Grain filling lasts for about twenty days in total. We have broken the time down into three sections, 11-16, 17-21 and 21-30 days after flowering and describe what happens to the endosperm, the cell layers surrounding the embryo sac and the embryo during each of these time periods. These periods correspond to the Medium Milk, Soft Dough and Hard Dough stages used by farmers to describe the grain in the field.
The first is the start of the grain filling period. The developing grain is still very soft with the maternal pericarp a mint green colour under the outer, clear epidermis. The green colour comes from chloroplasts in the 'cross cell' layer. The embryo is easy to dissect out. The grain is at the Medium Milk stage. Sectioned grains from 11 and 16 days after flowering illustrate the dynamic changes in microscopic structure that occur during the first stage of grain filling. The meristematic cells of the endosperm continue to divide and compartments are starting to form within the endosperm. The first large, Type 'A' starch grains are seen at about 16 days after flowering. Lipid and protein bodies are also seen at this time. The cell layers surrounding the embryo sac continue to change their character, cell walls become thickened and what will become the aleurone layer is recognisable for the first time. Typical aleurone cells are first visible near the nucellar projection at 12 days after flowering. The aleurone cells close to the ventral groove, which acts as transfer cells for the up-take of assimilates into the endosperm stop growing early and develop special characteristics. In comparison the cells of the dorsal aleurone are still dividing, enabling the grain to continue to expand. The aleurone cells that interface with the embryo near the scutellum are also changing even though the embryo is still physically separate.
During this period, the embryo is developing rapidly and has an elongated shape. The densely cytoplasmic cellular endosperm, still present at eleven days after flowering, has been completely consumed by the embryo by the sixteenth day. At 16 days after flowering the scutellum is clearly defined and the embryo now uses the endosperm starch reserves near the scutellum for its own development.