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 



