122 BOTANICAL GAZETTE [august 





one lacking both C and R; and each of these whites will behave 

 differently in certain crosses, though all are recessive to colors 

 and may be quite indistinguishable from one another when pure- 

 bred. Individuals lacking either C or R, when crossed with other 

 individuals having the same genotypic constitution, or when crossed 

 with individuals of the third type, which lack both C and R, will 

 produce only white offspring; but when recessive whites of the 

 first two types are crossed together, the complementary factors, 

 C and R, necessary for the production of color, are brought together 

 and a colored F t results, as exemplified by the classic case of 

 Emily Henderson" sweet peas, in which two white-flowered 

 plants, differing externally only in the form of the pollen-grains, 

 produced " reversionary" purple offspring when crossed together. 

 Many similar "reversions" have been discovered by experimental 

 breeders in a considerable number both of plants and of animals, 

 and the old riddle of "reversion on crossing," exemplified by these 

 phenomena, has been given a satisfactory solution in the "factor 

 hypothesis." In so many organisms have different kinds of reces- 

 sive whites been found, that their discovery in additional species 

 no longer occasions surprise. 



Less is known of the chemistry of dominant whites, but it is 

 conceivable that these may also be of several kinds. It is plain 

 that any pigment which is readily converted into an allied colorless 

 compound would give a basis for a dominant white in which the 

 pigment nucleus coexists with a factor which changes it to its 

 colorless derivative. A suggestive illustration in vitro of such a 

 reaction is the ready reduction of indigo blue (C l6 H IO N 2 2 ) in 

 alkaline solutions to indigo white (C l6 H I2 N 2 2 ). Spiegler (14) 

 believed that he had succeeded in isolating a "white melanin 

 from white wool and white horsehair, and while Gortner (5, 6) 

 has been unable to confirm Spiegler's conclusions in this regard, 

 the general type of reaction suggested by Spiegler may be retained 

 as possibly explaining some cases of dominant white. Gortner 

 (5) has proposed a very different hypothesis, namely, that as mela- 

 nin is the product of an oxidase acting on a chromogen (tyrosin), 

 dominant whites may be the result of anti-enzymes which inhibit 

 the action of the oxidase. The same hypothesis is applicable to 



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