TWIN HYBRIDS OF OENOTHERA HOOKERI T. AND G. 



The researches of Morgan on lethal factors, of Bartlett on mass 

 mutation, and of Renner on empty seeds, have led me of late to 

 the conception that lethal factors and hybrid mutants play a large 

 part in the splitting phenomena which accompany the normal 

 mutations of so many species of Oenothera. The question, which 

 part of those phenomena may be explained by this view, has to 

 be faced. The crosses of Oenothera Hookeri with the mutating species 

 behave in so many respects differently from the remainder of the 

 cases that they seem to afford suitable material for this study. 

 Oe. Hookeri x Oe. biennis produces a splitting in the second gene- 

 ration that does not follow the rules of Mendel and Oe. Hookeri x 

 Oe. Lamarckiana produces twins of the type of laeta and velutina, 

 but without their ordinary constancy, the laeta splitting in every 

 generation into the same two types. 



The discovery of mass mutation in Oe. grandiflora and of its 

 relation to the production of twin hybrids by this species affords 

 the clearest instance for this new conception and for this reason 

 I shall describe in the first place the results of my crosses of this 

 species with Oe. Hookeri. 



Crosses of Oenothera Hookeri with Oe. grandiflora 



My race of Oe. grandiflora was derived from a seed collected near 

 Castleberry, Ala. It splits in every generation into two large groups. 

 One is like the parent and repeats the splitting. The other is a weak 

 form with pale broad leaves and a low stature. Most of its specimens 

 die in early youth, some afterwards, but some reach the period of 

 flowering and ripening their seeds. From these the pale race is 

 reproduced pure. I consider this splitting, for reasons given elsewhere 

 {Opera VII, p. 317) as an instance of mass mutation and conse- 

 quently call the aberrant type Oe. grandiflora mut. ochracea. Further- 

 more I assume this mutation to be the cause of the production of 

 twins after crossing. These appear in about equal numbers. The 

 laeta come from the fertilization of the mutated gametes, but the 

 non-mutated sexual cells produce the velutina. The proof for this 



