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THE AMERICAN NATURALIST [Vol. XLIV 



latter are about half an inch in length. The progeny of 

 such varieties belong to the same classes as the progeny 

 of self-colored varieties. They give fewer self-colored 

 seedlings, however, which may be due to the action of 

 one or more unknown heritable factors. I have not at- 

 tempted to separate the self-colored from those showing 

 color in the young shoots, but have classified both as 

 colored varieties. 



The results of selfing varieties with different color 

 characters are as follows: 



Selfed purples gave either all purples (one variety) ; 

 purples, reds and colorless (two varieties) ; or purples 

 and colorless (three varieties). Selfed reds gave either 

 all reds (two varieties), or reds and colorless (two 

 varieties). Selfed red varieties gave no purple progeny. 

 Three colorless varieties (that is, no color in either the 

 tuber skin or young shoots) were selfed, giving all color- 

 less progeny. 



Without considering factors for limiting color, these 

 results seem to show that purple and red are separate 

 Mendelian units, each dominant to its absence, and that 

 purple is epistatic to red. 



It is an interesting fact that although the purple vari- 

 eties and the red varieties are distinct color types with- 

 out intermediates and that mosaic varieties of each are 

 known, yet in no case has a mosaic variety appeared in 

 which splashes of the two colors are found. We may 

 conclude therefore that the two colors are formed by the 

 action of other factors upon the same chemical constit- 

 uent. If we assume that the red color is a lower form 

 of oxidation than the purple color and that they are pro- 

 duced by different oxidases R and P acting upon the 

 same substance C, the results obtained are explained, for 

 the presence of P would oxidize all of the stubstrate to 

 the purple color. 



Shape of Tuber 

 Potato tubers vary in shape from a length six times 

 the median diameter to a length about the same as the 



