Development of Endosperm
- First of all, endosperm develops from the primary endosperm nucleus after the fertilization which stored food materials. It is utilized by the embryo during the early development then after at the time of seed germination.
- Food is present in the form of starch in the endosperm.
The endosperm is of three types on the basis of development:Different Types of Endosperm on the basis of Development
(i) Nuclear Endosperm
- This type of endosperm mostly found in Dicotyledon (Polypetalae). The nuclear endosperm is also present in Capsella. Such type of endosperm develops by free nuclear divisions of the nucleus of the primary endosperm nucleus. Thus a multinucleated endosperm is formed. Later on, cytokinesis takes place, so that multicellular endosperm is formed.
- This type of endosperm is the most common in Angiosperms.
- The milky fluid is found in green Coconut is an example of nuclear endosperm, which is called liquid syncytium.
- In Melastoma, cytokinesis never takes place so that it always remains nuclear endosperm.
(ii) Cellular Endosperm
- This type of endosperm is found in the Gamopetalae group.
- During the development, each division of the primary endosperm nucleus is followed by cytokinesis. So that endosperm remains cellular from the beginning.
(iii) Helobial Endosperm
- During the development of this type of endosperm, the first division of the primary endosperm nucleus is followed by unequal cytokinesis so that two unequal sized cells are formed (cell towards the micropyle is large).
- Now free nuclear divisions take place in each cell, which results in it becoming multinucleated. Eventually, cytokinesis takes place later on so that it is changed into a cellular endosperm. This type of endosperm is found in Monocots. It is an intermediate type of endosperm.
➢ Special Points
Development Of Embryo In Dicot
- The development of embryos in Capsella is first time discovered by "Hanstein".
In Angiosperm, Zygote undergoes a resting phase.
- When the endosperm is formed, the development of the zygote starts. In the beginning, it absorbs food from the endosperm and increases in size then after a layer secreted by itself. Now it is called Oospore. Development of Dicot Embryo
- The first division of Oospore is transverse, resulting in two cells are formed.
- The one cell that lies towards the micropyle is called basal cell or suspensor cell.
- The other cell is formed towards the Chalaza is called apical cell or terminal cell or embryonal cell.
- The basal cell and embryonal cell divide simultaneously.
- The basal cell divides transversely and the apical cell divides vertically resulting, in two suspensor cells and two embryonal cells formed. This stage is made up of four cells which are arranged in a 'T' shaped structure. Embryonal cells divide vertically to form four Embryonal cells. This is the quadrant stage of the embryo.
- The two suspensor cells divided by transverse divisions forming a 6-10 celled long filament like structure is termed suspensor. The main function of the suspensor is to push the developing embryo into a food-laden endosperm to provide nutrition.
- The micropylar cell of the suspensor swells up. This cell of suspensor is known as haustorial-cell.
- The cell of the suspensor lying near the embryonal cells is called hypophysis. This cell combines with radicle to form the apex of the root (Root cap).
- These four cells quadrant embryos further divide transversely to produce eight. The eight celled stage of the embryo is called the octant stage. The eight cells of octant are arranged in two tiers.
- The four cells of the octant embryo lying near the hypophysis are known as hypobasal cells and four cells present towards the chalaza termed as epibasal cells.
- The hypobasal cells give rise to radicle and hypocotyl and epibasal cells give rise to two cotyledons and plumule of the embryo.
- All the cells of octant divided by periclinal division so that a 16 celled globular embryo is formed (Proembryo).
- Due to the fast division of embryonal cells of the globular embryo, a heart-shaped embryo is formed. All the cells of this embryo are meristematic.
- Due to the fast growth in two lobes of the heart-shaped embryo, they develop into two cotyledons. Both the growing cotyledons turn in downwards due to the curved position of the body of the ovule of Capsella.
- The tissues are present above the joining place of both the cotyledons are responsible to form plumule and behind it, epicotyl is formed.
- The tissues present opposite to the plumule give rise to radicle.
