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Stem Anatomy and the Evolution of Woodiness in Piperales

Premise of research. Piperales displays a wide diversity of growth forms that appears to be linked with differences in cambial activity and subsequent derived wood production. To date, no overall synthesis of the evolution of woodiness in Piperales has been done and few studies have proposed an ancestral habit (woody/herbaceous). We provide anatomical data of all lineages within Piperales and reconstruct ancestral character states, focusing on the origin of woodiness within the order and on the ecological significance of key anatomical features. Methodology. Stem anatomical observations with special emphasis on wood anatomical features were performed on 28 species of the Piperales; by combining previously published studies with original data, we conducted phylogenetic reconstructions of cambial activity and vessel element perforation plates to assess the origin of woodiness and vessel evolution in Piperales. Pivotal results. Different patterns of cambial activity are observed in Piperales, from active secondary growth in both intra- and interfascicular areas in Aristolochia, Thottea (Aristolochiaceae), Saruma (Asaraceae), Manekia, and Piper (Piperaceae) to cambial activity mainly restricted to fascicular areas in Saururaceae and a complete lack of secondary growth in Verhuellia. Vessels in Piperaceae, Aristolochiaceae, and Asaraceae present simple perforation plates, while those of Saururaceae are mostly scalariform. A stem endodermis bearing a Casparian band—an atypical feature in aerial stems—is reported for all genera within the Piperaceae and for Saururus and Houttuynia in the Saururaceae. Conclusions. The common ancestor of the order likely had an active cambium and woody habit, including vessel elements with simple perforation plates. All woody species share several wood features, including wide and tall rays, suggesting a single origin of wood in the order. The high diversity of growth forms observed in Piperales is linked to frequent shifts in cambial activity and changes in habit-related features within the different lineages.