122 TEE AMERICAN NATURALIST [Vol. LVI 



play an important part in the formation of pigments, 

 or in changes in pigments already formed. Shade of 

 color, for example, is evidently often merely a function 

 of oxidase content. With a graded increase of oxidase, 

 therefore, a plant might be put through a regular gamut 

 of color effects. In general, the mere addition of lij- 

 drogen to dye-stuffs reduces them to the corresponding 

 leuco base. Armstrong, in his quinone theory of color, 

 makes much of the quinone s as the colored compounds 

 in dye-stuffs and maintains that the corresponding color- 

 less compounds are hydro-quinones. The structure and 

 size of the pigment molecule itself seems to be an impor- 

 tant factor in color. Hydrocarbon radicals, for instance, 

 deepen the tint ; the addition of hydrogen raises the tint. 

 In analyzing any particular case one has to take into ac- 

 count the original molecule, the position of any group 

 introduced and the number of groups introduced. It is 

 possible, for example, by the introduction of a tint-deep- 

 ening group to deepen the color, but by introducing two 

 or three more such groups to throw the absorption wholly 

 outside the visible spectrum and thus do away with tlie 

 color. From this it is clear that two compounds may be 

 closely related in constitution and yet one be colored, the 

 other colorless. As to why, physically, in both the aro- 

 matic and the aliphatic series, color is produced in certain 

 compounds and not in others, various investigators have 

 arrived at the conclusion that the cause of color is due 

 to " the making and breaking of contact between atoms, 

 thus giving them marked activity," a process known as 

 isorropesis, and Miss Wakeman goes on to explain : 



This change of linkage which must accompany the transforma- 

 tion of one modification of the compound to the other is the source 

 of the oscillations producing the absorption bands. If these oscilla- 

 tions are synchronous with light waves of a high frequency they 

 give rise to absorption bands in the ultra violet and the compound 

 is colorless. If, however, they are less frequent, the absorption band 

 apijcars in the visible portion of the spectrum and this absorption 

 of colored rays results in the compound taking on complementary 



