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.
