Post Fertilization Events:
After fertilization the zygote undergoes a series of mitotic divisions which leads to the formation of a multicellular embryo. The primary endosperm nucleus forms the endosperm for the nourishment of the embryo, the ovule changes into seeds, and the ovary forms the fruit. The Post Fertilization Events are given as follows:
-
1. Embryo Formation
2. Endosperm Formation
Embryo Formation:
Formation of the embryo from the zygote is called embryogeny. Embryo development is meroblastic and endoscopic.
i. Dicotyledonous embryo: The zygote secretes a wall made up of cellulose and divides by a transverse wall into an upper suspensor cell and a lower embryonal. The suspensor cell which lies towards the micropylar end divides transversely to form a row of seven to eight cells, the suspensor. It pushes the embryo deep into the embryo sac. The basal or the terminal cell of the suspensor attached at the micropylar end enlarges and becomes oval to spherical in shape. It serves to absorb food material. The lowermost cell of the suspensor adjacent to the embryonal cell is known as hypophysis. It forms the apex of the radicle.
ii. Monocotyledonous embryo: The zygote elongates and then divides transversely to form basal and terminal cells. The basal cell produces a large swollen, vesicular suspensor cell. It may function as haustorium. The terminal cell divides by another transverse wall to form two cells. The top cell after a series of divisions forms a plumule and single cotyledon, it is called Scutellum. It grows rapidly and pushes the terminal plumule to one shot. The middle cell, after many divisions, forms hypocotyl and radicle. It also adds a few cells to the suspensor.
Endosperm Formation:
Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This can make endosperm a source of nutrition in the human diet. In some cereals, both plumule and radicle get covered by sheaths developed from scutellum called coleoptile and coleorhiza respectively.
i. Nuclear endosperm: It is the common type of endosperm. The primary endosperm nucleus divides and redivides to form a large number of free nuclei. A vacuole appears in the center and pushes the cytoplasm containing the nuclei to the periphery. Wall formation or cytokinesis begins from the periphery and proceeds towards the center. It makes the endosperm a multicellular tissue such as Maize, Wheat, Rice, Sunflower, and Capsella.
ii. Cellular endosperm: Every division of the primary endosperm nucleus is followed by cytokinesis. Therefore, endosperm becomes cellular from the very beginning such as in Daturia, Petunia, Impatiens, and Magnolia.
iii. Helobial endosperm: It occurs in order Helobiales of monocots. The endosperm is of intermediate type between cellular and nuclear types. The first division of the primary endosperm nucleus is followed by transverse cytokinesis to produce two unequal chambers, larger micropylar and smaller chalazal. Subsequent divisions are free nuclear in both chambers. They are rapid in a micropylar chamber. Further development in both the chambers occurs like that of nuclear endosperm, multinucleate stage followed by wall formation. However, chalazal chambers often remain smaller and may degenerate.