Abstract: The present paper aims at discovering the characters of embryological development of Circaeaster agrestis, which makes up a monotypic genus, Circaeaster, to establish the phylogenetic relationships of the genus. The different opinions on its systematic position among botanists are briefly explained. The embryological studies show that the most important advanced characters of the genus are as follows. The ovule is amphitropous, unitegmic and tenuinucelar; the embryo sac formation is in accordance with the Polygonum type; endosperm formation is of the cellular type, the primary endosperm nucleus dividing to form two cells and the first wall vertical; embryo formation follows the variation of the Caryophyllad type; at the early stage of development of embryo, the integument has been already atrophied and at last disappeared, so that the seed coat is absent in the mature fruit. On the basis of the embryological and some morphological evidence, the authors consider that a close relationship between the genus and Ranunculaceae and its related families seems to be unlikely. The affinities of the genus Circaeaster are still uncertain.

Key words: Circaeaster agrestis, embryo, endosperm