Q.Describe in brief the embryo development process in Dicotyledonous Angiosperm.

The egg apparatus is the egg cell plus two synergids at the micropylar pole; dicot and monocot embryos differ mainly in the number of cotyledons and the presence of protective sheaths in monocots.

Egg apparatus: At the micropylar end of a mature (7-celled, 8-nucleate) embryo sac lies a group of three cells collectively called the egg apparatus: one egg cell (oosphere), flanked by two synergids. The synergids possess special thickenings called the filiform apparatus, which helps guide the pollen tube into the embryo sac for fertilisation.

Dicot vs monocot embryo (two differences):

1. A dicot embryo has two cotyledons, whereas a monocot embryo has only one cotyledon (often modified into an absorptive structure called the scutellum, e.g. in grasses). …

After fertilisation the zygote of a dicot divides to give a suspensor and an embryonal cell; the latter passes through proembryo → globular → heart-shaped stages, finally forming an embryo with two cotyledons, plumule and radicle.

Embryogeny in a dicotyledonous angiosperm:

1. Zygote (oospore): The diploid zygote formed by syngamy first undergoes a period of rest while the endosperm develops (to nourish the growing embryo).
2. First division: The zygote divides by a transverse wall into two cells — an upper terminal embryonal cell (epibasal) towards the chalaza and a lower basal cell (suspensor cell) towards the micropyle.
3. Suspensor: The basal cell forms a filamentous suspensor, whose lowest cell is the hypophysis and topmost cell forms part of the root; the suspensor pushes the embryo into the nutritive endosperm.
4. Proembryo: The embryonal cell divides repeatedly to form a globular (spherical) proembryo.
5. Heart-shaped stage: Two lobes grow out at the top, giving the characteristic heart-shaped embryo — these lobes become the two cotyledons (the dicot feature). …