The fusion of male and female gametes is called fertilization. Entry of pollen tube into the ovule is of 3 ways. They are
i. Porogamy: Pollen tube enters the ovule through micropyle. Eg: Ottelia.
ii. Chalazogamy: Pollen tube enters the ovule through chalaza. Eg: Casuarina.
iii. Mesogamy : Pollen tube enters the ovule through integument (or) funiculus.
Eg: Cucurbita.
* Pollen tube enters the ovule through any one of the above ways, and enters into the synergid through the filiform apparatus.
* The tip of pollen tube ruptures and two male gametes are released.
Double fertilization : It occurs by two events
1. Fertilization (or) syngamy
2. Triple fusion.
1. Fertilization (or) syngamy : The fusion of first male gamete with the egg cell to form diploid zygote is known as syngamy.
2. Triple fusion : The fusion of second male gamete with the secondary nucleus of central cell to form primary endosperm nucleus (3n) is called triple fusion.
Significance of Double fertilisation : The zygote develops into embryo and primary endosperm nucleus develops into endosperm. Double fertilisation is characteristic feature of angiosperms.