Is D. opposita valid? 
According to the Missouri Botanical Garden's TROPICOS database and the USDA ARS GRIN Taxonomy database, the plant known as 'Dioscorea batatas ' is more correctly Dioscorea polystachya Turcz. This is corroborated by the Flora of China database and the Flora of North America database. The print version of the Flora of China (PDF for Dioscorea chapter available here ) has the following note:
- The name Dioscorea opposita Thunberg (Fl. Jap. 151. 1784) has been widely used in the sense of D. polystachya, which is an earlier name for D. batatas. It was based on a misidentification of Japanese material as the strictly Indian species D. oppositifolia Linnaeus, which was cited as a synonym in the protologue, thus rendering D. opposita nomenclaturally superfluous and an illegitimate name.