Development of Embryo in plants

 

XII  BIOLOGY

CH-1-Reproduction in lower and higher plants 

PART-PART- Development of Embryo

Development of Embryo:

1)The process of development of zygote into an embryo is called embryogenesis.

2)The embryo is developed at the micropylar end of embryo sac.

3)The growth of embryo triggers only after certain amount of endosperm is formed. After fertilization the embryonic development begins.

4)The  zygote or oospore divides transversely  to  form two cells known as  proembryo;  the large cells towards the micropyle is called basal or suspensor initial cell . and the smaller cell towards   the  chalaza  is  called  terminal  or  embryonal initial cell .

5)The suspensor cell divides transversely to form a row of seven to eight cells called suspensor.

6) The first cell of the suspensor towards the micropylar end becomes swollen and function as a haustorium. The lowermost cell of suspensor is known as hypophysis.

7)the suspensor help  pushes embryo towards the centre of the embryo sac.                                                                                                      

8)Meanwhile the embryonal cell divides thrice to form an embryonal mass of eight celled octant stage in which cells are arranged in two tiers each containing four cells.

9)The planes of divisions are at right angles to each other.

10)The lower layer of four cells of octant give rise to hypocotyl and radicle whereas four cells of upper layer form the plumule and the one or two cotyledons .

11)The hypophysis by further division gives rise to the part of radicle and root cap.

12)Subsequently, the cells in the upper tier of octant divide in several planes so as to become heart shaped which then forms two lateral cotyledons and a terminal plumule.

13 Further enlargement of hypocotyl and cotyledons result in a curvature of embryo and it appears horse-shoe shaped.

14)The embryo development is similar in both dicots and monocots up to the octant stage. The difference appears later.

 In monocot embryo, single cotyledon occupies terminal position and plumule is lateral. The single shield shaped cotyledon is called as  scutellum. The protective sheath of  plumule is called  coleoptile and that of  radicle is  coleorhiza. Finally, ovule is transformed into the seed and ovary into the fruit.

15)In monocot embryo, single cotyledon occupies terminal position and plumule is lateral.

The single shield shaped cotyledon is called as scutellum.  

 The protective sheath of plumule is called coleoptile and that of radicle is coleorhiza.

Finally, ovule is transformed into the seed and ovary into the fruit

Embryo: 

• Embryo is the first stage of development for a multicellular organism.  The embryonic stage of sexual reproduction begins when the male sperm cell fertilizes the female egg cell.
• An embryo is a developed form of a zygote formed when male and female gametes fuse.
• The zygote divides and differentiates into an embryo
•  In plants, the embryo develops following syngamy, the fusion of pollen grains (male gametes) and egg cells (female gametes).
• The embryo forms in plants at the micropylar end of the female gametophyte, or embryo sac.
• Triple fusion leads to the formation of endosperm, which nourishes the developing embryo.
• The following article describes the development of embryos in monocots and dicots for the NEET Exam .

What is Embryo Development in Plants?

• Plant embryogenesis is the process by which a plant embryo develops after fertilising an ovule.
• This is an important plant life cycle stage; germination and dormancy follow.
• The zygote formed after fertilisation divides and changes into a mature embryo.

A mature embryo is made up of five major parts: 

1. The hypocotyl, shoot apical meristem, root meristem, root cap, and cotyledons.
2. Plant embryogenesis, unlike animal embryogenesis, produces an immature plant with no structures such as leaves, stems, or reproductive parts.
3. Both plants and animals go through a phylotypic stage, which limits morphological diversification and allows them to evolve independently.
4. Dicot and Monocot embryos continue to develop until the globular embryo stage.
5. Following this stage, the two groups differ.

Embryo Development in Dicots

1. Embryogenesis is the natural process of forming a seed in plants after fertilisation.
2. It involves the development of two key structures: the plant embryo and the endosperm, which later become the seed.
3. The zygote, formed from fertilisation, undergoes several cell divisions to form a mature embryo.
4.  This process establishes the basic cellular pattern for the plant's shoot-root body and primary tissue layers

Stages of Developement in Dicots

 Embryogenesis can be divided into the following stages of development:

1. Two Cell Stage: After fertilisation, the zygote divides into a small apical and a large basal cell. These cells are different and give rise to different parts of the plant, establishing polarity in the embryo.
2. Eight Cell Stage: The embryo now consists of eight cells, with distinct domains contributing to different parts of the plant, such as the shoot apical meristem, cotyledons, hypocotyl, and root apical meristem.
3. Sixteen Cell Stage: Additional cell divisions occur, and the four domains become more defined. The protoderm, which forms the epidermis, is introduced during this stage.
4. Globular Stage: The embryo becomes spherical or globular in shape. The ground meristem and procambium, which form the ground tissue and vascular tissue respectively, are initiated during this stage.
5. Heart Stage: The cotyledons start to form and elongate, giving the embryo a heart-shaped appearance.
6. Pro Embryo Stage: The cotyledons continue to grow, and parts of the suspensor complex are terminated to make space for the mature embryo.
7. Maturation: The cells mature, and macromolecules required for germination and seedling growth are stored in the embryo.

Embryo Development in Monocots

1. Monocotyledon embryos have only one cotyledon, known in the grass family as a scutellum.
2. The scutellum is located laterally along the embryonic axis.
3. A radical and root cap are enclosed in a sheath called coleorrhiza at the embryonal axis's lower end.
4. The epicotyl portion of the embryonal axis extends above the scutellum attachment.
5. The shoot apex and a few leaf primordia of the epicotyl are enclosed by the hollow foliar structure known as the coleoptile.
6.  Some monocots, such as Sagittaria sagittifolia, are studied for their embryo development.
7.   Dicot and monocot embryos develop similarly until the globular stage. After this stage, the difference between monocots and dicots becomes clear. 
8. Monocots have a single terminal cotyledon, while dicots have two cotyledons.
9.  Monocots have either no second cotyledon or one that is significantly smaller.
10. The scutellum refers to the single terminal cotyledon.
11. The reduced cotyledon is referred to as the epiblast.
12. During embryo development, a transverse wall separates the basal cell into two cells.
13. One of these cells divides into two, resulting in the formation of a suspensor and root cap. Another cell forms the rest of the root cap and a portion of the radical.
14.  The terminal cell has two perpendicular walls that form a quadrant structure, while periclinal cells are divided into outer and inner cells.
15.  Both groups of cells divide repeatedly, resulting in two zones.