14 FRIEDA COBB 



leaved plant. This segregation has been followed to the F 4 generation. 

 The f . typica plants descended from this cross have the constitution afiFF 

 or a'0Ff, and are called f. typica M (homozygous) and f. typica M (hete- 

 rozygous), respectively. 



9. The reciprocal cross, strain C X mut. formosa (a$FF X a'pff) also 

 gives only flat-leaved plants in the Fi generation; but in the F 2 generation 

 there is no Mendelian segregation, though there occurs by mutation a 

 small percentage (1.6 percent) of revolute-leaved plants. In this cross 

 the inheritance appears to be matroclinic. 



10. In reciprocal crosses between mut. formosa (a'pff) and f. typica E 

 (apff) the inheritance is purely matroclinic, as the two parents are alike in 

 regard to the Mendelian factors for flatness. 



11. The results, recorded in the tables of this paper, of various crosses 

 between f. typica M and f. typica C, f. typica E and mut. formosa are all such 

 as to favor the hypotheses of heterogametism and the presence of a pair 

 of Mendelian factors for flatness in Oenotkera pratincola. All the results 

 obtained could be correctly predicted on the assumption that: 



f . typica C = a@FF 

 f . typica E = afiff 

 mut. formosa a'ftfj 



12. The a of strain C may become mutated to a ', but in pure strain C 

 this change can find no expression, because of the Mendelian factors for 

 flatness for which this strain is homozygous. That the change does some- 

 times occur here, as in strain E, is shown by the occurrence of a few revo- 

 lute-leaved plants in the otherwise uniformly flat-leaved F 2 generation of 

 the cross f . typica C X mut. formosa, plants in which the a portion of the 

 constitution came from strain C and in which the Mendelian factors for 

 flatness are replaced by their recessive allelomorphs from strain E. 



13. The difference, with regard to Mendelian factors, between strains 

 C and E is paralleled by the difference, with regard to the same factors, 

 between mut. nitidissima, a type which has arisen in the experiment 

 garden, and strain E from which it arose. Strain E is recessive; mut. 

 nitidissima is a homozygous dominant. Evidently a homozygous dom- 

 inant strain can arise from a homozygous recessive strain ; the reverse proc- 

 ess has not as yet been known to take place in the garden. 



14. It is concluded that mutation may be masked by Mendelian factors, 

 and that the apparent induction of mutation by hybridization may be 

 merely the first appearance of changes which occurred in the past and 



