SHULL: DUPLICATION OF A LEAF-LOBE FACTOR 



433 



interest in certain peculiarities of inflorescence and capsule characters, 

 which will be discussed elsewhere, (b) The second fact which sug- 

 gested the probable occurrence of duplication of the B factor in this 

 strain was the frequent preponderance of rhomboidea-Vike plants in 

 the pure-bred families. A large portion of the plants which, because 

 of precocious development, did not reach the full typical adult con- 

 dition, showed, nevertheless, highly developed rhomboidea characters, 

 even when no trace of the elongated A lobes was discernible. Other 

 plants in the same pure-bred families showed conspicuous elongation 

 of the primary lobes, thus making certain that the A factor was 

 present. If two B factors were present, namely B and B', associated 

 with only one A factor, the greater ease with which the A character, 

 as compared with the B character, was suppressed by unfavorable 

 conditions, would be readily comprehensible. 



With this clue to a possible interpretation of the rosette characters 

 in the Tucson strain, especial note was taken of the F2 families from 

 crosses between the same Tucson biotype and B. Heegeri simplex, 

 but in every case the number of individuals which fruited with rel- 

 atively undifferentiated "juvenile" condition of the rosettes was so 

 great that the F2 ratios gave no fully convincing proof of the correct- 

 ness of the hypothesis. The results of such crosses are given in 

 Table 2. 



Table 2 



The Composition of the Fi Progenies from a Cross between Bursa hursa-pastoris heteris 



from Tucson, Arizona, and B. Heegeri simplex, grown at the Station for 



Experimental Evolution 



The results in the several families were quite consistent, but the 

 •deviation from the expected 15 : i ratio were in all cases very con- 

 siderable. If it is taken into account, however, that the demon- 

 strated difficulty in genetical studies with this Tucson strain arises 



