Seed develops from an ovule (consisting integument and nucellus) or megasporangium. Parts of ovule develop a haploid megaspore which grow into haploid female gametophyte. This female gemetophyte develops an egg or oospore. The latter is fertilized by a male gamete and develop into a zygote $(2 n)$. The zygote develops into an embryo or new sporophyte. The embryo $(2 n)$ is surrounded by female gemetophyte $(n)$, the latter by seed coat $(2 n)$. In this way, seed contains three generations locked one within another. Of these, two generations occur within embryo.