Theory:

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:
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Fertilization
  • The pollen grains from the anther fall on the stigma, they absorb a sugary substance and begin to swell and finally germinate.
  • The pollen grain develops a small tube-like structure known as a pollen tube by rupturing the intine. It then arises from the germ pore present in the exine. All the components of the pollen grain move into the tube.
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Development of pollen tube
  • The pollen tube secretes an enzyme that helps in the dissolution of the tissue of the stigma.
  • 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 pollen grains (male gametes) by mitosis.
  • On reaching the embryo sac of the ovule, the tip of the pollen tube bursts and the two pollen grains inside the tube enters the embryo sac.
  • After the entry of the sperm, one sperm fuses with the egg (syngamy) to form a diploid zygote, and the other sperm fuses with the secondary nucleus (triple fusion), the triploid primary endosperm nucleus (PEN).
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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 and is seen only in angiosperms.
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