240 MASS MUTATIONS AND TWIN HYBRIDS OF 



rare type, followed by secondary mutation in the succeeding gene- 

 rations. 



9. This initial mutability of 0. grandiflora must have yielded, 

 besides the ordinary mutants, hybrid mutants, produced by the 

 combination of a mutated sexual cell with a normal one. If then 

 the offspring of this fecundation is assumed to split in a manner 

 analogous to Mendel's formula for monohybrids, three types must 

 be the result. One of them is the mut. ochracea, which is now a 

 secondary mutant; the second is the mutant hybrid of the type of 

 the species, which will repeat the splitting; and the third must be 

 a constant form of the same type. This last does not appear, and 

 a lethal factor is assumed to answer for this gap. It must be linked 

 to the otherwise pure grandiflora gametes. It explains the absence 

 of the constant type, together with the presence of a corresponding 

 percentage of empty seeds. 



10. In this way the mass mutation as well as the empty grains 

 can be explained by the assumption of two initial mutations of the 

 ordinary type. One is that into ochracea, the other is the origin of 

 a lethal factor linked to the gametes which are not mutated into the 

 weak, yellow form. 



11. The twin hybrids, mentioned under 4, must be the result 

 of the same secondary splitting of the gametes. Those of the gran- 

 diflora type must yield the velutina and the laxa; those carrying the 

 characters of ochracea must give the laeta and the densa. 



12. The twins produced by the crosses of 0. grandiflora with 

 0. biennis, 0. syrticola, and 0. biennis Chicago are constant in their 

 progeny, but the laeta from crosses with 0. Cockerelli repeat the 

 splitting into the types of the twins. 



LITERATURE CITED. 



1. Bartlett, H. H., Mass mutation in Oenothera pratincola. Bot. Gaz. 60 : 



425-456.-1915. 

 2. , Mutations en masse. Amer. Nat. 49 : 129. 1915. 



3. Davis, B. M., Genetical studies on Oenothera. Amer. Nat. 44: 108—115. 

 1901; 45: 193-233. 1911. 



4. De Vries, H., Gute, harte, und leere Samen von Oenothera. Zeitschr. In- 

 dukt. Abst. und. Vererbungs!. 16 : 239. 1916. Opera VI, p. 109. 



5. , Gruppenweise Artbildung. Berlin. 1913. 



6. , On triple hybrids. Bot. Gaz. 47: 1-8. 1901. Opera VI, p. 496. 



7. , Die endemischen Pflanzen von Ceylon und die mutierenden Oeno- 



theren. Biol. Centralbl. 36: 1-11. 1916. Opera VII, p. 36. 

 8. , Oenothera gigas nanella, a Mendelian mutant. Bot. Gaz. 60 : 337— 



345. 1915. Opera VII, p. 27. 



