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THE AMERICAN NATURALIST [Vol. LIV 



mosomes themselves in these species are unusually favorable 

 objects of study, one of my correspondents going so far as to 

 predict that in time Crepis will become as famous and useful for 

 laboratory work as Ascaris is to-day. But the important consid- 

 eration in the present discussion is the fact that we have here 

 several species with the same chromosome number as Drosophila 

 melanogaster and at least one species with one less chromosome 

 pair. Obviously, if some of these species with the smallest 

 chromosome numbers are highly variable, existing in a large 

 number of distinct varieties or forms, they should serve as ex- 

 cellent material for genetic study especially if they possess the 

 other advantageous features already mentioned. 



For at least two such species I can report very great promise 

 as objects of genetic research. Crepis capillaris {virens) 2 with 

 three chromosome pairs (Rosenberg, 1909, 1918; Digby, 1914) 

 and C. tectorum 3 with four pairs ( Juel, 1905 ; Rosenberg, 1909, 

 1918) both exhibit polymorphism to a remarkable degree. This 

 is evidenced by the diversity of forms referred to these species 

 in the herbaria of the Royal Botanic Gardens at Kew and of the 

 Museum of Natural History in Paris. In both species it seems 

 to be merely a matter of sufficiently extensive seed collection 

 that is required in order to secure a sufficient number of allelo- 

 morphic pairs of characters to make possible the desired genetic 

 analysis. My cultures of C. virens, which have been grown from 

 seed secured from various foreign countries as well as in Cali- 

 fornia, have already yielded several pairs of contrasted char- 

 acters which will soon furnish a nucleus of genetic data on this 



These two species are also very prolific, considering the plant 

 as a whole, there being several or many heads on a plant and 

 each head bearing 5 to 15 fertile achenes in virens and 30 to 40 

 in tectorum. Unfortunately an individual flower produces but a 

 single seed and the flowers are so small as to make the work of 

 hybridization rather tedious when absolute control is exercised 

 through castration of the unopened flower. But, while this 

 method is essential in original crosses, it usually is not necessary 

 to castrate many flowers for any one cross, and when it comes to 



and Fernald (1908) and Britton and Brown (1918) name it C. capillaris 

 P. virens L. for this species. 



