Post Fertilisation Changes

After fertilization, the zygote undergoes a number of mitotic divisions and form a multicellular embryo. The primary endosperm nucleus also passes through a series of mitotic divisions and form a mass of endosperm cells which provide nourishment to the developing embryo.

Based on the kind of development, the endosperm is of 3 types namely nuclear endosperm, cellular endosperm, and helobial endosperm.

Nuclear type

This is the most common type of endosperm development. In this type, the nucleus undergoes repeated divisions. Each of the nuclei thus formed is arranged towards the periphery, enclosing a large central vacuole. Cytokinesis begins from the periphery towards the centre making it cellular at maturity.

Example: Maize, wheat, rice, sunflower etc.

Cellular type

In this type every nuclear division is followed by cytokinesis, making it cellular from the beginning it self.

Helobial type

In this type, the first mitotic division is followed by cytokinesis resulting in the formation of two unequal cells. Subsequent divisions are of the free nuclear type making the endosperm cellular after cytokinesis or at maturity.

Embryo Formation

The fertilised egg undergoes a regular sequence of divisions to form the embryo. The oospore divides to form a suspensor or basal cell and a terminal embryo cell.

The basal cell is towards the micropyle and the terminal cell is towards the chalaza. The basal cell divides repeatedly to produce a row of 4 to 8 cells. This constitutes the suspensor. The terminal cell divides in various planes to form a cluster of cells called the proembryo. The suspensor pushes the proembryo in to the endosperm, to enable the developing embryo to obtain nourishment.

(A - H). Stages in the development of dicot embryo in Capsella bursa-pastorisThe pro embryo cell divides to form the plumule, radicle and two cotyledons in the dicots. In monocots only one cotyledon develops.

As the embryo and endosperm mature, the integuments of the ovule becomes hard to form the seed coat, which protects the seed.

Growth of an embryo in a non-endospermous dicotyledon seed, such as shepherds purse (Capsella bursapastoris)