Development of Embryo

Embryogenesis, the development of the zygote into a fully formed embryo, begins only after a certain amount of endosperm has already formed around it, ensuring the developing embryo has a nutritive tissue to draw upon from the outset. The embryo itself develops at the micropylar end of the embryo sac.

The zygote's first division produces a two-celled proembryo made of a larger cell nearer the micropyle, the basal or suspensor initial cell, and a smaller cell nearer the chalaza, the terminal or embryonal initial cell. The suspensor initial then divides transversely, in a single plane, to build up a filamentous suspensor of about six to ten cells, whose job is to physically push the developing embryo deeper into the nutritive endosperm. The uppermost suspensor cell (nearest the micropyle) swells and functions as an absorptive haustorium, while the lowermost suspensor cell, immediately adjacent to the true embryo, is called the hypophysis and later contributes to the root.

Meanwhile, the embryonal initial cell undergoes three successive mitotic divisions, in mutually perpendicular planes, to form an eight-celled stage called the octant. The lower tier of four octant cells goes on to form the hypocotyl and radicle (embryonic root axis), while the upper tier of four cells forms the plumule (embryonic shoot) and one or two cotyledons (seed leaves); the hypophysis, by further division, contributes the remainder of the radicle and the protective root cap. As development proceeds, the upper-tier cells divide in multiple planes to become heart-shaped, differentiating into two lateral cotyledons flanking a terminal plumule; continued enlargement of the hypocotyl and cotyledons then curves the whole embryo, giving the mature dicot embryo its characteristic horseshoe shape (illustrated in Capsella, the classic dicot embryogenesis model).

…