MASS MUTATIONS AND TWIN HYBRIDS OF 

 OENOTHERA GRANDIFLORA AIT. 



(with six figures) 



Under the name of mass mutation, Bartlett has described a new 

 phenomenon observed by him in Oenothera pratincola and 0. Rey- 

 noldsii. Ordinarily, mutations occur in the species of Oenothera in 

 about 1 per cent or less of the offspring of self-fertilized individuals, 

 just as they do in the cases of Linaria and Chrysanthemum and in 

 horticultural instances. In the species studied by Bartlett (1, 2), 

 about one-half or even a larger number of the offspring were seen 

 to deviate from the parental type in a particular direction. These 

 are called mass mutations; they may appear in the same sowings 

 with normal mutations in other directions. 



Oenothera pratincola has produced four mass mutants: mut. For- 

 mosa, albicans, revoluta, and setacea; 0. Reynoldsii two, mut. semialia 

 and debilis. Bartlett has pointed out that the phenomenon bears 

 a certain degree of resemblance to Mendelian segregation, and 

 assumes that the fundamental mutation possibly occurred in only 

 one of the two gametes in a generation preceding the one in which 

 the diversity becomes manifest (2). 



Guided by these principles, I have studied the phenomenon of 

 mass mutation in Oenothera grandiflora in connection with its ability 

 to produce twin hybrids in certain crosses. This form of splitting 

 in the first generation after a cross was first discovered in 0. La- 

 marckiana (6, 9), but was shown by Davis (3) to occur in 0. grandi- 

 flora also. I found that the twin hybrids may be considered as a con- 

 sequence of the mass mutation, the mutated gametes producing one 

 of the twins and the typical sexual cells the other. This conception 

 evidently may be applied to 0. Lamarckiana and make some previous 



