Floral organogenesis in Phytolacca (Phytolaccaceae)

To provide data of floral development for the genus Phytolacca and to have a further insight into the systematic relationships of the family Phytolaccaceae, floral organogenesis of three species in this genus, viz., P. acinosa, P. polyandra and P. americana, was examined under scanning electron microscope (SEM). The first tepal primordium is initiated in non_median abaxial position and the second one in median adaxial position. Five tepal primordia arise in a 2/5 sequence on the periphery of the floral apex. Their initiations have anticlockwise or clockwise directions in different floral buds of the same species. The stamen and carpel primordia all arise on a ring meristem. In the species with monocyclic stamens 8-10 stamen primordia arise nearly simultaneously after the initiation of tepal primordia. In the species with dicyclic stamens 8 stamen primordia in the inner whorl first arise simultaneously and then 6-8 in the outer whorl irregularly initiate. Eight to ten carpel primordia arise simultaneously in alternate position with the stamen primordia. In the following developmental process the carpels in P. acinosa develop nearly into apocarpous gynoecium and in P. polyandra and P. americana into syncarpous gynoecium. Patterns of floral organogenesis and the flower development morphology in Phytolacca were analysed. Evidence from floral organogenesis supports the primitive position of Phytolaccaceae in the Caryophyllales.