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Explore the stages of pollen grain and embryo sac development in flowering plants. Understand the processes of meiosis, mitosis, and fertilization leading to seed formation.
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Learning Objectives • Outline pollen grain development from microspore mother cells • Outline embryonic sac development
Pollen grain development • An anther consists of four chambers called pollen sacs Pollen Sac
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
Fibrous Layer Tapetum Pollen
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
Pollen Grain development Microspore mother cell (2n) Tetrad of pollen Meiosis Separation Pollen Grains (n)
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
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
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
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?
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
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)
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)
Two of these form the polar nuclei in the embryo sac • The remaining female gamete forms an egg cell
Megaspore mother cell (2n) Meiosis Embryo sac (n) Mitosis (3 times) Polar nuclei Egg cell
Recall • Pollen grains divide by mitosis producing two haploid nuclei: • Tube nucleus – forms the pollen tube • Generative nucleus – form the male gametes
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)
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
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
Depth of treatment • Pollen grain development from microspore mother cells: meiotic division, mitotic division, generative and tube nuclei production, formation of pollen grain
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.
