Fertilization is the second step of sexual reproduction in flowering plants. It occurs with the union of egg and sperm. This may not occur until days, weeks or months after pollinaton has taken place, and sometimes will not occur at all.
See the short animation below for a simple overview of how pollination and fertilization work
For fertilization to occur the pollen grain must first produce a pollen tube which will then extend down through the stigma until it reaches the micropyle of the ovule. As the tube grows, most of the pollen grain’s contents are discharged along with it.
This entire structure and its contents are known as the mature male gametophyte.
The pollen tube’s journey may last less than 24 hours and usually does not take more than 2 days; however, there are some plants which take up to a year for their pollen tube to reach the micropyle ovule!
Now starts the interaction of the pollen tube (structure containing sperm) with the ovule (egg), fertilization is almost complete!
This part of the process is known as double fertilization or double fusion, and it describes the the way in which two sperm leave the pollen tube. One will fertilize the egg creating a zygote and then developing into an embryo, while the other sperm will fertilize the central cell nuclei and produce the endosperm. The endosperm is a tissue which forms around the developing embryo, serving an important role in the development of the seed.
When the pollen tube reaches the female gametophyte, it enters the egg through the micropyle, destroying a degenerating synergid (shown here in green) upon entrance, and then discharges its contents into the second synergid.
One sperm then migrates from the remaining synergid into the egg, the sperm cell unites with the egg nucleus and fertilization is said to have occurred! We now have a future seed-in-the-making! At this stage it is called a zygote.
The other sperm cell migrates from the synergid (green) to the to the central cell nuclei, also known as the polar nuclei, and unites with them (shown above in pink) and produces an endosperm nucleus. We now have a future endosperm-in-the-making!
Now both processes have completed, meaning that the “double” in double fertilization has occured: both fertilization events have been completed: sperm and egg fertilization and sperm and central cell nuclei fertilization.
Below is another diagram that illustrates pollen tube growth and double fertilization.
Remember that the endosperm and the embryo are two very important parts of a seed, and because of double fertilization their creation is made possible!
Here we can see that the result of double fertilization is the beginning of seed development. The ovule which houses the embryo and endosperm becomes the seed and the ovary which houses the ovule develops into fruit.