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.

Background: The presence of alternative patterns of secondary growth in stems of Nyctaginaceae has been known for a long time. Still, the interpretation of types of cambial variants are controversial. The knowledge on stem anatomical diversity in Nyctaginaceae, which is diverse also in habits, offers the unique opportunity not only to investigate the evolution of complex developments, but also to address how these anatomies shifted within habits and how the acquisition of novel cambial variants and habit transitions impacted the diversification of the family. Methods: We integrated developmental data with a phylogenetic framework to investigate the diversity and evolution of stem anatomy in Nyctaginaceae using phylogenetic comparative methods, reconstructing ancestral states, and examining whether anatomical shifts correspond to species diversification rate shifts in the family. Results: Two types of cambial variants, interxylary phloem and successive cambia, were recorded in Nyctaginaceae, which result from four different ontogenies. These ontogenetic trajectories depart from two distinct primary vascular structures (regular or polycyclic eustele) yet, they contain shared developmental stages which generate stem morphologies with deconstructed boundaries of morphological categories (continuum morphology). Unlike our a priori hypotheses, interxylary phloem is reconstructed as the ancestral character for the family, with three ontogenies characterized as successive cambia evolving in few taxa. Cambial variants are not contingent in habits, and their transitions are independent from species diversification.Conclusions: Our findings suggests that multiple developmental mechanisms, such as heterochrony and heterotopy generate the transitions between interxylary phloem and successive cambia. Intermediate between these two extremes are present in Nyctaginaceae, suggesting a continuum morphology across the family as a generator of anatomical diversity.