Bark and Wood Anatomy of the Tribe Hamelieae (Rubiaceae)

We studied wood and bark anatomy of six (Deppea, Hamelia, Hoffmannia, Omiltemia, Pinarophyllon, and Plocaniophyllon) of the seven genera of the tribe Hamelieae sensu Robbrecht, and Syringantha with as main purposes to determine if there are characters that support the boundaries of the Hamelieae, to evaluate the status of Syringantha as a member of the Hamelieae, and to evaluate the taxonomic position of Hamelieae within the subfamilies Rubioideae or Cinchonoideae. In addition, we studied for comparative purposes representative species of Psychotria (Psychotrieae, Rubioideae), Exostema, and Hintonia (Portlandia group, Cinchonoideae), Randia (Gardenieae, Ixoroideae), and Bouvardia (incertae sedis). Bark of most genera studied had a single periderm, while a rhytidome was observed in Exostema and few species of Psychotria. The mineral inclusions allowed recognizing related genera, for example, raphides in Hamelieae and Psychotria, prisms in Exostema, and druses in Randia. Members of Hamelieae showed wood type II, distinctive by the occurrence of libriform septate fibres, vessels in radial multiples of 2–6 vessels (80–90%, vessel grouping index 1.79–2.74), and diffuse apotracheal parenchyma. Syringantha shares with members of Hamelieae the presence of an endodermis, raphides in the bark, and wood type II. The combination of other wood characters mainly lend quantitative support to the taxonomic delimitation of some genera within Hamelieae. Raphides and wood type II supported a close relationship between Hamelieae and Hillieae within Cinchonoideae; characters that distinguish them from the other members of Cinchonoideae. Our results suggest independent origins of wood type II within the Rubiaceae. In addition, vessel density and diameter are discussed as possible adaptations to the different forest types where members of Hamelieae occur.