Development of embryo

 

DEVELOPMENT OF EMBRYO:-

Embryo develops from diploid zygote. Zygote is the product of syngamy or first fertilization. Fusion between haploid male gamete and haploid female gamete to from diploid zygote is syngamy.
Embryo consist of an axis and cotyledon. Axis consist of radical and plumule. Radical gives rise to root and plumule gives rise to shoot. Cotyledons may be one or two. Depending upon the number of cotyledons they are dicot embryo and monocot embryo.
The total changes that occurs during the development of mature embryo from zygote is called embryogeny.

DEVELOPMENT OF DICOT EMBRYO:-

•  The various steps in the formation of dicot embryo are -
• The zygote divides by a transverse wall into two unequal cells.
• They are -Large basal cell or suspensor cell and Small terminal cell or embryo cell
• Basal cell is towards micropyler end and terminal cell is towards antipodal.
• The suspensor cell divides transversely few times to produce a filamentous suspensor of 6-10 cells.
• The suspensor helps in pushing the proembryo in the endosperm.
• The upper most cell of this suspensor swells up to form a haustorial cell, which functions as haustorium.
• The lower most cell of the suspensor is called hypophysis. Hypophysis gives rise to root cap.
• The embryo cell undergoes two vertical divisions and one transverse division to form eight cells i.e. octant stage.
• Eight cells are arranged in two tiers, terminal epibasal and near the suspensor hypobasal.
• Epibasal cell form the two cotyledons and plumule.
• Hypobasal cell gives rise to hypocotyl and radical.
• The cells of octant stage divide by periclinal division forming out dermatogen, middle periblem and inner plerome.
• Dermatogen gives rise to epidermis, periblem to cortex and plerome to vascular tissue.
• After several divisions the embryo becomes heart shaped or cordate later the cotyledons enlarges and may even get curved...
• In this way a completely matured dicot embryo is developed. This type of embryo development is called Crucifer type or onagrad type.

 

DEVELOPMENT OF MONOCOT EMBRYO:-

Like dicotyledons the proembryo of monocotyledons also consist of large basal cell and a small terminal cell.
The basal cell divides transversely and forms three cells. The upper cell forms suspensor. Middle cell forms root cap. Lower cell forms radical.
The terminal cell or embryonic cell divides further and its lower half forms the lower part of single cotyledon and the upper half forms the upper part of single cotyledon, hypocotyl, radical and plumule.
The single cotyledon of monocot is called scutellum.
Both radical and plumule develop the covering coleorhiza and coleoptile respectively.

In this way a completely matured monocot embryo develops.
As the embryo and endosperm develop and mature, the integuments of the ovule becomes hard and lead to the formation of seed.

The following changes are usually marked in the formation of fruits and seeds: 

BEFORW FERTILIZATION	CHANGE AFTER FERTILIZATION
Ovary	Fruit
Ovary wall	Fruit wall or pericarp
Ovule	seed
Funicle	Stalk of seed
Hilum	Hilum of seed
Nucellus	Disintegrates or form perisperm
Integuments	seed coat
Micropyle	Micropyle of seed
Synergids of embryo sac	Disorganize
Egg cell of embryo sac	Embryo
Secondary nucleus of embryo sac	Endosperm
Antipodal of Embryo sac	Disorganize