MENDELIAN INHERITANCE IN OENOTHERA PRATINCOLA 11 



as in their absence. If such is the case, Mendelian factors hide the muta- 

 tion until a suitable cross occurs to remove the factors. This may have 

 some bearing on the question whether crossing induces mutation; it may 

 be that crossing merely makes possible an external expression, by removing 

 concealing Mendelian factors, of changes which occurred long since in the 

 germ-plasm and have been passed on from generation to generation giving 

 no visible sign of their presence. In the same way a single mutation, the 

 loss of one concealing factor, in a single chromosome, might bring to light 

 a whole series of new forms. (The several revolute-leaved types that have 

 occurred in the experiment garden can all be permanently concealed by the 

 single pair of Mendelian factors for flatness.) This may explain in part 

 the apparent periodicity of mutability. Perhaps the organism does not 

 have increased tendency to change, but hoards actual changes until chance 

 brings them to light, "gruppenweise," by the removal of inhibiting factors 

 through a mutation. 



Though the condition of the Mendelian factors in Oenothera pratincola 

 strains other than strains C and E has not been investigated, it seems 

 probable, since they have given no revolute-leaved plants, that the other 

 six strains are homozygous with regard to the factor for flatness. If strain 

 E is the only one of the eight strains carrying the recessive factors, it might 

 seem likely that it arose from one of the other strains by loss of a dominant 

 factor, and consequent Mendelian segregation, rather than that the reverse 

 change took place, were it not for the fact that strain E has produced in 

 the experiment garden a mutation (mut. nitidissima) which, as shown by 

 its behavior in crosses with mut. formosa, is a homozygous dominant in 

 regard to the Mendelian factors for flatness. Whether both dominant 

 factors were present in the original plant of mut. nitidissima and its FI 

 generation is a question which cannot be answered, but it is clear that they 

 are now present in the strain as it is carried on in the garden. 



That a pure dominant strain might arise from a pure recessive strain, 

 or the reverse, leaving no heterozygous plants to tell the tale, seems pos- 

 sible, for the heterozygotes, as they appear in the experiment garden, tend 

 to have poor and irregular leaf development as young rosettes, and might 

 easily be eliminated by natural selection. The modification of a factor 

 for flatness in a single chromosome of a homozygous dominant, or the 

 reverse modification in a homozygous recessive, would then be the only 

 change in the germ-plasm necessary to produce one homozygous strain 

 from the other. 



Some explanation should be made of the unusually low germination per- 

 centages occurring in the cultures recorded in this investigation. As the 



GENETICS 6: Ja 1921 



