S6 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 



