The primary endosperm nucleus, born out of fusion of the second sperm cell with the polar fusion nucleus during double fertilization, is the starting point for the development of the endosperm. Repeated divisions of this nucleus within the confines of the central cell generate a cluster of free nuclei or a cellular tissue known as the endosperm. Although the endosperm is reckoned as a triploid tissue in the vast majority of angiosperms, the ploidy level of the fusion nucleus obviously determines the final nuclear constitution of this tissue. Theoretically, the ploidy level of the endosperm might be expected to vary from diploid in the Oenothera type of embryo sac, to pentaploid in the Penaea, Fritillaria, and Plumbago types, and as high as 9N in the Peperomia type. Although both diploid and pentaploid endosperms have been described, it is, however, doubtful whether a 9N endosperm is formed as a permanent tissue in any seed types investigated.

The endosperm has been studied from a number of viewpoints using light and electron microscopy, tissue culture, and biochemical and molecular techniques. For many years, the patterns of division of the primary endosperm nucleus, the morphological nature of the tissue formed, and the interaction of the tissue with the developing embryo were of prime concern to investigators whose studies have been an important factor in revealing that the developing embryo is nourished by the food materials of the endosperm.