Double fertilization is fusion of

Double fertilization is a fusion of one male gamete with an egg and the other with secondary nucleus.
Double fertilization is a complex fertilization mechanism of flowering plants (angiosperms). This process involves the joining of a female gametophyte (megagametophyte, also called the embryo sac) with two male gametes (sperm).
The process of double fertilization is a unique biological process in which one sperm fuses with the egg to produce the embryo while the second fuses with the central cell to form the endosperm, a nutritive tissue needed for the growth of the succeeding generation.