OENOTHERA GRANDIFLORA AIT. 237 



The cross of the pollen of 0. grandiflora with the female gamete 

 of 0. biennis Chicago produces twins which are quite different from 

 the laeta and velutina, and are therefore called densa and laxa. 

 Formerly I assumed this difference to be due to the splitting of 

 another factor, but my results with 0. grandiflora, in connection 

 with the appreciation of the complicated nature of so many muta- 

 tions (7), open the prospect of considering it as due to the same 

 unit, only under the influence of different linkage. In fact, if we 

 assume the pollen of 0. grandiflora to be dimorphous before fecun- 

 dation, two types of hybrids must be expected in this cross as well 

 as in others. This conception simplifies the problem, although it 

 does not offer a direct proof against the presence of a special splitting 

 factor for densa and laxa. These twins are constant in their pro- 

 geny, even as the laeta and velutina just considered, and thereby 

 indicate the same special condition of the factors involved. When- 

 ever the differentiating characters of the twins are recessive to 

 those of the other parent, the twins must show the same external 

 marks. The hybrids will then be uniform instead of dimorphic, 

 as, for example, in our experiment with 0. grandiflora x 0. syrticola 

 (muricata). 



The cross 0. grandiflora x 0. biennis gave a dimorphic progeny, 

 which may evidently be ascribed to the presence of mutated and 

 normal gametes in the first species. If we assume this to be 

 the true interpretation, the pedigree may be written in this 

 form: 



0. grandiflora x biennis, or 



O. grandiflora tvpica x biennis -f 0. grandiflora ochracea x biennis 



I i 



First generation— biennis ochracea 



Second generation— biennis ochracea ochracea biennis 



Read in this way our experiments show that in the normal ga- 

 metes of 0. grandiflora the characters of the species are recessive 

 to those of 0. biennis, and that uniform and constant hybrids are 

 produced. The gametes which repeated the mutation into ochracea, 

 on the other hand, possessed dominant characters, and the offspring 

 was hardly distinguishable from normal ochracea. But after self- 

 fertilization it split off the biennis type, especially the character of 

 producing stout initial rosettes before making a stem, and this 

 splitting seems to conform to Mendel's law for monohybrids. 



