56 
COLOUR IN NATURE 
CHAP. 
Flowering Plants, and it is only here that it is 
seriously regarded as the result of a process of 
selection. The unequal distribution of chlorophyll 
is, however, well marked in some of the lower plants. 
As to the associated pigments, we find that in 
Flowering Plants the chlorophyll of green leaves is 
always mingled with a greater or less amount of a 
yellow lipochrome usually known as xanthophyll. 
The different tints of leaves are caused in great part 
by the varying amounts of the two pigments present, 
a large amount of xanthophyll giving the leaf a 
yellowish tint. The exact relation of this yellow 
pigment to the colouring-matter of leaves blanched 
by growing in darkness is uncertain, but the two are 
probably at least nearly related. We know from 
investigations on the lipochromes of animals that 
these pigments are not only peculiarly unstable, but 
that a number of them frequently occur simultane¬ 
ously in an organism, and are then very difficult to 
distinguish from one another. This fact enables us 
to understand readily how it is that the questions 
whether there is more than one lipochrome associated 
with chlorophyll, and whether etiolin (the colouring- 
matter of blanched leaves) is or is not identical with 
xanthophyll, remain undecided. In considering the 
colours of flowers and fruits we shall find that 
xanthophyll, or pigments closely related to it, may 
play an important part in the economy of the plant, 
but the function of the xanthophyll connected with 
chlorophyll is unknown, if indeed it possess any. 
The only suggestion that has been made in this 
matter is that the xanthophyll may have some 
function in protecting the protoplasm of the cells 
