According to Austin and Huaman ¹⁾, Ipomoea is divided mainly into 3 subgenuses; subg. Eriospermum, subg. Ipomoea, and subg. Quamoclit. In subg. Ipomoea, there are 2 sections; sect. Ipomoea and sect. Pharbitis. The latter sect. Pharbitis is divided into 3 series; ser. Pharbitis, ser. Heterophyllae, and ser. Tyrianthinae. I. nil, I. hederacea, I. indica, etc. belong to ser. Heterophyllae.

The ditribution of 6 species in ser. Heterophyllae of sect. Pharbitis examined are limited to the American continent, except for pantropical species of I. nil and I. indica ³⁾. Moreover, other most species in sect. Pharbitis are distributed in American continent. Thus, we can consider that I. nil would be of American origin.

The Japanese morning glory seems to have been originally introduced from China in Nara era (710-784). With regards to the Asian strains such as the Chinese Peking Tendan and the Nepal, it can be assumed that the place of origin of the Japanese morning glory is somewhere in Southeast Asia, including the area of Nepal ^9,10). However, I. nil has been collected widely in tropical to temperate regions of the world.

Yoneda¹¹⁾ recognized difference in mobility of a cathodal peroxidase isozyme between Asian strains and an African strain. This African strain has a strong resemblance to a Brazilian strain. What relationship does the Asian group have to the New World group?

According to Verdcourt ^6,7), the I. nil of the African continent is estimated to have escaped from cultivation and become naturalized. If this is the case, the origin of I. nil must be either Asia or the American continent. Fang & Staples ⁴⁾ studied the Convolvulaceae in China, and estimated that I. nil in China had been cultivated and had escaped, and that its original place might be the American continent.

The author preliminarily studied and compared various characters of 13 wild or local strains [Asia (5 strains), Australia (1 strain), South America (5 strains), Central America (1 strain), Africa (1 strain)] of Ipomoea nil (Yoneda, unpublished). Among these 13 strains, the high number of flowers per peduncle in the four Colombian strains, as well as in the Brazil and Africa strains, are considered to reflect the primitive characteristics of the morning glory. As for the leaf form, it is conceivable that the main lobe-narrow type of strains TKS (Tokyo Kokei Standard), Nepal, and Philippines, as well as the main lobe-wide type of strains Brazil, Africa, Mexico, and Iran, are differentiated from the main lobe-intermediate type of Australia and 3 strains of Colombia. Epidermal hairs are observed from the cotyledonary petiole (connecting the cotyledons to the hypocotyl), to the upper part of the hypocotyl. Cotyledonary petioles of 2 strains of Colombia have fairly many epidermal hairs, while strains TKS, Africa, Brazil, and Mexico have few or almost no hairs.

If these characteristics are taken into account, the author thinks that 2 strains of Colombia are interesting perspectives into the origin of the morning glory, Ipomoea nil. If I. nil has its origins in the tropical American continent, how could the morning glory enlarge its distribution to pantropical zones across the wide oceans ³⁾? There is no evidence that birds or sea currents dispersed morning glory seeds over the ocean. One possibility is that human beings influenced the dispersion. Because morning glory seeds had been used medicinally for a long time, the dispersion could have occurred along with the emigration of human beings ²⁾. However, because the Asian strains of the morning glory have been cultivated from ancient times, it is unlikely that propagation from the American continent occurred after Columbus' discovery. If we had any evidence of Yen's prehistoric kumara route from South America to the islands of the Pacific ocean by humans causing the dispersion of the sweet potato (Ipomoea batatas) ⁸⁾, it would be possible to assume that morning glory seeds had also dispersed along this ancient migration flow. Whether the place of origin of the morning glory is in America or Asia, it is necessary to study the mechanisms for the morning glory's dispersion over the wide seas ³⁾.

References

1. Austin, D. F. and Huaman, Z. (1996) A synopsis of Ipomoea (Convolvulaceae) in the Americas. Taxon 45: 3-38.
   
2. Austin, D. F. (2000). The search for "kaladana" (Ipomoea, convolvulaceae). Economic Botany 54: 114-118.
   
3. Austin, D. F., Kitajima, K., Yoneda, Y. and Qian, L. (2001) A putative American plant, Ipomoea nil (Convolvulaceae) in pre-Columbian Japanese art. Economic Botany 55: 515-527.
   
4. Fang, R.-ch. and Staples, G. W. (1995) Convolvulaceae pp. 271-325, In: Editorial Comittee (ed.). Flora of China. Vol.16. Missouri Botanical Garden, St. Louis, MO.
   
5. Ooststroom, S. J. V. (1940) The Convolvulaceae of Malaysia. 3. The genus Ipomoea. Blumea 3: 481-582.
   
6. Verdcout, B. (1957) The names of the morning glories cultivated and naturalized in East Africa. Taxon 6: 231-233.
   
7. Verdcourt, B. (1963) Convolvulaceae Pp. 1-161, In: Hubbard, C. E. and Milne-Redhead, E. (eds.) Flora of Tropical East Africa. Crown Agents for Overseas Governments and Administrations, London.
   
8. Yen, D. E. (1974) The sweet potato and oceania. Bishop Museum Press, Honolulu, Hawaii.
   
9. Yoneda, Y. (1995) Asagao, a gift from Edo-era: from dreams to science. Shoka-bo, Tokyo.
   
10. Yoneda, Y. and Takenaka, Y. (1981) Natural-Color Illustrated monograph of Japanese morning glory. New edition. Hokuryukan, Tokyo.
    
11. Yoneda, Y. (1970) Peroxidase isozymes in four strains of morning glory. Japan. J. Genetics 45:183-188.