Abstract: Southwesten of China is one of the two centers of distribution of Heracleum in the world. In this paper, fifteen species distributed in this area was dissected and observed. The anatomical characters of 15 species are very diverse and listed in Table 1 and Figure 1 to 3.Comparing the petiolar anatomy of populations belonging to 4 species which distributed in different area or altitude, it is shown that the anatomical charecters of petioles of Heracleum possess certain stability except with some variations in numbers of vascular bundles and size of the cross section within a species, andcan be used as a useful taxonomical evidence.According to the pattern of vascular bundles'distribution, combined with theshape of the cross section, the presence and shape of the groove on the adaxialside, smooth or undulate margin and the presence and shape of pith cavity, the anatomical structures of petioles of 15 species are divided into 5 forms (see fig. Ito 3). According to the charecters of 5 forms petiDles, combined with charecters ofeXterior shape, pollen morphology and cytology, the probable evolutionary relationships of the 5 forms of petiolar anatomy and the classification to genus Heracleumare discussed.In the end, by analysing the regular of distribution of Heracleum in China. the species with the 5 forms of petiolar anatomy and many other species of Herucleumare concentrated in Hongduan Montains of Southwesten in China. As a result, Hongduan Montains is not only the center of abundance but also the biggest centerof diversity of this genus.

Key words: Heracleum, Petiolar anatomy, Evolution, Center of diversity