Question

Explain double fertilisation briefly

Answer 1

Double fertilisation occur in plants
In this process involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm).
here  one sperm cell of a pollen grain fertilizes an egg cell while a second fuses with two polar nuclei to produce a triploid body that gives rise to the endosperm. 

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Answer 2

Double fertilisation is a complex fertilisation mechanism of flowering plants (Angiosperm).
This process involves joining of a female gametophyte with 2 male gametes. The pollen tube process to release the 2 sperm in megegametophyte