Many species are difficult to identify, however, and traditional infrageneric classification based on plant morphology alone, such as those of Stern (1956), Traub (1963) and Davis (1999, 2001), has not reflected what is known about its evolutionary history, due to the morphological similarities among the species and relative lack of easily discernible distinguishing characteristics. Stern divided the genus into three series according to leaf vernation (the way the leaves are folded in the bud, when viewed in transverse section, see Description);