Generic delimitation and infrageneric classification of Stewartia and Hartia (Theaceae; Stewartieae): Insight from pollen morphology

Tribe Stewartieae (Theaceae) consists of evergreen Hartia and deciduous Stewartia. Pollen morphology was investigated using light and scanning electron microscopy in 6 species of Hartia and 11 species of Stewartia. Hartia has a reticulate or microreticulate pollen sculpture pattern, a psilate or foveolate margo and a round or acuminate colpus tip, and no operculum. On the other hand, Stewartia (Old World species) has pollen with a rugulomicroreticulate sculpture pattern, a constrictive rugulomicroreticulate margo, an acuminate colpus tip, and a distinct or obscure operculum. Two New World species of Stewartia, however, exhibit pollen with some Hartia characteristics (e g., reticulate or microreticulate surface in S. malacodendron, round colpus tip in S. ovata, and no operculum in either species). Thus, two New World species have mixed pollen characteristics between Old World Stewartia and Hartia species. We argue that several significant differences in pollen characteristics between the two genera warrant the recognition of two distinct genera rather than one broadly defined genus. We found that the tribe Stewartieae, based on pollen morphology, can be divided into three groups, Hartia, Old World Stewartia, and New World Stewartia. Additional detailed phylogenetic studies are required for the understanding of the circumscription and biogeographic patterns of this group.