Why do you think the zygote is dormant for sometime in a fertilised ovule?
After syngamy ,diploid zygote is formed which develops into embryo .
PEN is formed after triple fusion which after repeated divisions forms the nutritive tissue endosperm.
The zygote is dormant in fertilized ovule for sometime because at this time, endosperm needs to develop, As endosperm is the source of nutrition for the developing embryo, the nature ensures the formation of enough endosperm tissue before starting the process of embryogenesis.