Amaranthus rudis and A. tuberculatus, One Species or Two?

PRATT, D.B. AND L.G. CLARK (Department of Botany, Iowa State University, Ames, IA 50011-1020). Amaranthus rudis and A. tuberculatus-one species or two? J. Torrey Bot. Soc. 128: 282-296. 2001.-Waterhemps traditionally have been divided into two species, Amaranthus rudis and A. tuberculatus, based on morphology, but the distinctions break down for specimens collected from a major portion of the total waterhemp geographic range. Two hypotheses of waterhemp species circumscriptions were explored using morphological and isozyme data sets. The first hypothesis holds that there are two waterhemp species as traditionally circumscribed, intermediate morphological materials being hybrids. The second hypothesis holds that there is a single, polymorphic species of waterhemp. Principal components analysis of pistillate morphological, staminate morphological, and isozyme data sets all show a single waterhemp continuum exhibiting geographic variation. A single waterhemp species, A. tuberculatus, is proposed.