On the left: Lethal factors located in the gametes of Oenothera muricata. The rigens complex is inviable due to a lethal factor within the pollen. The curvans complex is incapable of producing functional ovules.On the right: Zygotic lethal factors in Oenothera lamarckiana. The homozgotes velans x velans and gaudens x gaudens are not viable. Depicted are the genotypes of the F1 and the F2 generation. (R. E. CLELAND, 1935).