Résumé
The predominantly tropical and subtropical genus Aristolochia is known for its large number of climbing species. In addition, comparatively few herbaceous and shrub-like species are known. While climbers and herbaceous species occur in all three clades (subgenera) named Aristolochia, Pararistolochia and Isotrema, shrub-like species have only been reported in Isotrema. Our molecular phylogenies based on chloroplast and nuclear markers indicate that these shrubs are restricted to the monophyletic Central American lineage of the subgenus Isotrema, while its Asian/North American lineage contains exclusively climbing species. Recent field work shows that the rare shrub-like species are neither strictly climbing nor fully self-supporting; they develop a more-or-less upright growth with their branches leaning on the surrounding vegetation. After comparing the anatomy and biomechanics of these species to related climbers within Isotrema, Aristolochia and Pararistolochia, we investigate whether clades that become specialized as climbers can diversify further into self-supporting growth forms. Biomechanical and anatomical investigations reveal some differences between climbing and shrub-like species, but both growth forms show several stem characters typically found in climbers. These include large cross-sectional areas of ray parenchyma and a thick periderm, which are known to promote flexibility. Our study suggests that evolution of growth forms within the genus Aristolochia might be constrained by organizations favoring the climbing habit where shrub-like species do not develop the height, size and stability generally typical of self-supporting shrubs and trees. The insights into growth form evolution within the genus Aristolochia and Isotrema in particular, have encouraged us to enlarge the study on closely related clades. These include Thottea, sister genus to Aristolochia, which contains exclusively shrub-like species, as well as the monotypic genus Lactoris including the small shrub Lactoris fernandeziana, and the holoparasitic genus Hydnora. The latter clades have both been recently placed in close relationship to the genera Aristolochia and Thottea. By studying the anatomy and biomechanics of these lineages in a comparative approach, we intend to enlarge the knowledge about growth form evolution in Piperales