Embryogenesis

Embryogenesis

The zygote goes through a short resting period. During this time, zygote gets matured by some physical and chemical changes and the embryo –sac is enlarged. Normally, the first division of the zygote occurs after the division of the Primary Endosperm Nucleus. The zygote undergoes divisions to form proembryo and then embryo. The form and divisions are quite similar in both the groups – monocots and dicots up to the formation of proembryo.

      

Proembryo –

Zygote divides transversely and forms a terminal cell and basal cell.  The basal cell near the micropylar end of the embryo –sac is called the suspensor cell and the terminal cell develops into the body of the embryo. The terminal cell divides by two vertical  divisions at right angles to each other and forms the quadrant or 4 – celled stage now each cell divides transversely so as to form 8-celled or octant stage. It is at this stage the different types of embryogenesis may be recognized. Mainly, 5 types of patterns of embryogeny have been recognized- Crucifer type, Asterad type, Solanad type, Caryophyllad type and Chenopodiad type.

The embryo-

 Crucifer type of embryo development-

     

 Here, the divisions up to octant stage gives rise to two tiers of 4 cells each. Of these cells, lower four give rise to the stem tip and the cotyledons, while the upper four give rise to the hypocotyls.  These eight cells next divide periclinally to cut of 8 outer cells, which form the dermatogen, and 8 inner ones give rise to the initials of the periblem and plerome.

     

 Meanwhile, the basal cell divides transversely to form a row of 6-10 suspensor cells. The terminal most cell of the suspensor becomes large and vesicle like and probably serves as haustorium. The lowermost cell of the suspensor functions as the hypophysis. The cell divides to form two tiers of 4 cells each. Of these, the lower tier forms the root cortex while the upper gives rise to the root cap and the root epidermis.

    

At this stage, the embryo assumes a cordate shape. Subsequently, due to increase in size of hypocotyl and cotyledons, it acquires a horse- shoe shape because of curvature in ovule.

Chenopod type of embryo development or monocot embryo-  

        

 The zygote divides transversely and forms a terminal cell and basal cell. This is followed by the formation of a ‘T’ shaped proembryo. The terminal cell divides by a vertical division at right angles to the previous division and forms the quadrant or 4 – celled stage. These quadrates next divide periclinally to cut of 4 outer cells, which form the dermatogen, and 4 inner ones give rise to the central cells. These cells divide transeversly to form two tiers l and l’.

           

The tier l forms a part of the single cotyledon and the tier l’ forms the remaining part of the cotyledon, the hypocotyls and plumule. In the meanwhile, the basal cell divides transversely to form a row of three cells - m, n and n’. m and n forms the root and root cap while  n’ gives rise to a single celled suspensor.