Endosperm

[6] An endosperm is formed after the two sperm nuclei inside a pollen grain reach the interior of a female gametophyte or megagametophyte, also called the embryonic sac.

[8] One flowering plant, Nuphar polysepala, has diploid endosperm, resulting from the fusion of a pollen nucleus with one, rather than two, maternal nuclei.

[9] It is believed that early in the development of angiosperm lineages, there was a duplication in this mode of reproduction, producing seven-celled/eight-nucleate female gametophytes, and triploid endosperms with a 2:1 maternal to paternal genome ratio.

The evolutionary origins of double fertilization and endosperm are unclear, attracting researcher attention for over a century.

There are the two major hypotheses:[8] The triploid transition - and the production of antipodal cells - may have occurred due to a shift in gametophyte development which produced a new interaction with an auxin-dependent mechanism originating in the earliest angiosperms.

[13] Endosperm tissue also mediates the transfer of nutrients from the mother plant to the embryo, it acts as a location for gene imprinting, and is responsible for aborting seeds produced from genetically mismatched parents.

[7] In angiosperms, the endosperm contain hormones such as cytokinins, which regulate cellular differentiation and embryonic organ formation.

In some cases (e.g. wheat, rice) the endosperm is selectively retained in food processing (commonly called white flour), and the embryo (germ) and seed coat (bran) removed.

During germination, it secretes the amylase enzyme that breaks down endosperm starch into sugars to nourish the growing seedling.