What is fertilization?

ANGIOSPERM DIAGRAM

Before getting to the nitty gritty details of how plant’s get fertilized, first take a look at the diagram of a flower to get familiar with all of its components! 

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EASY GUIDE TO PLANT FERTILIZATION:

*** All underlines words will have a corresponding definition found at the bottom of this page***

Step 1: Pollen Tube Extension

Once landed on the stigma, the pollen grain will first produce a pollen tube which will then extend down through the stigma until it reaches the micropyle of the ovule

Visualization(Pollen Tube Components):

The vegetative nucleus stays at the tip while the generative nucleus lags behind, divides, and eventually produces two sperm cells. 

Fun Fact:

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 that take up to a year for their pollen tube to reach the micropyle in the ovule

Step 2: The Start of Double Fertilization

Now starts the interaction of the pollen tube(structure containing sperm) with the ovule(egg), fertilization is almost complete! 

Double Fertilization:

This is 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.

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Step 3: The Start of Double Fertilization

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.

Step 4: The completion of double fertilization

The other sperm cell migrates from the synergid(green) to the central cell nuclei, also known as the poler 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 occurred: both fertilization events have been completed: sperm and egg fertilization and sperm and central cell nuclei fertilization. 

Step 5: seed development

As a result of double fertilization, the creation of the endosperm and the embryo are made possible! Remember that both of these components are two very important parts of a seed. 

Visualization:

In the diagram to the right 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. 

DEFINITIONS

TERMDEFINITION
EndospermThe Tissue section of the seed that provides nutrients to the growing embryo
Generative NucleusThe nucleus that falls behind the vegetative nucleus and will eventually aid in sperm production
MicropyleA very small opening on the ovule where the pollen tube will enter
Polar NucleiTwo nuclei that will merge to become one and eventually merge again with the male gamete to help form the endosperm
Pollen TubeA tube that extends from the pollen grain, enters the micropyle of the ovule, and transports sperm for fertilization
OvuleA structure found within the ovary that will eventually become a seed after being fertilized by pollen
StigmaThe top part of the stamen that receives pollen
SynergidCells that are able to guide the pollen tube to the female gametophyte
Vegetative NucleusThe nucleus that stays at the tip of the pollen tube and helps the tube grow