Friday, July 21, 2017

MICROSPOROGENESIS

Microsporogenesis
Sporogenous tissue is present inner to anther lobe. Sporogenous tissue forms pollen mother cells. Microspore
mother cell divides meiotically to form four haploid microspores or pollen grains.
These are arranged in tetrads. The arrangement of pollen grains in a tetrad depends on the type of cytokinesis
taking place during meiosis.
It is of following two types
Simultaneous type : Here, the cytokinesis occurs only at the end of meiosis II. The resultant tetrad shows
tetrahedral arrangement and it is common in dicotyledons.
Succesive type Here, the cytokinesis occurs twice once at the end of meiosis l, forming two cells (dyad) and
then again at the end of meiosis ll to form four cells. The resultant tetrad shows isobilateral arrangement and
it is found in monocotyledons.
◆ In fact, there are five types of tetrads-Tetrahedral, lsobilateral, Decussate, T-shaped and Linear.
◆ Compound pollen grains are reported. In Drosera aud Typha, here the four pollen grains do not separate.
◆ In Mimosaceae, 8 to 64 pollen grains remain united called Messulae.
◆ In Calotropis and orchids all the pollen grains of an anther lobe form a typical structure called pollinium.
pollinia form translator and show translator mechanism of pollination.