1. Sexual reproduction in the flowering plant (Extended study)

2. Learning Objectives
Outline pollen grain development from microspore mother cells
Outline embryonic sac development

3. Pollen Grain development

4. Pollen grain development
An anther consists of four chambers called pollen sacs
Pollen Sac

5. Each pollen sac is protected by a fibrous layer
Inside the fibrous layer is the tapetum – a food store that supplies energy for cell divisions in the pollen sac
Fibrous Layer
Tapetum
Pollen

6. Fibrous Layer
Tapetum
Pollen

7. Pollen Grain development
Microspore mother cells (2n) are located in the pollen sacs
They are diploid
They divide by meiosis to produce a group of four haploid cells called a tetrad
Each tetrad breaks up to form four separate haploid pollen grains

8. Pollen Grain development
Microspore mother cell (2n)
Tetrad of pollen
Meiosis
Separation
Pollen Grains (n)

10. Pollen Grain development
Pollen grains divide by mitosis producing two haploid nuclei:
Tube nucleus – forms the pollen tube
Generative nucleus – form the male gametes
Tube nucleus
Mitosis
Generative nucleus

11. Pollen Grain development
When pollen grains have matured the walls of the anther dry and split
Pollen grains are then exposed and are ready for pollination

12. Pollen Grain development
After pollination the generative nucleus (n) divides by mitosis to form two sperm nuclei (n)
These will each play a role in double fertilisation

13. Pollen Grain development

14. Learning Check How many pollen sacs does an anther have?
What form of cell division takes place in a pollen mother cell?
What is the group of four pollen cells called before they separate?
What form of cell division takes place to produce the generative nucleus and tube nucleus?

15. Embryo sac development

16. Each ovary contains one or more ovules
An ovary has two walls called integuments
Between the integuments is a small opening (micropyle) through which a pollen tube can enter.
The nucellus provides nutrients for growth in the ovule
nucellus
Integuments
micropyle

17. Megaspore mother cell (2n)
One cell in the ovule, the megaspore mother cell (2n) divides by meiosis to form four haploid cells
Three of these cells disintegrate
The remaining cell is called the embryosac.
Meiosis
Embryo sac (n)

18. Gametes Polar nuclei (n)
The nucleus of the embryo sac (n) divides by mitosis three times forming eight haploid nuclei
These are still contained within the embryo sac
Five of these nuclei will later disintegrate
The remaining three are the gametes
Gametes Polar nuclei (n)
Gamete – Egg cell
(2n)

19. Two of these form the polar nuclei in the embryo sac
The remaining female gamete forms an egg cell

20. Megaspore mother cell (2n)
Meiosis
Embryo sac (n)
Mitosis (3 times)
Polar nuclei
Egg cell

22. Recall Pollen grains divide by mitosis producing two haploid nuclei:
Tube nucleus – forms the pollen tube
Generative nucleus – form the male gametes

23. Events leading to fertilisation
Once pollination has occurred the pollen tube nucleus forms the pollen tube which moves down the style to the ovule in the ovary
The tube nucleus moves towards chemicals released from the ovule (chemotropism)

24. Events leading to fertilisation
The generative nucleus (n) divides by mitosis as it moves down the pollen tube
Two haploid sperm nuclei result
These are the male gametes
On reaching the embryo sac a double fertilisation will take place

26. Fertilisation
One sperm nucleus (n) fuses with the egg nucleus (n) to form a diploid zygote. This will develop into an embryo
The second sperm nucleus (n) joins with the two polar nuclei (2n) forming a triploid (3n) endosperm nucleus

27. Syllabus

28. Depth of treatment
Pollen grain development from microspore mother cells: meiotic division, mitotic division, generative and tube nuclei production, formation of pollen grain

29. Depth of treatment
Embryo sac development: megaspore mother cell, meiotic division, cell disintegration, mitotic division in the production of eight cells of the embryo sac, one of which is the egg cell.