By means of artificial induction of flowering and seed setting, a breeding program of sweet potato plant may be established with definite purposes in view, such as to breed it for high productivity, starch content, disease resistancc, etc. all being significant for potato production in the middle portion of Japan. The present cxperiment was undertaken to get some information about the effect of overwintering of vines on the flowering and fruiting babit in sweet potato plant. Thirty varieties of Ipomoea Batatas Lam. were used as materials. On September 20th, healthy strong vines taken from our experiment field, were cut into pieces about 40 cm. long, and were transplanted in 27-cm. pots. These potted plants were transfered to the green house on November 25th, to be kept in it throughout the winter, and were shifted again to the field on April 25th. At that time, some of these vines were girdled, aslanting incision being made across the stem, at a distance of l0 cm. from the basal end. Fourteen varieties were induced to flowering as a sesult of overwintering of vines, of which nine set seeds by self-pollination. The amounts of carbohydrates such as starch and sugars were found markedly higher in the vines which had been induced to flower, while the moisture and nitrogen contents in those vines were much smaller than in the disbudded ones. In contrast to the non-flowering vines, the flowering ones had less active cambiums and somewhat deformed phloem cells, even though they produced many secondary-xylem elements. Thicker walls of the cells in the pericycle, xylem and phloem tissues were rather common in the flowering stems. The sensitivity of sweet potato plant to short-day treatment with 8- or 9-hour-daylength varied according to their age, some varieties being induced very readily to flower after overwintering of the plant. (Laboratory of Agronomy, Faculty of Agriculture, Tokyo Imperial University)