Endosperm is the tissue on the inside of seeds. It normally appears around fertilization. The purpose of the endosperm is to provide nutrients for the embryo and to protect it. The nutrients are normally starch, although there are also oils and protein. There are normally three types of endosperm development: nuclear, cellular, and helobial. The end result, the completed endosperm, then proceeds to aid the embryo while it is still in the seed casing.