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(days = days after flowering)

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Grain growth 4 to 10 days

The developing grain grows very rapidly in the ten days after fertilisation. We have divided this time into two sections, 1-4 and 4-10 days after flowering.
Between 4 and 10 days after flowering the grain is sometimes called 'Water-ripe' or 'Pre-milk'. This describes the observation that when squashed or cut open it appears to contain only water; the inside of the grain has very little in the way of cellular structure or content. However, the overall size has increased considerably.

In the first stage of grain growth the endosperm cells were in 'free nuclear division'; now, in the second stage, the internal structure of the endosperm is laid down. The nuclei of the endosperm continue to divide rapidly and cell walls start to grow near the inner nucellar epidermis. The first full ring of syncytial endosperm is formed by 7 days after flowering.

Depending on their position in the embryo sac, cells of the endosperm have different fates. The outer cell layer gives rise to aleurone initials whereas the inner cells will form the starchy endosperm. The number of cells in the endosperm will increase by division for several days yet, at least until 16 days after flowering and this will overlap with the next stage, the start of Grain Filling.
The antipodal cells, which are thought to have provided the machinery for protein synthesis to support the free nuclear division stage of 0-3 days, start to degenerate. They have completely disappeared by 5 days after flowering.
An internal structure, called the nucellar pillar, runs the full length of the grain just inside the ventral groove. The nutrients needed to sustain the next stages of embryo and grain growth are transported via the vascular tissues embedded within the nucellar pillar.

More information on the nucellar pillar is available here

The embryo can be dissected out easily, but until 5 days after flowering, even at a high magnification, it is still a globular structure with no clear differentiation of its cells. By 7 days after flowering, however, it starts to show compartmentalization and an early sign of differentiation is the appearance of a small cleft on the dorsal side at about 10 days after flowering. The densely cytoplasmic cellular endosperm, which has been nourishing the embryo at the bottom of the embryo sac, starts to degenerate at this time.