1919] DeVRIES— OENOTHERA RUBRINERVIS 25 



however, it is labile, since tall and low specimens appear in the first 

 generation of its cross with O. nanella. In many ternary crosses of 

 hybrids of this mutant the dwarfish stature dominates over the tall 

 condition, but the dominance is not always absolute and sometimes 

 3-5 per cent of tall specimens appear among the dwarfs, as I have 

 shown in Gruppenweise Arthildung. This fact evidently supports 

 our conception. 



The conclusion from this discussion is that since brittleness and 

 dwarfish stature are in some cases recessive to and in other cases 

 dominant over their antagonists, these latter must be sometimes 

 in the active and in other instances in the labile condition. 



Summary 



1. Oenothera rubrinervis is a half mutant, produced by the copu- 

 lation of a mutated gamete with a normal velutina gamete of O. 

 Lamarckiana. 



2. In consequence, it produces about one-fourth empty grains, 

 a mass mutation of about one-fourth pure or double mutants, 

 and one-half specimens of O. rubrinervis, which will repeat the 

 splitting. 



3. The pure or double mutant is called O. mut. deserens. It 

 is very similar to O. rubrinervis, but the leaves of its young rosettes 

 and the bracts of its flower spike are broader and more even. 



4. O. mut. deserens is constant from seed. It has no hereditary 

 empty grains. 



5. The formula for the self-fertilization of O. rubrinervis is 

 therefore O. {deserens -\-velutina) = des.Xdes. -jrvelu. Xvelu. -\-des. X 

 velu. The first combination gives the mass mutation, the second 

 the empty grains, the third the normal plants of O. rubrinervis. 



6. In crossing with other species the two kinds of gametes will 

 produce twin hybrids, as, for example, laeta and velutina. This 

 assertion has been controlled by making the corresponding crosses 

 of 0. mut. deserens and O. mut. velutina. The first produce the 

 laeta and the second the hybrid velutina. The result of a cross of 

 0. rubrinervis is equal to the sum of these two crosses. 



7. Outside of the mass mutability into O. deserens, O. rubrinervis 

 is not known to mutate to any noticeable degree. This shows that 



