Dicot and Monocot Embryo PDF

Dicot and Monocot embryo Fr G Victor Roch SJ  I. Development of Dicot Embryo After fertilization, the fertilized egg is called zygote or oospore which develops into an embryo. The oospore before it actually enters into the process, undergoes a period of rest which may vary from few...

Dicot and Monocot embryo Fr G Victor Roch SJ  I. Development of Dicot Embryo After fertilization, the fertilized egg is called zygote or oospore which develops into an embryo. The oospore before it actually enters into the process, undergoes a period of rest which may vary from few hours to few months. Generally the zygote (oospore) divides immediately after the first division of the primary endosperm nucleus (PEN). Development of Dicot Embryo 1. The zygote divides transversely to form a two-celled proembryo. The cell towards the micropyle is known as the basal cell and the other is known as terminal cell. 2. The basal cell undergoes several transverse divisions to form a long suspensor. 3. The terminal cell divides longitudinally twice to form four cells. This four-celled stage of terminal cell is called quadrant stage. 4. The four cells of the quadrant stage now divide transversely to form an octant stage of eight cells arranged in two tiers of four cells. The lower tier gives rise to the stem tip and cotyledons, while the upper tier is meant for the formation of hypocotyl. 5. This is followed by periclinal division in the octant cells to give rise to eight outer cells, and eight inner cells. 6. The eight outer cells form the dermatogen, which divide anticlinally and develop into the epidermis. 7. The eight inner cells form the periblem and plerome. 8. The cortex develops from the periblem and stele from the plerome. The basal cell divides several times to form a long suspensor of six to ten cells. 9. The lowermost cell of the suspensor nearest to the developing embryo is known as hypophysis. The hypophysis, by repeated divisions, gives rise to root cap, epidermis and cortex of the root. 10. Further enlargement of hypocotyl and cotyledon result in a curvature of the cotyledons. The embryo at this stage appear as a horse-shoe shaped structure. In the mature embryo, the stem tip is terminal and the two cotyledons occupy the lateral position.  Stages-Development of a dicot embryo (i) Embryo formation starts after a certain amount of endosperm is formed. (ii) Zygote divides by mitosis to form a proembryo. (iii) Formation of globular and heart-shaped embryo occurs, which finally becomes horse shoe-shaped forms a mature embryo. (iv) In dicot plants, embryo consists of two cotyledons and an embryonal axis between them. (v) The portion of embryonal axis above the level of attachment of cotyledons is epicotyl and it terminates in the plumule. (vi) The portion of embryonal axis below the level of attachment of cotyledon is the hypocotyl, it becomes radicle (root tip).  II. Monocot Embryo Development Development of Embryo in Monocots There is no essential difference between the monocotyledons and the dicotyledons regarding the early cell divisions of the proembryo, but the mature embryos are quite different in two groups. Here the embryogeny of Sagittaria sagittifolia has been given as one of the examples. 1. The zygote divides transversely forming the terminal cell and the basal cell. 2. The basal cell, which is the larger and lies towards the micropylar end, does not divide again but becomes transformed directly into a large vesicular cell. 3. The terminal cell divides transversely forming the two cells. Of these, the lower cell divides vertically forming a pair of juxtaposed cells, and the middle cell divides transversely into two cells. 4. In the next stage, the two cells once again divide vertically forming quadrants. 5. The cell next to the quadrants also divides vertically and the cell next to the upper vesicular divides several times transversely. 6. The quadrants now divide transversely forming the octants, the eight cells being arranged in two tiers of four cells each. 7. With the result of periclinal division, the dermatogen is formed. 8. Later the periblem and plerome are also differentiated. 9. All these tissues formed from the octants develop into a single terminal cotyledon afterwards. 10. The lowermost cell L of the three-celled suspensor divides vertically to form the plumule or stem tip. 11. The cells R form radicle. 12. The upper 3-6 cells contribute to the formation of suspensor.