482 COBB AND BARTLETT: INHERITANCE IN OENOTHERA 



II. On the basis of the preceding assumption it can be under- 

 stood why the phenomenon known as mass-mutation should 

 occasionally occur. Mass-mutation is a term descriptive of the 

 origin of mutations in such inordinately large numbers that the 

 cultures show no resemblance to Mendelian segregation. 



III. In Oenothera pratincola the only mutations which have 

 occurred through mass mutation are a series in which the leaves 

 are revolute instead of flat. These revolute-leaved mutations 

 have appeared in only one strain of the species, to the eye indis- 

 tinguishable from other strains. 



IV. It has been discovered that the difference between the 

 mass-mutant and nonmass-mutant strains is that the latter 

 carry a Mendelian factor determining flatness, in addition to the 

 unbalanced determiner for flatness in the a. portion of the a 

 gamete. In the mass-mutating strain there is no Mendelian 

 factor for flatness. The constitutions of the zygotes are there- 

 fore a^ff and a/SFF, respectively, the former mutable with re- 

 gard to flatness, giving rise to revolute-leaved forms with the 

 constitution a'^E. 



V. In Oenothera pratincola the a gametes are female. Recip- 

 rocal crosses are therefore unlike with regard to unbalanced 

 characters (those confined to the a portion of the a gamete), 

 being matroclinic in each case. Reciprocal crosses are alike as 

 far as Mendelian characters are concerned. Therefore: 



a'/Sff X ajSFf — > a'^Vi, flat-leaved, and heterozygous 

 with regard to the Mendelian factor for flatness, segregating in 

 the F2 generation in the ratio i homozygous flat : 2 heterozygous 

 flat : I homozygous revolute. 



a/SFF X a'/Sff — ^ o;/3Ff, flat-leaved, and heterozygous with 

 regard to capability for giving rise to mutable stocks, segregating 

 in the Fo generation in the ratio i homozygous dominant, flat, 

 immutable : 2 heterozygous, flat, like the Fi generation : i homo- 

 zygous recessive, flat only through the a determiner for flatness, 

 and therefore mutable with regard to revoluteness. 



VI. The results not only afford one of the best examples of 

 Mendelism in Oenothera hitherto adduced, but likewise give an 



