............ take part in reproduction.

The androecium is the third reproductive whorl of the flower and is made up of stamens. A stamen is made up of a stalk called the filament and an anther which contains pollen grains. Pollen grains are the male gametophyte. The pollen grain, when deposited on the stigma, forms the pollen tube. The pollen tube carries the male gametes that fertilize the female gametophyte. Hence pollen grain takes part in reproduction.

So, the correct answer is 'Pollen grains'