has part pollen development microgametophyte development GO:0009555 biological_process male gametophyte development GO:0009564 formation of generative and vegetative cells male gametophyte formation The process whose specific outcome is the progression of the pollen grain over time, from its formation to the mature structure. The process begins with the meiosis of the microsporocyte to form four haploid microspores. The nucleus of each microspore then divides by mitosis to form a two-celled organism, the pollen grain, that contains a tube cell as well as a smaller generative cell. The pollen grain is surrounded by an elaborate cell wall. In some species, the generative cell immediately divides again to give a pair of sperm cells. In most flowering plants, however this division takes place later, in the tube that develops when a pollen grain germinates. GO:0048231 pollen grain formation microsporogenesis gametophyte development