Endosperm is:
Endosperm is triploid or generally polyploid nutritive tissue arising from fusion of second sperm cell with homodiploid central cell containing two or more polar nuclei depending on species and embryo sac type. After fertilization primary endosperm nucleus undergoes rapid mitoses either free nuclear forming syncytium then cellularizing, entirely cellular, or helobial patterns generating tissue rich in starch, storage proteins, lipids, and hormones sustaining embryo development. Polyploid nature permits vigorous metabolic activity and genomic imprinting regulates resource allocation between maternal and paternal genomes, analogous to mammalian placenta providing nutrition and regulatory signals within seed environment.
Ref: Li & Berger, Curr Opin Plant Biol 2012: angiosperm endosperm is polyploid nutritive tissue from central cell fertilization.