Abstract: Cytoembryological studies were carried out to determine the apomictic mechanisms of Boehmeria silvestrii (Pamp.) W.T. Wang. The results showed that the modes of reproductiou conforms diplospory. But the development of unreduced embryo sac in B. silvestrii differs from the known types. The first meiotic division in megasporocyte reached diakinesis, but never advanced further. The chromosomes exhibited in the univalent state which maintained at a longer time. Before reaching the metaphase I which was characterized by the disappearance nuclear membrane and the appearance of spindle, the megasporocyte "jumped" bom diakinesis into interkinesis without reduction of chromosome number. The second meiotic division proceeded normally and gave rise to in a dyad of diploid megaspores. The micropylar megaspore degenerated, while the ebalazal megaspore underwent three mitoses to form an eight-nucleated mature embryo sac including an egg, two synergids, two polar nuclei and three antipodals. The embryo and the endosperm respectively originated from egg and secondary nucleus autonomously divided without pollination. The endosperm was of the nuclear type and developed earlier than embryo.

Key words: Boehmeria silvestrii, Apomixis, Diplospory, Obligate apomixes