Spermatophyte pollen grains and Pteridophyte or Bryophyte spores are homologous structures. They develop during the alternation of generations in the sporophyte. Pollen synthesis occurs as follows: Pollen mother cells are developed from the archespore (the pollen-producing tissue of the pollen sack) during mitosis (cell division). During the subsequent meiosis (reduction division) microspores are formed as a tetrad. Tetrad formation occurs either by the division of cells in one plane (tetragonal tetrad formation; in some Pteridophytes, Gymnosperms, Monocots and primitive Dicots) or at right angles to one another (tetrahedral tetrad formation; in Bryophytes, some Pteridophytes and Dicots). A further mitotic division results in one generative and one vegetative nucleus in each member of the tetrad. Once the tetrad has broken up, the new pollen exist as single grains. In some genera, for example Alpenrose (Rhododendron) or heather (Calluna) the single pollen grains remain grouped together as tetrads.
Spore production occurs in a similar manner.

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