A Comparison of Floral Structures of Anisophylleaceae and Cunoniaceae and the Problem of their Systematic Position

Flowers of Anisophyllea (Anisophylleaceae, Cucurbitales) and Ceratopetalum (Cunoniaceae, Oxalidales) are surprisingly similar in appearance. To date, these families have never been interpreted as closely related, and even in present molecular (rbcL) studies they appear in different orders of eurosids I (APG, Annals of the Missouri Botanical Garden 85:531-553, 1998). In this investigation, flowers of selected taxa of both families are morphologically and anatomically compared. In addition, previous work on the two families is reviewed. The results strongly emphasize the great similarity in all floral organs. Some special similarities include the occurrence of trimerous flowers, isomerous organ whorls (including the gynoecium), valvate sepals, digitate petals, obdiplostemony, incurved filaments in bud with similar anthers, similar pollen, similar nectaries, carpels with free styles, a canal in the centre of each individual carpel as well as in the centre of the entire gynoecium along the symplicate zone, and similar ovules with a slit-shaped micropyle. In addition, recently recovered Late Cretaceous floral fossils that share features of both families further emphasize a potential close relationship. However, if more extensive molecular studies are performed in the future that support the current disparate position of the two families, then an explanation of the biological/functional similarities in floral structure should be attempted: specifically, whether this suite of features is a symplesiomorphy for basal rosids, or an autapomorphy for each family.