The grain grows very rapidly during the first ten days after fertilisation. For convenience, this has been subdivided into two periods, 1-4 and 4-10 days after flowering. We describe the external changes in the whole grain and the internal changes, which can be studied in detail in microscope sections.
The the first four days post-fertilisation, the seed increases about three fold in size. There is a rapid swelling of the tissues of the carpel, both the pericarp and the embryo sac; these tissues surround the fertilised embryo. This growth is achieved by expansion of the cells, rather than cell multiplication, in the cell layers that enclose the embryo sac. These complex changes are explained in greater detail on a separate page, 'Cell layers surrounding the Embryo Sac'.
After fertilisation, nuclei from the second polar event divide synchronously inside the embryo sac but at this stage cell walls are not formed. The nuclei remain in a state of 'free nuclear division' for 3-4 days and the duration of this phase will, in part, determine the final number of cells in the endosperm. The number of these cells is closely related to the final weight of the grain and control of this process is an important target for cereal biologists. The first divisions of the endosperm nucleus produce the so called 'coenocytic endosperm'. The 'free nuclear division' stage is fuelled by very large secretory cells at the top of the embryo sac called the antipodal cells.
The zygote formed at fertilisation has divided only once or twice. It is positioned at the base of the embryo sac where it is nourished by the densely cytoplasmic 'cellular endosperm' cells.