Theory:

What is fertilization?
The process by which a male gamete fuses with the female gamete to form a zygote is called fertilisation.
The process of fertilization in plants:
Figure320207w800.jpg
  • The pollen grains from the anther fall on the stigma and begin to germinate.
  • The pollen grain develops a small tube-like structure known as a pollen tube that arises from the germ pore. All the components of the pollen grain move into the tube.
116px-Pollen_Tube.svg.png
Development of pollen tube
  • The pollen tube developed grows through the stigma and style tissues and eventually reaches the ovule, via the micropyle.
  • The vegetative cell degenerates inside the pollen tube, and the generative cell splits into two sperms (male gametes).
  • On reaching the embryo sac of the ovule the tip of the pollen tube bursts and the two cells inside the tube enters the embryo sac.
  • After the entry of the generative cell, it fuses with the egg (syngamy) to form a diploid zygote, and the vegetative cells fuses with the secondary nucleus (polar nuclei - 2n) and forms as the triploid primary endosperm by triple fusion.
 
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Double fertilization
  • As two types of fusion such as syngamy and triple fusion, takes place in the embryo sac. This process is known as double fertilization.
A video explaining the process of fertilization in plants
Reference:
https://commons.wikimedia.org/wiki/File:Figure_32_02_07.jpg
https://commons.wikimedia.org/wiki/File:Pollen_Tube.svg
https://upload.wikimedia.org/wikipedia/commons/a/af/Double_Fertilization.jpg