The fertilized egg is called zygote. Following a predetermined mode of development (embryogeny) it gives rise to an embryo, which has the potentiality to form a complete plant.
An embryo comprises an embryonal axis with two cotyledons attached to it laterally. The portion of embryonal axis above the level of cotyledons is called epicotyl and the portion below the level of cotyledons is known as hypocotyl. The epicotyl terminates into plumule (embryonic shoot), and the hypocotyl at its lower end bears radicle (embryonic root). The embryo of a monocotyledon differs from that of a dicotyledon mainly in having only one cotyledon.
In the majority of angiosperms the zygote divides transversely, resulting in a small apical cell toward the interior of the embryo sac toward the micropyle. Rarely, the division of the zygote may be vertical or oblique. The variations in the developmental pattern of embryo during early embryogeny are common to monocotyledons and dicotyledons. Differences appear when the initials of plumule and cotyledons are laid down. From the 2-celled stage until the initiation of organs the embryo is commonly called proembryo. It is at the octant stage of the proembryo that the destinies of various cells become determined.
Embryogeny in Dicotyledons:
Based on the plane of division of the apical cell in the 2 celled proembryo, and the contribution of the basal cell and the apical cell in the formation of embryo proper five types of embryogeny have been recognized:
A. The apical cell of the 2 celled proemryo divides longitudinally:
���a) The basal cell plays only a minor role or none in the subsequent development of the proper- Crucifer Type or Onagrad Type (eg Annonaceae, Brassicaceae, Onagraceae).
���b) The basal cell and apical cell both contribute to the development of embryo- Asterad Type (eg Asteraceae, Balsaminaceae, Violaceae, Vitaceae).
B. The apical cell of the 2 celled proembryo divides transversely.
���i) The basal cells plays only a minor role or none in the subsequent development of the embryo proper.
������a) The basal cell usually forms a suspensor- Solanad Type (eg Campanulaceae, Linaceae, Solanaceae, Theaceae)
������b) The basal cell undergoes no further division, and the suspensor, if present, is always derived from the apical cell- Caryophyllad Type (eg Caryophyllaceae, Crassulaceae, Haloragaceae)
���ii) The basal and apical cells both contribute to the development of embryo- Chenopodiad Type (eg Boraginaceae, Chenopodiaceae).
Nutrition of Embryo
Emdosperm- The chief source of nutrition of embryo inside the ovule is the endosperm. There is ample evidence to suggest that the endosperm, which mostly forms the immediate environment of the embryo, is rich in nutritive and growth regulating substances. Chemical analysis of coconut milk (liquid endosperm) has revealed that it is rich in inorganic ions and amino acids. It also contains sugars and growth regulators. Corn endosperm at the milk stage has been shown to contain large quantities of carbohydrates, amino acids, and protein complexes. Another feature of endosperm which enables it to nourish the embryo is its capacity to develop haustorial structures. They penetrate into various ovular tissues, absorb nutrients from there and transfer them to the embryo. In the absence of endosperm, or when it is not sufficiently well developed, the embryo depends for its nutrition on some other structures. Mostly it is the suspensor haustorium, which substitutes the endosperm, Mostly it is the suspensor haustorium which substitutes the endosperm.
Pseudo Embryo Sac- It contains several free nuclei and dense cytoplasm which are generally consumed by the developing embryo.
Perisperm- The nucellus is usually consumed by the developing endosperm. But in certain families, Amaranthaceae, Cannaceae, Zingiberaceae, the nucellar tissue surrounding the embryo sac persists in the seed and becomes densely packed with food materials. The persistent nucellus is called perisperm. In such cases the endosperm functions as an intermediary tissue in between the perisperm and the embryo, absorbing food stored in the former and passing it on to the latter.
Chalazosperm- In Cyanastrum the endosperm and the major part of the nucellus disappearduring seed development. However, the nucellar cells in the chalazal region of the ovule, above the vascular bundle, divide actively forming a prominent tissue called chalazosperm. The cells of this tissue, which are loosely arranged, get filled with fat and starch, and serve as a substitute for the endorsperm.
