66 
COLOUR IN NATURE 
CHAP. 
slow and partial, giving rise to the appearance of a 
variegated leaf. Further, owing either to the for¬ 
mation of several lipochromes, or to the unequal 
distribution of a single lipochrome, parts of the 
leaf become a deeper shade of yellow or orange, 
sometimes becoming intensified to a dull red, while 
in some cases there is produced a special red 
anthocyan pigment. There are therefore three 
main factors in the production of the tints of 
autumn : (i) the disappearance of the chlorophyll- 
green, (2) the increasing prominence of the lipo¬ 
chromes, and (3) the development of anthocyan. 
Other changes of minor importance also occur. 
Thus the general effect is often heightened by the 
dull brown colours assumed by the leaves of such 
trees as the oak and the beech. These colours are 
produced by the oxidation of the tannins of which 
these trees contain such an abundant supply. These 
substances are probably useless, and are got rid of 
in the falling leaves and the bark. Although these 
changes tend to occur with great regularity every 
autumn, it is a matter of common experience that 
they are to a large degree dependent upon the 
weather, a fine dry autumn with a touch of frost 
being specially favourable to the development of 
brilliant colouring. Autumn colouring is of great 
interest in a comparative study of coloration. There 
is no reason to suppose that the colouring is of the 
slightest use to the trees, and yet it often displays 
to an extraordinary degree that beauty and perfectness 
which we are accustomed to regard as the result of 
the action of Natural Selection. It is further of 
fundamental importance in the investigation of the 
