Sapindaceae lianas are remarkable for the diversity of cambial variants found in their stems. One of the family’s exclusive cambial variant is the divided vascular cylinder, which occurs in eight species of the genus Serjania. This cambial variant is marked by 5 peripheral vascular cylinders around a large pith. We performed a comparative developmental analysis, integrating traditional plant anatomy techniques with high-resolution X-ray micro-computed tomography to investigate the structure and development of the stems of three species with divided vascular cylinder. Our observations showed that the initial stages of stem development were similar to those described in the literature, however, on later developmental stages a central vascular cylinder appears in all species. The ontogeny of these stems are marked by three main processes: (i) dissection of vascular tissue from the peripheral vascular cylinders; (ii) development of new cambial arcs through the redifferentiation of pith cells; and (iii) recruitment of cambial cells from the inner portions of the vascular cambium of the peripheral vascular cylinders, forming a novel central vascular cylinder where the pith was, surrounded by five initial peripheral cylinders. As an ulterior developmental stage, some older stems also develop neoformations and connections between the different vascular cylinders. While our findings support previous descriptions of divided vascular cylinders, this is the first study illustrating the formation of the central vascular cylinder in this cambial variant. Our observations further corroborate that Serjania is the lineage with the highest and some of the most complex forms of cambial variants among all vascular plants.

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Background and Aims The tribe Paullinieae have the highest diversity of vascular variants among the seed plants. The developmental diversity is better understood in the species rich genera Paullinia and Serjania, however, the phylogeny and diversity of vascular variants in the smaller genera of Paullinieae remain understudied. Here we investigate the evolution of development of stem vasculatures in the small genus Urvillea. Methods We generate the first molecular phylogeny of Urvillea derived from 11 markers using a maximum likelihood and Bayesian approach. In combination with phylogenetic reconstruction, stochastic character mapping is used to assess evolutionary changes in stem ontogenies, determined from developmental anatomy of stems collected in the field or from herbarium and wood collections. Key Results Urvillea is supported as a monophyletic group and sister to Serjania. There are five stem ontogenies in Urvillea, including a typical growth and four vascular variants. Most stem ontogenies initiate with lobed stems. Lobed adult stems are conserved in Urvillea, but this ontogeny was lost multiple times. A reversal to typical growth occurred in non-climbing species. Phloem wedges, fissured stems and ectopic cambia evolved independently once. Phloem wedges is an intermediate developmental stage in the formation of fissured stems, which is characterized by a continuous fragmentation of vascular tissues. Lobed stems may generate constriction zones and lobes may split or not. Conclusions Urvillea stands out as the third most diverse genus in number of vascular variants within Paullinieae, but only one ontogeny (fissured stems) is exclusive for the genus. Differential cambial activity and ectopic cambia are the main ontogenetic processes generating stem diversity. The evolutionary history of vascular variants demonstrates the large developmental plasticity of the cambium in such a small genus and corroborates a scenario of repeated evolution of complex anatomies within Paullinieae lianas.