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  • Describe the process of megasporogenesis upto fully developed embryo sac formation in an angiosperm.        

Describe the process of megasporogenesis upto fully developed embryo sac formation in an angiosperm.

 

 

 

 
 
 
 
 

Answers (1)

1. The process of formation of megaspores from the megaspore mother cell is called megasporogenesis.

2. Ovules generally differentiate a single megaspore mother cell (MMC) in the micropylar region. of the nucellus. It is a large cell containing dense cytoplasm and a prominent nucleus.

3. The MMC undergoes meiotic division. Meiosis results in the production of four megaspores

4. In a majority of flowering plants, one of the megaspores is functional while the other three degenerate.

Only the functional megaspore develops into the female gametophyte (embryo sac). This method of embryo sac formation from a single megaspore is termed monosporic development.

Formation of the embryo sac –

1. The nucleus of the functional megaspore divides mitotically to form two nuclei which move to the opposite poles, forming the 2-nucleate embryo sac. Two more sequential mitotic nuclear divisions result in the formation of the 4-nucleate and later the 8-nucleate stages of the embryo sac.

2. These mitotic divisions are strictly free nuclear, that is, nuclear divisions are not followed immediately by cell wall formation. After the 8-nucleate stage, cell walls are laid down leading to the organisation of the typical female gametophyte or embryo sac.

3. Distribution of cells inside the embryo sac - Six of the eight nuclei are surrounded by cell walls and organised into cells; the remaining two nuclei, called polar nuclei are situated below the egg apparatus in the large central cell. There is a characteristic distribution of the cells within the embryo sac.

Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus, in turn, consists of two synergids and one egg cell. The synergids have special cellular thickenings at the micropylar tip called filiform apparatus, which play an important role in guiding the pollen tubes into the synergid. Three cells are at the chalazal end and are called the antipodals. The large central cell, has two polar nuclei. Thus, a typical angiosperm embryo sac, at maturity, though 8-nucleate is 7-celled.

 

Posted by

Priyanka Kumari

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