Abstract: The sequence of organ initiation and the change in number and position of stamens and carpels are compared among the four genera in the Saururaceae in this paper. Several evolutionary trends of organogenesis are revealed and summarized as follows: The first trend is that the first appearance of stamens and carpels converts from the median sagittal ones to the lateral ones. In Saururus the median sagittal stamens and carpels all appear first, while in Gymnotheca the median posterior stamen arises first, the lateral pair second, the median anterior one third, and the lateral carpels appear first. In Anemopsis and Houttuynia the lateral stamens have changed to arising first. The second trend is the conversion of initiation of stamens and carpels from the median sagittal ones in pair as in Saururus to singular, acompanied by delayed development in Gymnotheca and to final degeneration of the anterior stamen and carpel in Anemopsis and Houttuynia. The latter two genera have a pair or only one stamen at the posterior position, but during early development there is a gap as a trace of degenerating stamen at the anterior position. Moreover, in Gymnotheca with four carpels, a gap exists at the anterior position, which is the place of the delayed anterior carpel; in the genera with three carpels, a gap is present at the anterior position in Anemopsis, but absent in Houttuynia. The other trends involve the change of the two lateral stamens from separate initiation in Saururus to initiation from a common primordium in Gymnotheca and Anemopsis and to reduction of one stamen in Houttuynia; the conversion of single posterior stamen to a pair from a common primordium in Anemopsis; the change of initiation of three carpels from separate primordia in Anemopsis to from a ring primordium in Houttuynia. The facts seem to show that the genus Houttuynia is more advansed than Anemopsis. The evolutionary trends of flowers in the Saururaceae can be explaned as the results of fusion, reduction and multiplication. The systematic relationships among the four genera were inferredin a diagram.

Key words: Saururaceae, floral organogenesis, systematic significance