- This curved position of the embryo is called the Torpedo or Chordate stage.
- An axis is present between plumule and radicle is called the embryonal axis. It is also called Tigellum (main embryonal axis).
- Both the cotyledons are present at the lateral position of the embryonal axis and the plumule is formed in the terminal position in the Dicotyledon embryo.
- This type of development of the embryo is known as the Crucifer type or Onagrad type. It is the most common type of development in Dicots.
- Crucifer type of development is also found in Capsella so that it is considered as typical Angiosperm for the study of embryonic development of Angiosperms.
- During the development of the embryo, the embryo is formed from some part of the zygote so that is called meroblastic development.
- Ovule modified into a seed in which testes formed by the outer integument and tegmen formed by the inner integument.
- Only the micropyle of the ovule remains unchanged and also present in the seed.
- Entire ovary modified into a fruit. This fruit is formed by fertilized ovary so that it is called true fruit.
- In some of the Angiosperms, the fruit is formed from the ovary without fertilization known as parthenocarpic fruit.
- In some fruit, parthenocarpy is useless (If the edible part is endosperm or seed).
Stages of Embryo Development in Capsella Bursa-Pastoris (Dicot)
Development of Embryo in Monocotyledon
- The Lilium type of embryonic development is found in monocotyledons. The first division is the transverse division in Oospore. Stages in the Development of a Typical Monocot Embryo in Sagittaria
- Results two cells are formed, the upper cell chalazal is called the embryonal cell and the lower micropylar cell is termed as the basal cell.
- The basal cell does not divide further and later on it increases in size and form a single-celled vesicular suspensor. The only embryonal cell divides transversely in which terminal cell is called a cotyledon cell and the lower (middle) cell is known as embryonal axis cell.
- A transverse division takes place in the embryonal axis cell to gives rise to two cells.
The one cell out of two gives rise to plumule initial and another gives rise to radicle initial.
- The plumule initially divides to form a plumule of the embryo.
- Radicle initial divides to form radicle. In this, both the initials are responsible to form an embryo in a Lateral position.
- In Angiosperms, the development of the embryo is meroblastic and endoscopic (towards the chalaza).
- In the grass family, the cotyledon is called the scutellum that is situated towards one side (lateral) of the embryonal axis. At its lower end, the embryonal axis has the radical and root cap enclosed in an undifferentiated sheath called coleorhiza.
- The portion of the embryonal axis above the level of attachment of the scutellum is the epicotyl. Epicotyl has a shoot apex and a few leaf primordia enclosed in a hollow foliar structure, the coleoptile.
Reproduction in Plant
➢ Development of Floral Organs in Arabidopsis
- Arabidopsis thaliana is a small weed belonging to the family Brassicaceae.Arabidopsis thaliana
- It contains approximately 26000 genes and the complete gene sequence has been studies by plant biologists.
- Like other plants in Arabidopsis, the initiation of flowering is influenced by various environmental factors. Of these photoperiodism and temperature are more important. Under the influence of the two factors, the apical meristem starts producing flowers instead of vegetative structure. The development of calyx, corolla, androecium and gynoecium is controlled by specific genes. These are called homeotic selector genes.
Example: Apetala-2, Apetala-3, Agamous etc.
- As a result of the effect of these genes, the development of leaves is replaced by the development of flowers.
- Similar to leaves, the floral organs developed by periclinal divisions in protoderm or sub-protoderm cells. Initially, these cells show periclinal divisions followed by periclinal as well as anticlinal divisions resulting in the formation of an outgrowth. Cells of the outgrowth divide, develop and differentiate to form different floral organs.
- In Arabidopsis calyx are the first to develop followed by corolla, androecium and gynoecium. The order of development of floral may vary from plant to plant.
For example, in members of the family Umbelliferae, the sequence of development of floral organs is - androecium, corolla, calyx and gynoecium.
Table: Difference between Embryo and Endosperm
Table: Difference between Egg Cell and Secondary Nucleus
Table: Number of Chromosomes in Different Parts of Plant