Julius von Sachs (Germany, 1832〜1897), a pioneer of plant physiology, said in the late 1800s that a specific organ-forming substance was involved in the formation of each plant organ. One of them was a flower-forming substance. Sachs covered a part of a plant with a light-proof box and illuminated another part of the plant. He found that the plant part in the darkness formed flowers. He explained this phenomenon by assuming the existence of a flower-forming substance, which was produced by light, and transferred to another part of the plant to produce flowers. He used the morning glory, a species closely-related to the Japanese morning glory, Pharbitis nil. The flower-forming substance of Sachs would be equivalent to florigen proposed later on.
　　The study of flowering in the sense of modern plant science started from the finding of photoperiodism by W. W. Garner and H. A. Allard (U.S.A.) in 1920. They found that plants flowered in response to the change in day length. Early work on flowering was mainly done from the view point of ecology. Photoperiodic responses of vegetable crops were examined and the results compared with the daylength in certain areas at different latitudes to determine if those vegetables could be cultivated in those areas. In the 1930s, J. Bonner, K. C. Hamner and others (U.S.A.) started the physiological study on flowering, and M. Kh. Chailakhyan (Soviet Union) proposed the florigen theory.
　　In Japan, Yoshiji Yoshii, Tohoku Imperial University, published his first paper on photoperiodism in 1925. Shidai Nakayama soon started studying flowering physiology with Yoshii. Nakayama used Pharbitis nil as an experimental material. On the other hand, Shun-ichiro Imamura, Kyoto University, started studing flowering in the mid-1930s. Imamura and his coworkers used several plant materials in their early days, but soon concentrated their studies on P. nil. This is the beginning of the history of study of flowering in P. nil. The selection of plant material is an important factor influencing the direction of study. P. nil is a quite sensitive short-day plant, and therefore it is very suitable material for the study of flowering. Imamura's choice of P. nil contributed greatly to the development of flowering physiology.
　　Imamura was stimulated by the paper of Garner and Allard (1920) and became interested in flowering. The time was just before World War �Uand there was a demand to increase crop yield. Sweet potato (Ipomoea batatas) was a good candidate, and efforts were made to select high yielding cultivars. Sweet potato flowers only occasionally and its reproduction is mainly by vegetative propagation, and therefore hybridization breeding is difficult. Accordingly, several methods had been tried to promote sweet potato to flower. It had been known that sweet potato could flower if grafted to P. nil. Imamura interested in this story started studies on flowering in sweet potato. His interest soon changed to P. nil. He used several varieties as experimental materials in early works, such as ‘Violet’、‘Tendan’ and ‘Kidachi’． Imamura selected ‘Violet’ as the most suitable material, and then pursued studies on flowering physiology in P. nil mainly with ‘Violet’． A nearly isogenic line of ‘Violet’was established. Today, this Kyoto University line of ‘Violet’ is used all over the world.
　　The results of the study by Imamura on flowering physiology in P. nil were first reported at the Japanese Society of Botany in 1941. Atsushi Takimoto, a coworker of Imamura, started studies on flowering in P. nil in 1949. Takimoto and Imamura began to publish their papers in 1953. Thereafter, many studies on flowering in P. nilwere actively pursued by Imamura and his ex-students, Takimoto, Kiyomi Wada (Shizuoka University) and Yukiyoshi Ogawa (Mie University). When Imamura retired from Kyoto University, the knowledge accumulated were published in ”Physiology of Flowering in Pharbitis nil" from the Japanese Society of Plant Physiologists in 1967. The plant physiologists who established the basis of flowering physiology of P. nil other than Imamura and his colleagues and Nakayama mentioned above were Yukindo Oota (Nagoya University) and Hiroshi Harada (Tsukuba University). Those who are currently working in this area have been influenced directly or indirectly by these pioneers.