434 



BROOKLYN BOTANIC GARDEN MEMOIRS 



from the suppression of dominant characters it will be obvious that 

 these ratios are modifications from a higher ratio {e. g., 15 : i) rather 

 than from the 3 : i ratio to which the empirical results in fact more 

 closely approximate. 



A new attempt to test the constitution of the rosette in the shep- 

 herd's-purse of Tucson, was made with seeds received from Dr. 

 Forrest Shreve on November 29, 1914. The same care was given to 

 the culture of the families involved in this new experiment as was 

 exercised in those recorded in Table 2. The better greenhouse facili- 

 ties available at Princeton as compared with those at Cold Spring 

 Harbor, where the previous cultures had been grown, made it possible 

 to secure a closer approximation to the expected ratios, as seen in 

 Table 3, the deviation being the same as before in direction but less 

 in amount. 



Table 3 



The Composition of the F2 Progenies from a Cross between Bursa bursa-pastoris heteris 

 from Tucson, Arizona, and B. Heegeri simplex grown at Princeton University 



Fortunately for genetical studies on the rosette characters of 

 shepherd's-purse, such extensive suppression of characters occurs in 

 nearly all of the other biotypes which have been under observation, 

 only as a result of distinctly unfavorable environmental conditions. 

 The study of a large number of these biotypes from other regions, 

 in crosses with B. Heegeri simplex, has brought to light several other 

 cases of probable duplication of the B factor, as shown in Table 4. 



The cultures in Table 4 also show for the most part distinctly 

 defective ratios, due certainly in the main to the fact that they were 

 being grown in an extensive study of the capsule determiners, and that 

 in consequence suitable conditions were not provided in many cases 

 for full development of the rosette characters. Here and there a 

 ratio closely agrees with the expected F2 ratio, 15 : i, the best fits 

 being seen in certain families grown from seeds from Bremen, Germany. 

 By chance these families from Bremen grew under more favorable 

 conditions than many of the other cultures listed in this table and 

 this fact doubtless explains in part why they show a closer approxima- 



