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MICRO-SPORANGIUM is the chamber where microspores (pollen grains) are formed.
Inside the MICRO-SPORANGIUM which is present at anther, the head of stamen. There is a homogeneous mass of cells known as SPOROGENOUS TISSUE.
This sporogenous tissue cells, get specialized and form MICROSPORE MOTHER CELL (MMC). And this is the cell that undergoes meiosis to form Microspore Tetrad, i.e. 1 Meiocyte (diploid) = 4 cells (haploid).
This meiosis is of two types:-
(No need for explanation. The picture below is enough to address the basic differences between the two.)
These microspores get separated, during dehiscence, to form pollen grain.