Explain the process of double fertilization in angiosperms.

Double fertilization is observed only in angiosperms. It involves two types of fusions in the embryo sac: 
- Fusion of sperm cell (male gamete) with egg cell (female gamete) to form a diploid zygote. This process is known as syngamy or generative fertilization.
- Fusion of the other sperm cell with two polar nuclei to form primary endosperm cell containing triploid nucleus also known as primary endosperm nucleus (PEN). This process is known as triple fusion as it involves fusion of three haploid nuclei. The zygote gradually develops into an embryo and central cell becomes primary endosperm cell (PEC) and eventually develops into endosperm.
- Double fertilization gives stimulus to other cells of embryo sac to resume their growth and form nutritive tissue to ensure that developing embryo gets proper nutrition for formation of viable seed.
- Since formation of endosperm takes place after zygote formation, angiosperms ensures that energy used in formation of endosperm is not wasted if fertilization fails.