The use of heterochromatic regions, containing moderately repetitive DNA as markers, has allowed to follow the course of development of the generative and vegetative nuclei in the pollen grains of some species ofTradescantiaandNarcissus.It has been found that the chromocentres, relatives to the NOR-heterochromatin, are not randomly distributed. This implies the existence of a special mechanism governing the orientation of the centromeres in the spindle-shaped generative nuclei: a predisposition which may facilitate the mitotic process in the pollen tube.