lo Bulletin 392 



Experimental tests of both a chemical and a genetical nature have 

 demonstrated that most whites are due to the absence of some part of 

 the mechanical device for the formation of pigment and are recessive to 

 the pigmented forms when crossed with them. 



Some plants show by their genetical behavior that either of two factors 

 in pigment formation — chromogen or oxidase — may be lacking from a 

 variety. It is possible, for example, in such plants as sweet peas, stock, 

 and lychnis, by mating certain white-flowered individuals, or in the aleu- 

 rone of maize by crossing two white-seeded strains, to bring together the 

 two complementary factors and thus produce a reversionary, colored Fi 

 generation. 



In the case of the so-called "dominant whites" the lack of color is 

 supposed to be due to an inhibitor which prevents the formation of the 

 pigment altho all the necessary substances for its formation may be 

 present. The action of the inhibitor may be complete, as in the dominant 

 whites, or it may be only partial, resulting in dilute colors — as in stocks, 

 where it is supposed to be responsible for the dilute varieties of rose, 

 flesh, and other pale colors. The factor in these cases is supposed to be 

 of a limiting nature and to prevent entire reaction. 



In some cases the inhibiting factor appears to be of such a nature that 

 it acts locally, resulting in parti-colored "flaked" forms, or in various 

 color patterns, or in the dominance of what appears to be a lower grade 

 of pigmentation over a higher grade. In many cases the inhibition is not 

 quite complete, and dominant whites are often distinguishable by the 

 presence of patches or washes of color not found in the recessive whites 

 (Shull, 1912:121). In Papavar Rhoeas the presence of a white margin on 

 the petals is dominant over its absence. It is probable, therefore, that 

 the white margin is due to the presence of an inhibitor localized in the 

 margin of the petals (Shull, 191 2 : 128). There is also an inhibitor which 

 affects the body of the petals, prodiicing what is essentially a dominant 

 white, tho the inhibition is often very imperfect, in which case the flowers 

 are more or less washed and striated with color but generaUy whitish 

 (Shull, 1912 : 134). 



Miss Marryat (1909 : 45) reports that in her experiments with Mirahilis 

 jalapa, flaking proved dominant thruout; and Correns (1910:424) con- 

 firms her results. In the writer's experiments with morning-glories the 

 same condition has been found to obtain. The distribution and behavior 

 of color in such cases as these would seem to indicate a localization of 

 the color-producing substances rather than the presence of an inhibitor. 



The chemical substances that react together to form anthocyanic pig- 

 ments can be interpreted in mendelian terms if it is considered that the 

 mendelian factors involved in the various stages of color production are 

 in the nature of enzymes, oxidases, and chromogens. Their activities 



126 



