Welcome back to my channel, in today’s video we are going to
go over the process of double fertilization. Double fertilization is a process that occurs in
angiosperms, also known as flowering plants, where there are two fertilization events.
One of these like in other plants results in the zygote, the second results in the endosperm.
Let’s start with some general structure of the flower parts involved in this process.
The female reproductive organs can be found in the inner most whorl of a flower,
and are collectively called the gynoecium. The gynoecium is made up of one or more pistils,
this flower we are looking at only has a single pistil.
Each pistil consists of the stigma at the top,
the ovary at the bottom, and a style which is the stalk connecting the two.
Within the ovary are the ovules. These ovules have two integument layers
forming the outside of the ovule and within this is tissue called the megasporangium.
Each ovule has a chalazal end and a micropylar end which has the micropyle
which is an opening at one end of the ovule. Inside the ovule there is originally a single
cell, known as the megasporocyte, or as the megaspore mother cell.
This megasporocyte undergoes the process of meiosis to produce four haploid megaspores;
one of these is larger than the other three, only this larger one survives.
The remaining megaspore undergoes three mitotic divisions to create eight nuclei.
Three of these nuclei form the antipodal cells and move to the chalazal end of the embryo sac.
Two nuclei form the synergid cells and move to the micropylar end of the embryo sac.
Another nuclei also moves to the micropylar end and forms the egg cell.
The remaining two nuclei move to the centre of the embryo sac.
The male reproductive organs can be found in the whorl surrounding the female reproductive organs,
and is called the androecium. The androecium is made up of the stamens.
Each stamen consists of a filament with an anther on the end.
The anther is made up of pollen sacs that hold the pollen grains.
Each pollen grain has a generative cell and a tube cell.
The process of double fertilization starts when a pollen grain lands or gets deposited
onto the stigma. The pollen then
germinates under the right conditions. The tube cell then grows down through the stigma,
down the style, through the ovary and then into the ovule through the micropyle.
This growth of the tube towards the micropyle part of the embryo sac
is guided in a way by chemicals secreted by the synergid cells also present near the micropyle.
Once arriving at the embryo sac the pollen tube causes the death of one of these synergid cells
and then uses the space where it had been as its way into the embryo sac.
As the pollen tube is growing the generative cell also divides to form two sperm cells.
These sperm cells move down the pollen tube and into the ovule.
And this is where the double fertilization takes place.
One of the haploid sperm fuses with the haploid egg cell to form a diploid zygote.
But a second fertilization event occurs.
The second haploid sperm fuses with the two haploid polar nuclei to form
the triploid endosperm, which provides the nutrition for the growing embryo.
The rest of the cells in the embryo sac degenerate.
The diploid zygote then continues to divide and go through the stages of embryogenesis.
And that concludes this video on double fertilization.