microsporogenesis: Process of pollen formation.
Stages
- Microspore Mother Cell → Meiosis → Four Microspores (haploid).
- Microspore → Divides → Generative Cell (forms sperm) + Tube Cell.
- Pollen Wall Formation: Exine (outer, tough) + Intine (inner, soft).
- Maturation: Generative Cell forms two sperm cells.
- Dispersion: Pollen released from anther for fertilization.
Ovule Development #
- Ovule: Female part in flower’s ovary; becomes a seed after fertilization.
- Megaspore Mother Cell: Makes megaspores through meiosis.
- Embryo Sac: Female gametophyte formed from surviving megaspore.
- Cells in Embryo Sac:
- Antipodal Cells (3), Synergids (2), Egg Cell (1).
- Central Cell with Polar Nuclei (fuse later).
Double Fertilization #
- Angiosperms: Flowering plants with double fertilization.
- Two Fertilization Events:
- Sperm + Egg → Zygote (future plant).
- Sperm + Polar Nuclei → Endosperm (nutrient-rich tissue).
Steps
- Pollen lands → Tube grows → Sperm released.
- Sperm + Egg → Zygote.
- Sperm + Polar Nuclei → Endosperm.
Significance of Double Fertilization #
- It leads to formation of nutritive endosperm which provides food for developing embryo.
- It allows for coevolution of endosperm and embryo with parent plant for better viability.
- It is considered an evolutionary adaptation that gave reproductive advantage to flowering plants.
- It is unique to angiosperms and triples the ploidy of primary endosperm nucleus, increasing amount of stored nutrients.
Describe the structural changes that occur after fertilisation, leading to the development of the seed and fruit. #
- zygote divides to become embryo;
- seed:
- cotyledons, plumule, radicle;
- mitosis;
- ref (single) basal cell;
- suspensor/foot, forms;
- endosperm nucleus forms endosperm;
- endosperm develops into food store of cotyledons;
- endosperm persists in some cases;
- nucellus disappears;
- changes in micropyle/described;
- integuments become testa;
- aleurone layer develops;
- becomes, dry/harder/waterproof
- Fruit:
- ovary wall becomes pericarp;
- modified for dispersal;
- dries/becomes fleshy
Gametogenesis in Plants #
- In males
- Takes place in the anther;
- Pollen mother cells divide by meiosis forming 4 haploid cells;
- Each of these cells divide by mitosis;
- cytokinesis does not take place resulting in a cell with two haploid nuclei;
- These cells mature into pollen grain;
- one of the nuclei is the tube nucleus and the other is generative nucleus
- In females
- Takes place in the ovules
- Similar to the male, Spore mother cell divides by meiosis to produce four haploid cells
- All but one degenerates, this cell develops into an embryo sac;
- Embryo sac divides by mitosis 3 times forming 8 haploid nuclei, of which one becomes the female gamete
State the structural features that are characteristic of a wind-pollinated flower and explain how each feature contributes to successful pollination. #
- lots of pollen grains made
- so more chance of pollination;
- pollen grains, light/ smooth/ aerodynamic,
- so easily transported;
- no/ small, petals/ corolla/ perianth,
- so stamens/ anthers/ stigma, outside of flower;
- long filaments
- so anthers outside of flower;
- anthers outside of flower/ versatile anthers,
- so pollen released;
- long style
- so stigma outside of flower;
- stigma outside of flower/ stigma has large surface area,
- so traps pollen
