Describe the development of endosperm after double fertilisation in an angiosperm. Why does endosperm development precede that of zygote? [3]

After double fertilisation, the Primary Endosperm Nucleus (PEN) undergoes successive nuclear divisions to give rise to free nuclei (free-nuclear endosperm). Subsequently cell wall formation occurs and the endosperm becomes cellular. [2]

The cells of the endosperm tissue is filled with reserve food materials and are used for the nutrition of the developing embryo. Hence it develops before the zygote. [1]