Wakeman—Pigments of Flowering Plants. 779 
4. The existence of series of pigments related to similar sym¬ 
metrical, or almost symmetrical hydrocarbons of different de¬ 
grees of saturation. 
1. The influence of unsaturation upon the production of color 
in a molecule. 
In this connection it should be pointed out that all organic 
pigment molecules are unsaturated. The highest degree of sat¬ 
uration known in a pigment molecule is C n H 2n _ 4 , and visible 
color exists in substances of this degree of saturation only 
when the quinone grouping is present, the quinone grouping 
being one of the best known and most reliable of the chromo- 
phorous groups. 
Among substances referable to hydrocarbons of the degree of 
saturation C n H 2n _ 6 no substances colored in the ordinary sense 
are known to exist but several pigment producing substances 
are known. All substances, however, having a benzenoid group¬ 
ing are colored in a physical sense, since they all exhibit selec¬ 
tive absorption, not, it is true in the visible portion of the 
spectrum but just beyond it in the ultra violet. 
The largest number by far of plant pigments are referable to 
hydrocarbons of the degrees of saturation C n H 2n — 14 and 
C n H 2n _ 16 , through colored substances of known constitltion refer¬ 
able to hydrocarbons of higher unsaturation, up to C n H 2n _ 34 , 
have been isolated from plants. Moreover, all colored hydro¬ 
carbons, in which the production of color cannot be attributed 
to the usual chromophorous groups, are highly unsaturated, 
caroten and lycopen being of the degree of saturation C n H 2n _ 24 , 
while the blue hydrocarbon from oil of milfoil, having the for¬ 
mula C 15 H 18 , is apparently of the degree of saturation C n H 2n _ 12 . 
2. Influence of co-called chromophorous groups upon the pro¬ 
duction of color. 
As has been pointed out elsewhere in this paper, but little 
has been contributed to our knowledge of pigmentation in plants 
by a study of chromophorous groups, since only the corbonyl 
of the so-called chromophorous groups and the hydroxyl of the 
auxochrome groups are of at all frequent occurrence in plant 
pigments. Neither is the mere presence of either or both of 
these groups, or of multiples of one or both, sufficient to explain 
the phenomenon of color in any known plant pigment. In 
