Nutritive tissue that develops in the embryo sac of most angiosperms. It usually forms after the fertilisation of the two fused primary endosperm nuclei of the embryo sac with one of the two male gamete nuclei. In most diploid plants, e.g., cereals, the endosperm is triploid (3n), but in some (e.g., lily) it is often consumed as the seed matures.