Natural and Artificial Hybridization Between Helianthus mollis and H. occidentalis

The classification of species of Helianthus has long been a source of difficulty for amateur and professional botanists alike. Watson (1929), in his contribution toward a monograph of the genus, commented on the tendency of certain species groups to intergrade imperceptibly with one another. Heiser (1947, 1949, 1951) has shown that much of the variation encountered in annual sunflowers is due to interspecific hybridization. In diploid perennial species of Helianthus lack of genetic barriers to interspecific hybridization have been demonstrated by Long (1955 a,b) and Jackson (1955, 1956). As a result of recent work in the perennial species several supposedly rare taxa have been shown to be naturally occurring interspecific hybrids or their derivatives. The first putative hybrids between H. mollis Lam. and H. occidentalis Riddell were recognized and collected by G. Moore in Laclede Co., Missouri. During a recent study of certain perennial sunflowers, the writers encountered numerous populations of natural hybrids between these two species. Several were selected for analysis and are presented here.