Why does the zygote begin to divide only after the division of primary Endosperm cell (PEC)?

Explanation

The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue. The cells of this tessue are filled with resserve food materials and are used for nutrition of the developing embryo.
Embryo develops at the micropylar and of the embryo sac where zygote is situated . Most zygotes divide only after certain amount of endosperm is formed . This is an adaptation to provide assured nutrition to the developing embroyo.