Floral structure of Cardiopteris (Cardiopteridaceae) with special emphasis on the gynoecium: systematic and evolutionary implications

Cardiopteris, a small herbaceous genus, had long been placed in its own family, Cardiopteridaceae. However, the family was recently broadly circumscribed to include more genera in Aquifoliales. To better understand the morphological relationships of the genus and the family, I studied the floral anatomy and development of Cardiopteris using C. quinqueloba. As has been previously described, flowers are 5-merous with a unilocular gynoecium. I confirmed that the gynoecium is bicarpellate, possessing two dissimilar styles. An analysis of the development, structure, and vasculature of the gynoecium showed that it is pseudomonomerous, consisting of one fertile adaxial carpel and one solid sterile abaxial carpel. The adaxial carpel forms a thin style with a capitate stigma, whereas the abaxial carpel has a thick style, which develops into a freshy fruit appendage. Comparisons with flowers of other genera (Citronella, Gonocaryum, and Leptaulus) of Cardiopteridaceae as well as the other families (Aquifoliaceae, Helwingiaceae, Phyllonomaceae, and Stemonuraceae) of Aquifoliales showed that the pseudomonomerous gynoecium is very likely a synapomorphy to support the sister-group relationship between Cardiopteridaceae and Stemonuraceae. Moreover, contrary to all previous descriptions, Cardiopteris flowers were found to have an annular nectariferous disk at the base of the gynoecium.