116 Shull. 







pected and the actual numbers in the F 2 from the new crosses is much 

 closer than in the original F 2 , largely due, I believe, to the fact that 

 we have continued to learn how best to meet the cultural requirements 

 of the plant. In the recent cultures nearly every individual could be 

 classified, while in the earlier cultures the plants became diseased after 

 growing long under unnatural conditions, and many died unclassified. 

 Our present interest lies in the frequency of occurrence of the two 

 kinds of capsules; the rosette -characters are given in the table 

 only for the purpose of completing the parallel with the previously 

 published results of the original F 2 . They serve, however, to demon- 

 strate again the complete independence of the capsule-characters from 

 the leaf-characters. It is seen that, with respect to the capsules in all 

 four families of the new F 2 , the agreement with the monohybrid ratio is 

 striking, whereas the original F2 showed in each family an approximation 

 to the ratio 15 : 1. 



Discussion. 



We have seen that considerable departures from the expected 

 ratios have appeared in many of the families, and yet, that in every 

 test the essential features required by the assumption that there are 

 two independent determiners for the triangular capsule have been 

 strikingly manifest. I take it, therefore, that this hypothesis must be 

 in a general way correct, although the deviations in the ratios have 

 raised other questions which will need still further investigation for 

 their definite solution. 



The deviations from the expected ratios chanced in F2 and Fs to 

 be all in the same direction, indicating in every family a deficiency in 

 the number of individuals of the Heegeri type, and two hypotheses were 

 suggested (SHULL 1911) as possibly accounting for this condition, namely, 



(a) that B. Heegeri is a constitutionally weaker type and that in con- 

 sequence there is a differential elimination of plants of this type; or 



(b) that there is a selective mating in which the union of unlike gametes 

 is favored. 



In the F 4 and F 5 , however, the Heegeri plants have been in excess 

 of expectation about as frequently as they have been deficient. This 

 result might seem to dispose of the first of the two hypotheses, - - the 

 one which I thought the more probable, for if the Heegeri type were 

 much weaker at an early stage of its development, than the bursa- 

 postoris type, we might expect that there would always be a deficiency 

 in the number of Heegeri plants reaching maturity. This would be an 



