1. The abnormal course of nuclear division induced by colchicine in the root tip cells of Allium cepa is described.2. Since the effect of colchicine differs not only in different tissues in the same root, but also even in the same tissues of the same root tip, the root treated with colchicine is of mixoploid nature.3. Tetraploid plants are produced in both Lycopersicum esculentum and L. pimpinellifolium; the former has high self-sterility, while the latter has high self-fertility. Besides, among the polyploid are usually found some mixoploid plants which are of no practical value from the breeding point of view.In conclusion, I have great pleasure in recording my gratitude to Hon. Professor K. Fujii, for valuable criticism and guidance. My thanks are also due to Professor T. Nakai, Director of the Botanical Institute, for the facilities given me for studying at the Institute, and to Professor Y. Sinotô for much helpful advice. The expenses for carrying out of this study were partly defrayed out of a grant from the Japan Society for Advancement of Cytology, to which body the writer wishes to express his best thanks.