Ranalisma rostratum Stapf is a rare and endangered species．  This paper deals  with the fertilization and the development of embryo and endosperm in this plant．The embryogenesis is of Caryophyllad type and the development of endosperm belongs to Heobial  type．Before fertilization，the two polar nuclei are located respectively at both ends of embryo  sac. In most angiosperms with two polar nuclei，the polar nuclei may fuse eiher before fertilization to form a secondary nucleus or during fertilization called triple fusion．  In Ranalisma  rostratum Stapf，  however，  it is found that only in case when the micropylar polar nucleus is  fertilized，it can move to the chalazal end and fuse with the chalazal polar nucleus．This phenomenon is  very rare and the process must take more time to fulfil fertilization both polar  nuclei．  This feature of fusion of polar nuclei is therefore thought as a primitive character  from the view of phylogeny．