JOURNAL OF THEARNOLD ARBORETUMVOLUME 66 OCTOBER 1985 NUMBER 4 A REVISION OF AMMANNIA (LYTHRACEAE) IN THE WESTERN HEMISPHERE SHIRLEY A. GRAHAM AMMANNIA L. is a genus of about 25 species of aquatic or marsh-inhabitingherbs distributed in both the Temperate and Tropical zones. It is best repre-sented in Africa (16 species), with a maximum of seven species occurring oneach of the other continents. The only monograph of the genus (Koehne, 1903)is outdated because of accumulated changes and additions to the taxonomyand nomenclature and the more extensive collections now available. Many species ofAmmannia'are distinguished from one another by seeminglyminor qualitative differences that are difficult to recognize in practice. In somecases, species limits are based more on geographic disjunctions than on mor-phological distinctions. Relationships of morphologically similar species oc-curring on different continents are unknown. The need to resolve the confusionsurrounding the variability in species in the New World in order to preparetreatments for several floras now underway has stimulated this revision. Thedifficulty in obtaining viable seeds necessary for biosystematic investigationsof the narrowly endemic African and Asian species also prompted restrictionof the study to the species occurring in the Western Hemisphere. MORPHOLOGY The genus Ammannia was divided by Koehne (1880b) into two subgeneraand two sections. Subgenus Cryptotheca (Blume) Koehne, comprising the singlespecies A. microcarpa DC., is unique in the Lythraceae by virtue of its parietalplacentation. The remainder of the genus comprises subgenus Ammannia (for-merly subg. Euamrnannia Koehne), which is further divided into two sectionsand four series, all highly artificial in nature. Five species, representing bothsections of subgenus Ammannia, occur in the Western Hemisphere. SectionAmmannia (formerly sect. Astylia Koehne), with short or included styles, isrepresented by A. latifblia L. and the adventive A. baccifera L.; section Eustyliac President and Fellows of Harvard College, 1985.Journal of the Arnold Arboretum 66: 395-420. October, 1985.