Development and evolution of basal cauline placentation Basella-rubra

In Angiosperms placentae or ovules are fomed on carpels or the floral apex. Hence, in a developmental sense, there are carpellate and acarpellate gynoecia. The latter occur in about 11% of all Angiosperm families. Basella rubra is an example of the noncarpellate condition. Its single basal ovule is formed directly from the floral apex. In young developmental stages it even retains the tunica-corpus organization of the floral apex. In later developmental stages, three septa arise only at the base of the ovule. The single vascular strand of the ovule is symmetrically derived from the bases of all six strands that supply the ovary wall, i.e., it is not associated with the vascular strand of only one of the three gynoecial appendages. Hence, neither development nor vascularization support a carpellate interpretation of the Basella gynoecium. With regard to the evolution of basal placentation in Basella and other taxa of Angiosperms three possibilities exist: 1) It is derived from the carpellate condition, 2) It is primitive and the carpellate condition is derived, 3) Both carpellate and noncarpellate organizations have coexisted during the evolution of Angiosperms which may have been monophyletic or polyphyletic.