HYBRIDIZATION IN THE ANNUAL SUNFLOWERS: HELIANTHUS ANNUUS x H. DEBILIS VAR. CUCUMERIFOLIUS

Abstract

In 1945 seeds of sunflowers from three localities in Texas were sent to the writer by E. Mortensen. When plants from these seeds were grown they were found to approach H. debilis var. cucumerifolius in many characters although on the whole they were nearer to H. annuus in appearance (fig. 1 ). Subsequently a large amount of herbarium material was studied and it was concluded that natural hybridization between these two species probably occurs in eastern Texas. Field work in Texas in 1949 verified this conclusion, and some observations on the variation of these two species with an analysis of both natural and artificial hybrids is presented in the following paper. Helianthus annuus is a wide-spread weed in much of North America and is morphologically quite diverse. Preliminary remarks concerning its variability have already appeared (Heiser, 1949). Plants of H. annus from eastern Texas are usually very distinctive. The lemon yellow rays, the deep irregular serration (fig. 3, e) and somewhat undulate margin of the leaf, the purple mottled stems, and a tendency to produce long lateral branches from the base of the plant rather effectively distinguish most eastern Texas populations of this species from populations of H. annuus occurring elsewhere, although these traits occasionally appear outside of eastern Texas. In addition, the eastern Texas plants of H. annuus generally have few rays, small disks and bracts, and small achenes. These three