One of the interesting Biology Topics is the study of animal behavior and how it is influenced by genetics and the environment.
Post Fertilization Events in Plants – Development of Embryo and Endosperm Formation
After fertilization, the zygote or oospore undergoes a series of mitotic divisions which leads to the formation of a multicellular embryo. The primary endosperm nucleus forms the endosperm for the nourishment of the source, the ovules change into seeds and the ovary forms the fruit. These changes can be discussed under the following heads: Embryo formation and Endosperm formation.
1. Embryo Formation
The formation of the embryo from the zygote is called embryogeny. Embryo development is meroblastic (from a part of the zygote) and endoscopic (towards the interior due to the presence of a suspensor).
(i) Dicotyledonous Embryo:
The zygote or the oospore secretes a wall made up of cellulose and divides by a transverse wall into an upper suspensor cell and a lower embryonal cell. The suspensor cell which lies towards the micropylar end, divides transversely to form a row of seven to eight cells, the suspensor. The suspensor pushes the embryo deep into the embryo sac. The basal or the terminal cell of the suspensor attached at the micropylar end enlarges and becomes oval to spherical in shape. It serves to absorb food material. The lowermost cell of the suspensor adjacent to the embryonal cell is known as hypophysis. It forms the apex of the radicle.
Meanwhile, the embryonal cell divides thrice to give rise to an eight-celled octant or embryonal mass. The first two divisions are vertical and the third one is transverse. The four outer cells of the octant form the plumule and the cotyledons. The four basal cells of the octant towards the suspensor from the hypocotyl and part of the radicle. By further repeated divisions, these embryonal cells form a spherical mass which later on becomes heart-shaped. The two lobes continue to grow by further divisions into the two cotyledons.
A group of cells in the groove of the two lobes forms the plumule. Thus a fully mature dicot embryo consists of an embryonal axis with two cotyledons, the plumule which gives rise to the shoot, and the radicle at the basal end, which develops into the root system. The above description of the typical mode of development of the dicot embryo takes place in Capsella bursa-pastoris (Cruciferae).
Development of Dicotyledonous Embryo:
(ii) Monocotyledonous Embryo:
The zygote elongates and then divides transversely to form basal and terminal cells. The basal cell (towards the micropylar end) produces a large swollen, vesicular suspensor cell. It may function as haustorium. The terminal cell divides by another transverse wall to form two cells—the top cell after a series of divisions forms a plumule and a single cotyledon. Cotyledon called scutellum, grows rapidly and pushes the terminal plumule to one side. The middle cell, after many divisions forms hypocotyl and radicle. It also adds a few cells to the suspensor.
In some cereals, plumules, and radicles are covered by sheaths developed from scutellum called coleoptile and coleorhiza respectively. Thus a mature monocotyledonous embryo consisting of one cotyledon is called the scutellum. It is situated towards the lateral side of the embryonal axis. This axis at its lower end has a radicle and root cap enclosed in a sheath called coleorhiza. The part of the axis above the level of attachment of the scutellum is called epicotyl. It has a shoot apex enclosed in a sheath called a coleoptile.
Development of Monocot Embryo:
Difference between Coeloptile and Coleorrhiza:
|1. Coeloptile is the terminal upper end of epicotyl with shoot apex and leaf primordia that are enclosed in a hollow foliar structure. (Mainly a protective seath around plumule).||1. Coleorrihiza is the lower end of the embryonal axis that has a radicle and root cap enclosed in a sheath (around the radicle).|
|2. It is a pore-like structure from which the first leaf emerged.||2. From here a radicle emerges.|
|3. Usually negatively geotropic.||3. Usually positively geotropic.|
|4. Usually helps in photosynthesis.||4. The non-green part of the root cannot do photosynthesis.|
Difference between Hypocotyl and Epicotyl:
|1. It is part of the embryonic axis below the level of the cotyledon.||1. It is part of the embryonic axis above the level of the cotyledon.|
|2. Radicle or root tip can be formed from hypocotyl.||2. Plumule can be formed from epicotyl.|
2. Endosperm Formation
Endosperm is the tissue produced inside the seeds of most flowering plants around the time of fertilization. It surrounds the embryo and provides nutrition in the form of starch, though it can also contain oils and protein. This can make endosperm a source of nutrition in the human diet. For example, wheat endosperm is ground into flour for bread (the rest of the grain is included as well in whole wheat flour), while barley endosperm is the main source of beer production. Other examples of endosperm that form the bulk of the portion are coconut ‘meat’ and coconut ‘water’ and corn. Some plants, like the orchid, lack endosperm in their seeds. Depending upon the mode of its formation angiosperms endosperm is of three types – nuclear, cellular and helobial.
(i) Nuclear Endosperm:
It is the common type of endosperm. The primary endosperm nucleus divides and redivides to form a large number of free nuclei. A vacuole appears in the centre and pushes the cytoplasm containing the nuclei to the periphery. Wall formation or cytokinesis begins from the periphery and proceeds towards the centre. It makes the endosperm a multicellular tissue, e.g., Maize, Wheat, Rice, Sunflower, and Capsella. In several cases, the wall formation remains incomplete. For example, coconut has multicellular endosperm (coconut meal) in the outer part and free nuclear as well as vacuolate endosperm (coconut milk) in the centre.
(ii) Cellular Endosperm:
Every division of the primary endosperm nucleus is followed by cytokinesis. Therefore, endosperm becomes cellular from the very beginning, e.g., Datura, Petunia, Impatiens, and Magnolia.
(iii) Helobial Endosperm:
It occurs in order Helobiales of monocots. The endosperm is of intermediate type between cellular and nuclear types. The first division of the primary endosperm nucleus is followed by transverse cytokinesis to produce two unequal chambers, larger micropylar and smaller chalazal. Subsequent divisions are free nuclear in both chambers. They are rapid in the micropylar chamber. Further development in both the chambers occurs like that of nuclear endosperm, i.e., multinucleate stage followed by wall formation. However chalazal chambers often remain smaller and may degenerate.
Difference between Dicot and Monocot Embryo:
|Dicot Embryo||Monocot Embryo|
|1. In this of embryo basal cell forms 6-10 celled suspensor.||1. In this type of embryo basal cell produces a single-celled suspensor.|
|2. This type of embryo has two cotyledons.||2. This type of embryo has only a single cotyledon called the scutellum.|
|3. Here, the terminal cell produces an embryo except for the radicle.||3. Here terminal cell produces the whole of the embryo.|