The endosperm is the tissue inside of a seed that protects and nourishes the embryo of an angiosperm (flowering plant). When pollen attaches to the stigma of a flower, two haploid male gametes travel down the pollen tube to the embryo sac. One of these gametes fuses with a haploid oosphere to produce a diploid embryo. The second gamete fuses with a central cell that contains two haploid nuclei. This second fertilization produces the triploid endosperm that will surround the embryo inside of a seed.