Abstract
Taxonomic decisions presented in this study of Vallisneria are founded on the consistency of comparable staminate and pistillate floral structures considering the geography and dioecious nature of the genus. Field studies, realized in southern Europe, eastern North America, Central America, northern South America and the Greater Antilles, formed the basis for the development of the present knowledge of characteris. Umbel and spike-like inflorescences were discovered from three localities in the Americas. Staminodia were encountered in the female flowers of Vallisneria spiralis. Two species including four varieties are recognized, mapped and illustrated. Infraspecific taxa are V. spiralis L. var. spiralis, V. spiralis var. denseserrulata Makino, V. americana Michaux var. americana and V. americana var. biwaensis (Miki) Lowden, comb. nov. These taxa are delineated according to the degree of connation of fertile filaments in the staminate flower and adnation of staminodia to stigma—style surfaces in the pistillate flower. Both species converge along what appears to be a continuous gradient in floral variation.
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