Cyto[~eop, raphy of Eupatorium chinense subsp.

The examination of 1976 herbarium specimens of Eupatorium chinense subsp. sachalinense var. oppositifolium and 908 of var. sachalinense revealed their distinct geographical and ecologial distribution patterns. Var. oppositifolium includes two distinct groups: the diploid type and the polyploid one. The distribution of the diploid type is restricted to the "Sohayaki-Region", and its habitat appears to be restricted to fragile gravitational slopes and rocky areas which lack tall competitors. In contrast, the polyploid type is widely distributed throughout the Japan Archipelago. The polyploids grow successfully in recently cleared dry habitats and in repeatedly disturbed tall grass and forb communities. Climatic warming after the last~glaciation combined with explosive agricultural developments and the polyploids rapid and tall growth habits with agamospermous reproduction seem to have facilitated their north-ward migration and distributional expansion. Karyotype analyses were made on 29 populations of var. oppositifolium which comprised eight cytotypes: 2x, 3x, 4x, 5x based on x = 10, 3 ehromosomally deficient polyploids and an aneuploid with 2n=39. Most populations exhibited various com-binations of polyploid cytotypes. Different polyploid cytotypes how no apparent habitat preferences. None of the polyploid cytotypes appear to compete with each other in colonizing and exploiting newly disturbed habitats. This, together with their agamospermous propagation, rare sexuality and random association when colonizing, can result in an intricate mixture of various cytotypes within local populations