Wakeman—Pigments of Flowering Plants. 777 
type of oscillations within the molecule by the entrance of a 
new group, by the formation of a molecular compound, or by 
the associations of the molecule of a solute with those of its 
solvent. Three cases are possible: 
1. The new oscillation may coincide with and increase the 
original oscillation; then the absorption band will move further 
toward the ultra-violet and the compound will remain, or be¬ 
come visibly colorless. 
2. The new oscillation may be an entirely different type from 
the original, in which case new bands will appear, and since 
the original oscillation will be retarded to some extent, there 
will be a change but not a very considerable one in the visible 
color of the substance. 
3. The new oscillation may combine with the original and 
retard it greatly, when there will be a considerable sharp 
change in color. 
PREFACE. 
Work upon plant pigments was begun by the writer during 
the summer of 1907 when, as an undergraduate student, her 
attention was directed to pigmentation in the Monarda species. 
Since that time, though the subject has been sometimes tem¬ 
porarily pushed aside by other interests, it has never been lost 
sight of and it has usually been the subject of most absorbing 
interest. At times the work has appeared to be of a purely 
chemical nature, without any biochemical significance, as in the 
study of thymoquinone, hydrothymoquinone, and the oxidation 
products of thymoquinone. Its object at these times has been 
to elucidate the behavior of certain plant pigments, i. e. those 
of the Monarda species. Sometimes it has been of a character 
usual in the study of plant pigments, the extraction of pigments 
from the plants themselves and an examination of the pro¬ 
ducts obtained. At other times it has been of what is gener¬ 
ally considered of a more purely biochemical character, a study 
of oxidases, water content, etc., and their relation to the for¬ 
mation of pigments in plants. 
It has been found in the course of these investigations that no 
adequate and satisfactory knowledge of plant pigments can be 
