100 



Shall. 



The reappearance of the Heegeri-type of capsule in only 111 indi- 

 viduals out of a total of 2540 plants in the F 2 of crosses between 

 B. Heegeri and an American biotype of B. bursa-pastoris was at first 

 accepted as evidence of a non-Mendelian behavior (SHULL 1907, 1909 b). 

 This ratio, 21*9 : 1, did not approximate so closely the ratio 15 : 1 as 

 to suggest clearly by itself the existence of two determiners for the 

 triangular capsule, and it was not until the appearance of NILSSON- 



Fig. 3. Anatomical details of the capsule-wall of Bursa bursa-pastoris (A) and of 

 B. Heegeri (B). X 100. Drawn by J. MARION SHULL from microtome sections. 



ELBE'S papers in which duplicate determiners were demonstrated for 

 the presence of a ligula in Avena and for red pericarp- color in Triticum 

 that a similar explanation was recognized as possibly available for the 

 unexpected ratio for capsule-form in Bursa. 



The phenomena expected in the first four generations following any 

 cross in which duplicate determiners are involved and in which dominance 

 is complete, may be considered with advantage before proceeding to the 



