The present paper reports the chromosome numbers and karyotypes of nine species and one variety of Heracleum in the Hengduan Mountains of South-western China. Among them the somatic chromosome number and karyotypes of seven species and one variety are reported for the first time. Their chromosome parameters and karyotypes formulae are shown in Tables 1, 2, and their karyotypes and idiograms are shown in Plates 1, 2 and Fig. 1.
The chromosome number of all the species is 2n=2x=22 except H. millefolium var. longilobum, which has both 2n=22 and 2n=24 (see Plate 2: T--W). The karyotypes of the nine species and one variety of Heracleum consist of metacentric and submetacentric chromosomes. The karyotypes of two species, H. wolongense and H. stenopterum, belong to “lA” type, while the karytypes of the rest belong to “2A” type in Stebbins1 classification. Four species have two chromosomes with satellites. And one B-chromosome is dis-
covered in somatic cells of some H. vicinum individuals (see Plate 1: G).
Classification is discussed based on the analysis of the chromosome number and karyotype of Heracleum. The cytotypes are found different between H. millefolium var. millefolium and H. millefolium var. longilobu. The karyotypic formula of the former one is K (2n) = 22 = 18m+4sm, whereas that of the latter is K (2n) = 22 = 16m+6sm and K (2n) = 24= 18m+6sm. They also differ from one another in order number of the submetacentric chromosomes. Although they are small low-growing plants and have 3-4 pin-
natipartite leaves and seem closely related, they differ greatly in length and density of arrangement of the ultimate lobelets and in indumentum of ovaries. Under SEM, the sculpture pollen exine is shown differentiated between the two varieties. Hence it is suggested that H. millefolium var. longilobum be treated as an independent species. Furthermore, based on the interspecific relationship and variation in chromosome number and karyotypic symmetry, the hypothesis is proposed that the genus Heracleum has been evolving in two directions, from diploid to polyploid and from symmetrical to asymmetrical in karyotype. Finally, according to the data available the correlation is found between the change of chromo-
some number and karyotypic symmetry on one side and the latitudinal distribution of the genus on the other, as 26 species and one variety of Heracleum occur in the Hengduan Mountains, and their external morphological characters and cytotypes have differentiated distinctly, it is considered that the Hengduan Mountains is both the center of frequency and the center of diversity of Heracleum. Moreover, we speculate that the Hengduan Mountains or the Caucasus is the original center of this genus.