Byttneria is one of the few climbing genera in Malvaceae. Some Byttneria are known for their lobed stems. In a recent article in Annals of Botany, Lorena Luna-Márquez et al. explore the development of these stems, how they have evolved within the group and their relevance in the evolution of the climbing growth form in Malvaceae.
Each lobe coincides with one of the five vascular bundles. By augmented activity of the fascicular cambium in the lobes coupled with reduced activity of the interfascicular cambium in the interlobes, secondary growth increases the lobulation already present during primary growth. Within Byttneria and allies, lobed young stems appeared at least three times, once in Ayenia and twice in the paraphyletic Byttneria. Lobed adult stems were conserved in Byttneria s.s., where lobed adult stems in combination with prickles were shown to have evolved as a climbing mechanism within the group; prickles were lost once within Byttneria s.s., in a shrubby subclade. Byttneria Clade 2 comprises climbers with twining cylindrical adult stems and no prickles, which constitutes a different climbing mechanism in the group.
Luna-Márquez et al. provide evidence of one of the few cambial variants known whose secondary body reflects the primary body vasculature and show that lobed adult stems and prickles in Byttneria could be used in the new delimitation of genera in the group. Lobed stems independently appeared in climbing Grewia, suggesting a convergence favouring the climbing growth form.
Luna-Márquez, L., Sharber, W.V., Whitlock, B.A., Pace, M.R., 2021. Ontogeny, anatomical structure and function of lobed stems in the evolution of the climbing growth form in Malvaceae (Byttneria Loefl.). Annals of Botany. https://doi.org/10.1093/aob/mcab105