A New Interpretation of the Successive Cambia of Some Nyctaginaceae as Interxylary Phloem

Abstract

Premise of research. The alternative patterns of secondary growth (vascular cambial variants) in stems of Nyctaginaceae are outstanding and have been widely investigated since the late nineteenth century. However, there are controversial interpretations in the literature regarding the existence of either one or two types of cambial variants in this family (successive cambia vs. interxylary phloem). We aim to explore the anatomical diversity of stems in Nyctaginaceae to document the real nature of the cambial variant present in most species of the family. Methodology. We analyzed 60 species, focusing on 18 species from 12 genera, for developmental studies. Anatomical and ontogenetic features were characterized from images produced by standard plant techniques for macro- and microscopic analyses. Pivotal results. Our analyses reveal that most species of Nyctaginaceae present stems with polycyclic eusteles, which later develop a single cambium that produces secondary xylem and secondary phloem at unequal rates around the stem circumference. This unusual activity results in the absence of a regular cylinder of secondary vascular tissues and in the formation of secondary phloem strands (surrounded by variable amounts of sheathing axial parenchyma) embedded within the secondary xylem. In cross section, adult stems can exhibit different tissue arrangements (i.e., phloem islands/strands, patches, or concentric bands) that result from differences in rates of production of phloem and associated sheathing axial parenchyma forming the strands. The cambial variant in these stems is described as interxylary phloem, as similarly observed in other eudicot lineages. Conclusions. Our examination of the stem development of Nyctaginaceae confirms the presence of interxylary phloem, which has been overlooked in the family as most previous studies have reiterated descriptions of successive cambia as the common cambial variant within the family. These findings emphasize the importance of developmental studies encompassing a representative number of genera to further our understanding of stem macromorphologies and to highlight the complexity and diversity of stem architectures in Nyctaginaceae.