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THE AMERICAN NATURALIST [Vol. XLVIII 



occur in Europe. Finally there is X C. Bur so-past oris 

 arachnoidea, a sterile, linear-leaved form, with a weak 

 stem and which frequently shows fasciation (Figs. 17-21). 

 To facilitate a distinction between these forms, a key is 

 appended : 



a. Early leaves broad* 



d. Secondary lobes pronounced. Treleaseana. 

 dd. Secondary lobes absent. Setchelliava. 



It was found that, besides the genes A, B, C and D, 

 whose existence was shown by Shull, there exists another 

 gene, N, responsible for the narrow character of the 

 earlier leaves. For the various forms, mentioned here, 

 the following zygotic constitutions have been tentatively 

 determined : simpler, bhun; rhomboidea , BBnn and Bonn; 

 Setchelliana, BbNn; Treleaseana, BBNn; attenuata, 

 bbNn; arachnoidea, BBNN, BbNN and bbNN. The zygo- 

 tic constitution of orbicularis has not been determined. 



As to the probable origin of X C. Bursa-pastoris Setch- 

 elliana, little can be said. It most probably results from a 

 cross between rhomboid 'va and attenuata (BBnn X bbNn). 

 This seems the most plausible explanation since, judging 

 from herbarium specimens, both attenuata and rhom- 

 boidea occur throughout the United States. Unfortu- 

 nately such an assumption necessitates an explanation of 

 the origin of attenuata. 



My thanks are due to the regents of the University of 

 Michigan for the facilities placed at my disposal, to head 

 gardener Adolph Weiner for his constant care of the ex 

 perimental plants, to Messrs. J. H. Ehlers, A. Povah, C. 

 Oberlin and A. W. Murdock for assistance in classifica- 

 tion of the seedlings and to the director of the Missouri 

 Botanical Garden for the loan of herbarium material. 



