formation of pollen grains, embryo sac, and process of double fertilisation SPM notes

Formation of pollen grains

1.     Pollen grains are formed in the anther

2.     An anther has 4 pollen sacs

3.     Each pollen sac contains hundreds of cells called pollen mother cells

4.     Each pollen mother cell undergoes meiosis to form 4 haploid microspores

5.     The nucleus of each microspore divides by mitosis to form a tube nucleus and a generative nucleus

6.     The microspores develop into pollen grains

Formation of embryo sac

1.     Ovule develops from ovarian tissue

2.     Ovule has diploid embryo sac mother cell

3.     Embryo sac mother cell undergoes meiosis to form a row of four haploid cells called megaspores

4.     3 of 4 megaspores degenerate, leaving 1 in the ovule

5.     Megaspore continues to grow and enlarges

6.     Nucleus of megaspore undergoes mitosis 3 times to form 8 haploid nuclei

7.     3 of the 8 nuclei migrate to one end to form antipodal cells

8.     2 move to the centre to form polar nuclei

9.     1 of the 3 nuclei nearest to the micropyle develops into an egg cell flanked by 2 synergid cells

10.  The structure formed is called an embryo sac

Process of Double Fertilisation

When the pollen grain lands on the stigma, fertilisation is initiated.

Sucrose solution secreted by the stigma stimulates the pollen grain to germinate and form a pollen tube.

The pollen tube grows down the style towards the ovule.

The generative nucleus divides by mitosis into two male nuclei.

The male nuclei move down the pollen tube led by the tube nucleus.

When the pollen tube reaches the ovary, it penetrates the ovule through the micropyle.

The tube nucleus disintegrates, leaving a clear passage for the male nuclei to enter the embryo sac.

Double fertilisation occurs in the ovule. One male nucleus fuses with the egg cell to form a diploid zygote while the other male nucleus fuses with the two polar nuclei to form triploid nucleus.

After fertilisation, the diploid zygote divides repeatedly by mitosis to form an embryo while the triploid nucleus forms an endosperm which provides nutrients for the growing embryo.

The ovule develops into a seed while the ovary develops into a fruit.