Aristolochia thotteaeformis, a distinct new species from southern Vietnam

Aristolochia thotteaeformis, a distinct new species from southern Vietnam, is described here. The new species clearly differs from all the known Aristolochia species in Vietnam by its erect shrubby habit and zig-zag young branches. The vegetative phenotype resembles the genus Thottea (Aristolochiaceae). Morphological characters such as the 1-lipped limb, the 6-lobed gynostemium, and the flower with a stipe between the utricle and ovary place this new species in Aristolochia subgenus Aristolochia section Diplolobus subsection Podanthemum. Morphologically, Aristolochia thotteaeformis is most similar to A. philippinensis and A. humilis by sharing a cuneate to rounded leaf base, a straight perianth in pre-anthesis, then slightly curved upward, an ovoid to ellipsoid utricle, and a 5???10 mm long, cylindrical tube. However, the new species clearly differs from these two species by some morphological characters of young branches, leaves, petiole, inflorescence axis, bracteole, perianth, limb, and seed. In addition to the detailed description, illustrations, information on ecology, conservation status, and comparison with its similar species, a key to all the known species of Aristolochia subsection Podanthemum from Vietnam is provided.