MEGASPOROGENESIS & MEGAGAMETOGENESIS

 

STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

MEGASPOROGENESIS & MEGAGAMETOGENESIS

Development of Ovule ( Megasporogenesis ) . 

• Ovule develops as primordium and then mound of nucellus over placenta 
• Initials of integuments develop from its base . 
• They grow and come to surround the nucellus on all sides except at the tip or micropylar region . 
• In the hypoder mal region of nucellus towards the micropyler end develops a primary archesporial cell . 
• It grows in size and develops a prominent nucleus . 
• The archesporial cell often divides once into outer primary parietal or wall cell and inner primary sporogenous cell . 
• Primary parietal cell may divide one or more times . 
• The primary sporogenous cell commonly functions directly as diploid megaspore mother cell or megasporocyte . 
• The megaspore mother cell ( MMC ) undergoes meiosis and forms a linear tetrad of 4 haploid megaspores . 
• The process of meiotic formation of haploid me gaspores from diploid megaspore mother cell is called megasporogenesis . 
• Commonly the chalazal megaspore remains functional while the other 3 degenerate . 

Development of Female Gametophyte ( Megagametogenesis ) . 

• The functional megaspore is the first cell of female gemetophyte . 
• The cell enlarges and undergoes three free nuclear mitotic divisions . 
• The first division produces two nucleate embryo sac . 
• The two nuclei shift to the two ends and divide there twice forming four nucleate and then eight nucleate structure . 
• One nucleus from each side moves to the middle . 
• They are called polar nuclei . 
• The remaining three nuclei form cells at the two ends , 3 celled egg apparatus at the micropylar end and three antipodal cells at the chalazal end . 
• The middle binucleate par organises itself into central cell . 
• Embryo sac developed from a single megaspore is called monosporic . 
• Maheshwari ( 1950 ) has also distinguished bisporic and tetrasporic embryo sacs in which two and four megaspore precursors are involved respectively in the formation of embryo sac .

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