Copyright 1998-2022 Yoshiaki Yoneda
The plants in section Pharbitis have the most primitive characteristics in subgenus Ipomoea, and most species have three carpellar pistils (three-lobed gynoecia).
In the history of taxonomy, Choisy proposed the genus Pharbitis to be independent of and separate from genus Ipomoea, since the sections of Pharbitis have three carpellar pistils. Later, Clark proposed Ipomoea subgenus Pharbitis. After the introduction of botany as a Western science to Japan, the academic name of Pharbitis nil Choisy has been broadly applied to Asagao in Japan. In the field of horticulture, this academic name becam commonly used, and the author also adopted this name until a recent date. However, in the plant classification, Choisy's system is not supported by most taxonomists. The closely related Marubaasagao has been called Ipomoea purpurea (L.) Roth for a long time, and the academic name of Asagao is Ipomoea nil (L.) Roth from the taxonimical point of view". From both historic and gardening points of view, Asagao, the morning glory, is described as Ipomoea nil (Pharbitis nil) in this morning glory database.
Austin and Huaman (1996) investigated plants of Convolvulaceae on the American continents and classified plants of the Ipomoea genus into several subgenera, sections and series. The following shows section Pharbitis in subgenus Ipomoea L.
Subgenus Ipomoea L.
__Section Pharbitis (Choisy) Griseb.
______Series Pharbitis (Choisy) D.F. Austin
__________Marubaasagao Ipomoea purpurea (L.) Roth is included in this series.
______Series Heterophyllae (House) D.F.Austin
__________Amerikaasagao Ipomoea hederacea Jacq.,
__________Noasagao Ipomoea indica (Burm.f) Merr., and
__________Asagao Ipomoea nil (L.) Roth are also included in this series.
______Series Tyrianthinae (House) D.F. Austin