64 ESTABLISHMENT OF VARIETIES IN COLEUS 



Riddle (1909), in a comprehensive survey of the chemical and phys- 

 iological facts regarding the origin and nature of melanin pigments, 

 points out (p. 323) that "a single chromogen acted upon by a single 

 enzyme usually produces several colors depending upon the degree of 

 oxidation involved." He gives conclusive evidence that " the power to 

 oxidize tyrosin compounds is not dependent primarily upon germinal 

 segregation, but rather upon active tissues, relations, and conditions," 

 and that local conditions, especially in pathological cases, determine the 

 production of melanin. In general Riddle shows the inadequacy of a 

 strict interpretation of color inheritance in animals on the basis of unit 

 factors and gametic purity. 



In the light of all the chemical studies on pigmentation, it seems 

 clear that qualitative reactions are concerned which involve the pro- 

 duction, flow, and assembling of substances through the relations and 

 interactions between cells. That these interactions should be so widely 

 readjusted in a hybrid progeny resulting in such varied expression of 

 color in quantity, quality, and distribution is suggestive that funda- 

 mental readjustments may occur more readily with characters that are 

 dependent on cellular interactions than with those that are strictly meti- 

 dentical. As already pointed out, the variations in Cokus propagated 

 vegetatively give numerous patterns differing widely in regard to quan- 

 tity and distribution of the pigments concerned and present the same 

 sort of phenomena of readjustment seen in seed progeny of hybrids. 



In questions relating to the development of color patterns, the 

 Liesegang precipitation phenomena, especially as developed by 

 Gebhardt (1912) and Kiister (1913), seem to me most illuminating. By 

 the various phenomena associated with rhythmic precipitation and 

 crystallization of mineral solutions in gelatin plates, Gebhardt was able 

 to produce simple and multiple eye, line, and flaked patterns strikingly 

 similar to various markings in butterfly wings. By varying the sub- 

 stances used, modifying the amounts, the distribution, the degree of 

 concentration, and providing for interaction between areas of different 

 concentration, a wide range of markings were produced. Gebhardt 

 points out that such physical and chemical phenomena indicate that 

 the distribution of pigment even in the intricate markings of butterfly 

 wings may be due to an epigenetic regulation of the quality and quan- 

 tity of such substances as chromogen, oxydase, and oxygen and the 

 reciprocal influences of different centers of action. It is pointed out that 

 the cell boundaries and especially the position of veins may determine 

 the distribution of the substances involved and determine the relative 

 locations of centers of action as well as areas and centers of no action. 



Kiister (1913) extended the study of Liesegang precipitation pheno- 

 mena to the effects produced in capillary tubes, obtaining various types 

 of banded precipitation patterns which, as he emphasizes, suggest that 

 similar chemical and physical processes may determine many types of 

 variegation in both monocots and dicots. 



