Essential Parts of Flower: Androecium - Microsporogenesis

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Notes

Microsporogenesis:

  • The stages involved in the formation of haploid microspores from diploid microspore mother cell through meiosis is called Microsporogenesis.
  • The primary sporogenous cells directly, or may undergo a few mitotic divisions to form sporogenous tissue.
  • Sporogenous tissue is present inner to anther lobe. 
  • The last generation of sporogenous tissue functions as microspore mother cells.
  • Each microspore mother cell divides meiotically to form a tetrad of four haploid microspores (microspore tetrad).


    Enlarged view of a pollen grain tetrad

  • Each forms tetrahedral tetrads, which depends on the type of cytokinesis taking place during meiosis. 
  • Cytokinesis is of two types:  
  1. Simultaneous type: Cytokinesis occurs at the end of meiosis II. The resultant tetrad shows a tetrahedral arrangement and it is common in dicotyledons. Each nuclear division in microspore mother cell is followed by cell wall formation.  
  2. Successive type: Cytokinesis occurs twice once at the end of meiosis I, forming two cells (dyad) and then again at the end of meiosis II to form four cells. Results in isobilateral pollen tetrad and found to be monocotyledons. There are five types of tetrads: 
  • Microspores soon separate from one another and remain free in the anther locule and develop into pollen grains.
  • In some plants, all the microspores in a microsporangium remain held together called pollinium. Example: Calotropis. In family Asclepiadaceae and Orchidacae, microspores are present as pollinium. Pollinia are attached to a clamp or clip like sticky structure called corpusculum. The filamentous or thread like part arising from each pollinium is called retinaculum. The whole structure looks like inverted letter 'Y' and is called translator. As the pollinium is attached to the pollinating agents like insects, the entire mass of pollen grains is transferred as a unit.
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