Successive cambia in liana stems of Paullinieae and their evolutionary significance in Sapindaceae

In Sapindaceae tribe Paullinieae, most genera contain species with cambial variants, and five types have been commonly described. However, new, undescribed cambial variants have been observed in species of Paullinieae, including successive cambia. Although successive cambia are common in many families, their structure and development have never been studied in detail in Sapindaceae. The aim of this work was to survey the ontogeny of successive cambia in some members of Paullinieae to better understand the diversity of the group. Stems of four Paullinia spp. and Serjania pernambucensis were studied using anatomical procedures and micro-computed tomography analyses. At the onset of secondary growth, the stems exhibit regular activity and later new cambia arise from the multiseriate pericycle. In Paullinia spp. new cambia arise from the outermost pericyclic cells, whereas in S. pernambucensis they arise from the innermost pericyclic cells at the phloem border. Cambia with reverse products arise in both genera, whereas neo-formed vascular tissue and vascular connections occur only in Paullinia spp. At the final stage of development, the stem of S. pernambucensis has continuous successive cambia, whereas Paullinia spp. have discontinuous successive cambia. This is the first report of neo-formation of vascular tissue in rays of lianas of Paullinieae and the first report of cambia with reverse activity for the family. Finally, this study elucidates the distribution and diversity of cambial variants in Paullinieae and shows their importance to the systematics of the tribe.