The first division of oospore is transverse. Of the two cells thus formed the one which lies towards the interior of the embryosac is called terminal cell (embryonal cell) and the other basal cell (suspensor cell). The terminal cell may divide vertically. The suspensor cell divides by several transverse divisions forming a 5-10 celled suspensor.

The uppermost cell of this or haustorial cell. The suspensor pushes the developing embryo into food laden endosperm and the upper cell functions as haustorium. The lowermost cell of suspensor is called hypophysis,. This cell gives rise to the apex of radicle.

In the mean time, two cells formed from terminal cell divide by a transverse division forming four embryonal cells (Quadrant stage). These four cells divide by another vertical division forming eight cells (octant stage). Four cells at the apex (epibasal cells) give rise to plumule and cotyledons. Four cells in between the epibasal cells and suspensor (called hypobasal cells) give rise to hypocotyl and most of the radicle.

All cells of octant stage, divide by a periclinal division forming one layered outer (epidermal) cells which after anticlinal divisions form dermatogen. The inner (central cells) divides by several transverse and vertical divisions forming plerome in the centre and periblem in between plerome and dermatogen.

ADVERTISEMENTS:

Dermatogen gives rise to epidermis, periblem to cortex and plerome to vascular tissue. After several more divisions, the embryo becomes cordate. Later the cotyledons enlarge and may even get curved. In this way a completely mature embryo with two cotyledons attached to an axis are formed. The portion of axis in between the cotyledons is called plumule and the other part as radicle. The above given description of development of embryo takes place in Capsella bursa pastoris, a member of family Cruciferae. This type of embryo development is called Crucifer type or Onagrad type and is considered as typical mode of development of embryo.

Monocotyledonous Embryo: There is no difference between dicotyledons and monocotyle­donous embryo upto octant stage. Luzula forsteri (Juncaceae) is characteristic of simple type of embryogeny. The zygote divides by a transverse division into a basal cell and a terminal cell.

The terminal cell of the two celled proembryo divides by a vertical wall to produce two juxtaposed cells and a little later basal cell divides by a transverse division. Thus a T-shaped proembryo is differentiated. Terminal cell divides by another vertical division to produce four cells.

Cell basal forms suspensor and all the remaining cells contribute to the formation of embryo. 6-cells divide further to give rise to a tissue T. The lower half part of this tissue go to form the lower half portion of single cotyledon, hypocotyl, plumule and radicle. From a part of basal cell, terminal part of radicle is formed which forms apical part of root cap and from other portion of basal cell remaining part of root cap is formed.

ADVERTISEMENTS:

Polyembryony:

Polyembryony is the phenomenon of development of more than one embryos in a seed. It was discovered by Leeuwenhoek in 1719 in orange. It may be of two types:

I. True or II. False

I. True: When embryos arise in the same embryosac in which zygotic embryo has developed.

ADVERTISEMENTS:

II. False: When extra embryos are developing in one embryosac other than the one in’ which zygotic embryo has developed.

More than one embryo may be developed in the seed by one of the following methods:

(1) Cleavage Polyembryony:

When more than one embryo are formed by cleavage or splitting of normal zygote, e.g. Erythronium americanum, Nymphea advena, Nicotiana rustica.

ADVERTISEMENTS:

(2) Embryo from Synergid:

Synergid is fertilized by sperm e.g. Saggitaria.

(3) Embryo from Antipodal: e.g. Ulmus americana. ,

(4) Embryo from Endosperm: e.g. Balanophora.

ADVERTISEMENTS:

(5) Adventive Polyembryony : Embryo formed form cells of nucellus which penetrate into embryo sac. Eugenia, Mangifera.

(6) Embryo originating from other embryosac in the ovule: e.g. Citrus.

Apomixis:

In normal sexual cycle, fertilization (Amphimixis i.e., fusion of male and female gametes), leads to the formation of embryo and later on seed. Seed germinates to produce new plant where male and female gametophyte are formed after meiosis. This normal sexual cycle involves fertilisation and meiosis.

ADVERTISEMENTS:

However, substitution of usual sexual reproduction by a form of reproduction which does not involve meiosis and-syngamy is called apomixis a term introduced by Winkler in 1908. There are two main types of apomixis:

I. Vegetative Reproduction:

New plant is produced from stem, leaf, buds, etc. (Seed is not required).

II. Agamospermy:

Seeds are produced and embryo is formed but meiosis and fertilization are eliminated. It may be of three types:

(a) Adventive Embryony:

Embryo formed from cells of nucellus or integuments.

(b) Diplospory:

Megaspore mother cell develops into an unreduced embryosac. The embryo is formed from unfertilized egg (partheno­genesis), or some cell of embryo sac (apogamy).

(c) Apospory:

A somatic cell in the nucellus directly forms an unreduced embryosac and the diploid egg parthenogenetically develops into embryo.