VOLUME LXV NUMBER 5 
THRE 
BOTANICAT GAZETTE 
MAY 1918 
MASS MUTATIONS AND TWIN HYBRIDS OF 
OENOTHERA GRANDIFLORA AIT. 
Htuco DEVRIES 
(WITH SIX FIGURES) 
Under the name of mass mutation, BARTLETT has described a 
new phenomenon observed by him in Oenothera pratincola and 
O. Reynoldsii. 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 Chrysanthe- 
mum and in horticultural instances. In the species studied by 
BarTLert (x, 2), about one-half or even a larger number of the off- 
spring 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. 
Ocnothera. pratincola has produced four mass mutants: mut. 
formosa, albicans, revoluta, and: setacea; O. Reynoldsii two, mut. 
‘Semtalia and debilis. BARTLETT has pointed out that the phenome- 
non 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 
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