314 
COLOUR IN NATURE 
CHAP. 
that lipochromes are of great importance both as 
reserve stuffs and as oxygen carriers in the process 
of assimilation. When the metabolism of the cell is 
active, oxygen is withdrawn from the yellow pigment, 
and it becomes converted into green chlorophyll. 
When metabolism diminishes, the green chlorophyll 
becomes oxidised and is converted into a lipochrome, 
and thus the colours of autumnal leaves, of fruits, and 
of flowers are produced. The fact that chlorophyll 
is commonly regarded as a nitrogenous compound, 
which the lipochromes are certainly not, is nowhere 
alluded to. Lipochromes, Simroth regards as pig¬ 
ments of relatively great simplicity, especially char¬ 
acteristic of plants as the simpler organisms. When 
they occur in animals, they are to be looked upon 
as evidences of a primitive condition, though they 
may be utilised for purposes of warning coloration, 
mimicry, etc., such colours, according to the author, 
being always of simple nature. If lipochromes are, 
however, evidences of a primitive condition, it is 
difficult to understand why they should be so frequent 
in birds ; but the author does not touch upon this. 
The third kind of evidence upon which Simroth 
bases his thesis is the order of appearance of the 
colours which either belong to the right half of the 
spectrum, or are not pigmental colours at all. Such 
pigments are characterised by their chemical com¬ 
plexity, and are associated with complex tissues. 
Thus the greater intensity of animal life expresses 
itself in the nature of animal pigments ; the masses 
of simple colours, like red, yellow, and green, which 
are so common in plants, being rare in animals (but 
Crustacea ?). 
