114 Shull. 



of the bursa-pastoris plants of the F 2 will give again the 15 : 1 ratio 

 in the F 2 from a new cross with Heegeri, though seven of them will 

 continue to breed true, indefinitely, as long as they are selfed. Only 

 by crossing can the differences among these seven be discovered. 



As crossing the Bursas is a rather tedious process, and the 

 results from selfing the hybrids during these five generations have so 

 abundantly demonstrated the method of inheritance of the triangular 

 capsule, I have made no attempt to analyze the extracted dominants, 

 but a new cross made for another purpose has incidentally given proof 

 of the central fact to be expected from such an analysis, namely, that 

 the extracted dominants are not all identical with the original dominant 

 parent in their hereditary behavior. As already indicated, the most 

 satisfactory method of testing these various extracted dominants is to 

 make new crosses between them and the recessive Heegeri. Certain 

 crosses among the extracted dominants themselves would also yield very 

 characteristic results, as, for instance, a cross between CCdd and ccDD, 

 which would produce the original Fi genotype and yield 15 : 1 in the 

 F 2 . When crossed with Heegeri the extracted dominants from the 

 original F 2 should all yield uniform Fi progenies of B. bursa-pastoris, 

 of course, but among every seven extracted dominant plants from the 

 F 2 of the original cross, 



1 (CCDD) should yield only 15 : 1 ratios in the new F 2 families; 



I should yield 15:1 in half of the new F 2 families 



/-v>n,7\ I an d 3 : 1 ratios in the other half; and 

 I (2 L>L>Ud) I 



should yield only 3 : 1 ratios in the F 2 of the new 



2 \ and 



. cross. 



( (ccDD) j I 



In crosses between B. Heegeri and extracted B. bursa-pastoris plants 

 from the Fs and later generations of the original cross, the same 

 distribution of dominant and recessive types as represented in the tabu- 

 lation just given, would appear only when the extracted dominants used 

 in the new crosses were themselves included in a family having a 15 : 1 

 ratio. In families of whatever generation, showing the 3 : 1 ratio, the 

 extracted dominants could be of but one type in any given family; 

 they must be either CCdd or ccDD and these in new crosses with 

 Heegeri would yield only the monohybrid ratio 3:1. 



In 1911 a cross was made between an extracted Bursa Heegeri 

 simplex and an extracted B. bursa-pastoris heteris, the latter being 



