10 FRIEDA COBB 



grees may be, act in the same way. For instance, an f. typica from mut. 

 grisella from f. typica C behaves exactly as f. typica C with no mutations 

 in its direct ancestry, at least during the period in which the strain has 

 been carried in the garden. 



It may also be noted that all plants of mut. formosa appear the same 

 and behave the same genetically regardless of extraction. Those used in 

 crosses were of five different types of extraction: (1) directly from f. typica 

 E, by self-pollination; (2) from f. typica E, first by mutation to angusti- 

 folia, then to mut. nitidissima and finally to mut. formosa, all by self- 

 pollination; (3) from the cross mut. formosa X f. typica E, by matroclinic 

 inheritance; (4) from the cross mut. formosa X strain C, by segregation; 

 (5) from crosses with both strain C and strain E, by segregation and matro- 

 clinic inheritance, e.g., ( (mut. formosa X mut. latifolia C} mut. formosa 

 X f . typica E) mut. formosa. 



In many thousand offspring, mut. formosa has produced nothing but 

 revolute-leaved plants. 



DISCUSSION 



It may seem to those who are used to working with organisms in which 

 clear Mendelian inheritance is the usual thing, that this case in Oenothera 

 pratincola has been worked out with unnecessary elaboration. But several 

 considerations should be borne in mind: first, that evident Mendelian 

 inheritance is so rare in Oenothera that only two indisputable cases have 

 been recorded, that of mut. brevistylis (DE VRIES 1901, p. 223; 1903, pp. 

 151-179, 429) and that of the dwarf mutation from mut. gigas (DE VRIES 

 1915 b) and that all instances deserve therefore to be thoroughly examined; 

 second, that in this case the Mendelian inheritance is apparently modified 

 by inheritance of another kind, working simultaneously with and inde- 

 pendently of the Mendelian inheritance; and third, that the hypothesis of 

 heterogametism put forth to explain this other type of inheritance needs 

 further testing. 



If the explanations offered here are correct, we have, in addition to 

 Mendelian inheritance masked by heterogametism, mutation masked by 

 Mendelian factors. That the a of strain C can undergo mutation to a.' is 

 shown by the presence of a few revolute-leaved plants in the F 2 generation 

 of the cross f . typica C X mut. formosa. These plants derive their a from 

 strain C, but have the dominant Mendelian factors which are present in 

 strain C replaced by their recessive allelomorphs (see table 8). There 

 seems no reason to doubt that this mutation in a occurs just as frequently 

 in the presence of the Mendelian factors for flatness (i.e., in pure strain C) 



